Patent Publication Number: US-8111265-B2

Title: Systems and methods for brightness preservation using a smoothed gain image

Description:
RELATED REFERENCES 
     This application is a continuation-in-part of U.S. patent application Ser. No. 11/460,907, entitled “Methods and Systems for Generating and Applying Image Tone Scale Corrections,” filed on Jul. 28, 2006 now U.S. Pat. No. 7,982,707; which is a continuation-in-part of U.S. patent application Ser. No. 11/293,562, entitled “Methods and Systems for Determining a Display Light Source Adjustment,” filed on Dec. 2, 2005 now U.S. Pat. No. 7,924,261; which is a continuation-in-part of U.S. patent application Ser. No. 11/224,792, entitled “Methods and Systems for Image-Specific Tone Scale Adjustment and Light-Source Control,” filed on Sep. 12, 2005 now U.S. Pat. No. 7,961,199; which is a continuation-in-part of U.S. patent application Ser. No. 11/154,053, entitled “Methods and Systems for Enhancing Display Characteristics with High Frequency Contrast Enhancement,” filed on Jun. 15, 2005; U.S. patent application Ser. No. 11/224,792 is also a continuation-in-part of U.S. patent application Ser. No. 11/154,054, entitled “Methods and Systems for Enhancing Display Characteristics with Frequency-Specific Gain,” filed on Jun. 15, 2005; U.S. patent application Ser. No. 11/224,792 is also a continuation-in-part of U.S. patent application Ser. No. 11/154,052, entitled “Methods and Systems for Enhancing Display Characteristics,” filed on Jun. 15, 2005 now U.S. Pat. No. 7,800,577; Ser. Nos. 11/154,052. 11/154,053 and 11/154,054 all claim the benefit of U.S. Provisional Patent Application No. 60/670,749, entitled “Brightness Preservation with Contrast Enhancement,” filed on Apr. 11, 2005 and U.S. Provisional Patent Application No. 60/660,049, entitled “Contrast Preservation and Brightness Preservation in Low Power Mode of a Backlit Display,” filed on Mar. 9, 2005 and U.S. Provisional Patent Application No. 60/632,776, entitled “Luminance Matching for Power Saving Mode in Backlit Displays,” filed on Dec. 2, 2004 and U.S. Provisional Patent Application No. 60/632,779, entitled “Brightness Preservation for Power Saving Modes in Backlit Displays,” filed on Dec. 2, 2004; U.S. patent application Ser. No. 11/224,792 also claims the benefit of U.S. Provisional Patent Application No. 60/710,927, entitled “Image Dependent Backlight Modulation,” filed on Aug. 23, 2005; this application is also a continuation-in-part of U.S. patent application Ser. No. 11/460,940, entitled “Methods and Systems for Color Preservation with Image Tonescale Corrections,” filed on Jul. 28, 2006 now U.S. Pat. No. 7,515,160. 
    
    
     FIELD OF THE INVENTION 
     Embodiments of the present invention comprise methods and systems for image brightness preservation using a spatially-smoothed gain map. 
     BACKGROUND 
     A typical display device displays an image using a fixed range of luminance levels. For many displays, the luminance range has 256 levels that are uniformly spaced from 0 to 255. Image code values are generally assigned to match these levels directly. 
     In many electronic devices with large displays, the displays are the primary power consumers. For example, in a laptop computer, the display is likely to consume more power than any of the other components in the system. Many displays with limited power availability, such as those found in battery-powered devices, may use several illumination or brightness levels to help manage power consumption. A system may use a full-power mode when it is plugged into a power source, such as A/C power, and may use a power-save mode when operating on battery power. 
     In some devices, a display may automatically enter a power-save mode, in which the display illumination is reduced to conserve power. These devices may have multiple power-save modes in which illumination is reduced in a step-wise fashion. Generally, when the display illumination is reduced, image quality drops as well. When the maximum luminance level is reduced, the dynamic range of the display is reduced and image contrast suffers. Therefore, the contrast and other image qualities are reduced during typical power-save mode operation. 
     Many display devices, such as liquid crystal displays (LCDs) or digital micro-mirror devices (DMDs), use light valves which are backlit, side-lit or front-lit in one way or another. In a backlit light valve display, such as an LCD, a backlight is positioned behind a liquid crystal panel. The backlight radiates light through the LC panel, which modulates the light to register an image. Both luminance and color can be modulated in color displays. The individual LC pixels modulate the amount of light that is transmitted from the backlight and through the LC panel to the user&#39;s eyes or some other destination. In some cases, the destination may be a light sensor, such as a coupled-charge device (CCD). 
     Some displays may also use light emitters to register an image. These displays, such as light emitting diode (LED) displays and plasma displays use picture elements that emit light rather than reflect light from another source. 
     SUMMARY 
     Some embodiments of the present invention comprise systems and methods for varying a light-valve-modulated pixel&#39;s luminance modulation level to compensate for a reduced light source illumination intensity or to improve the image quality at a fixed light source illumination level. 
     Some embodiments of the present invention may also be used with displays that use light emitters to render an image. These displays, such as light emitting diode (LED) displays and plasma displays use picture elements that emit light rather than reflect light from another source. Embodiments of the present invention may be used to enhance the image produced by these devices. In these embodiments, the brightness of pixels may be adjusted to enhance the dynamic range of specific image frequency bands, luminance ranges and other image subdivisions. 
     In some embodiments of the present invention, a display light source may be adjusted to different levels in response to image characteristics. When these light source levels change, the image code values may be adjusted to compensate for the change in brightness or otherwise enhance the image. 
     Some embodiments of the present invention comprise ambient light sensing that may be used as input in determining light source levels and image pixel values. 
     Some embodiments of the present invention comprise distortion-related light source and battery consumption control. 
     Some embodiments of the present invention comprise systems and methods for generating and applying image tone scale corrections. 
     Some embodiments of the present invention comprise methods and systems for image tone scale correction with improved color fidelity. 
     Some embodiments of the present invention comprise methods and systems for brightness preservation using a smoothed gain image. 
     The foregoing and other objectives, features, and advantages of the invention will be more readily understood upon consideration of the following detailed description of the invention taken in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL DRAWINGS 
         FIG. 1  is a diagram showing prior art backlit LCD systems; 
         FIG. 2A  is a chart showing the relationship between original image code values and boosted image code values; 
         FIG. 2B  is a chart showing the relationship between original image code values and boosted image code values with clipping; 
         FIG. 3  is a chart showing the luminance level associated with code values for various code value modification schemes; 
         FIG. 4  is a chart showing the relationship between original image code values and modified image code values according to various modification schemes; 
         FIG. 5  is a diagram showing the generation of an exemplary tone scale adjustment model; 
         FIG. 6  is a diagram showing an exemplary application of a tone scale adjustment model; 
         FIG. 7  is a diagram showing the generation of an exemplary tone scale adjustment model and gain map; 
         FIG. 8  is a chart showing an exemplary tone scale adjustment model; 
         FIG. 9  is a chart showing an exemplary gain map; 
         FIG. 10  is a flow chart showing an exemplary process wherein a tone scale adjustment model and gain map are applied to an image; 
         FIG. 11  is a flow chart showing an exemplary process wherein a tone scale adjustment model is applied to one frequency band of an image and a gain map is applied to another frequency band of the image; 
         FIG. 12  is a chart showing tone scale adjustment model variations as the MFP changes; 
         FIG. 13  is a flow chart showing an exemplary image dependent tone scale mapping method; 
         FIG. 14  is a diagram showing exemplary image dependent tone scale selection embodiments; 
         FIG. 15  is a diagram showing exemplary image dependent tone scale map calculation embodiments; 
         FIG. 16  is a flow chart showing embodiments comprising source light level adjustment and image dependent tone scale mapping; 
         FIG. 17  is a diagram showing exemplary embodiments comprising a source light level calculator and a tone scale map selector; 
         FIG. 18  is a diagram showing exemplary embodiments comprising a source light level calculator and a tone scale map calculator; 
         FIG. 19  is a flow chart showing embodiments comprising source light level adjustment and source-light level-dependent tone scale mapping; 
         FIG. 20  is a diagram showing embodiments comprising a source light level calculator and source-light level-dependent tone scale calculation or selection; 
         FIG. 21  is a diagram showing a plot of original image code values vs. tone scale slope; 
         FIG. 22  is a diagram showing embodiments comprising separate chrominance channel analysis; 
         FIG. 23  is a diagram showing embodiments comprising ambient illumination input to the image processing module; 
         FIG. 24  is a diagram showing embodiments comprising ambient illumination input to the source light processing module; 
         FIG. 25  is a diagram showing embodiments comprising ambient illumination input to the image processing module and device characteristic input; 
         FIG. 26  is a diagram showing embodiments comprising alternative ambient illumination inputs to the image processing module and/or source light processing module and a source light signal post-processor; 
         FIG. 27  is a diagram showing embodiments comprising ambient illumination input to a source light processing module, which passes this input to an image processing module; 
         FIG. 28  is a diagram showing embodiments comprising ambient illumination input to an image processing module, which may pass this input to a source light processing module; 
         FIG. 29  is a diagram showing embodiments comprising distortion-adaptive power management; 
         FIG. 30  is a diagram showing embodiments comprising constant power management; 
         FIG. 31  is a diagram showing embodiments comprising adaptive power management; 
         FIG. 32A  is a graph showing a comparison of power consumption of constant power and constant distortion models; 
         FIG. 32B  is a graph showing a comparison of distortion of constant power and constant distortion models; 
         FIG. 33  is a diagram showing embodiments comprising distortion-adaptive power management; 
         FIG. 34  is a graph showing backlight power levels at various distortion limits for an exemplary video sequence; 
         FIG. 35  is a graph showing exemplary power/distortion curves; 
         FIG. 36  is a flow chart showing embodiments that manage power consumption in relation to a distortion criterion; 
         FIG. 37  is a flow chart showing embodiments comprising source light power level selection based on distortion criterion; 
         FIGS. 38A  &amp; B are a flow chart showing embodiments comprising distortion measurement which accounts for the effects of brightness preservation methods; 
         FIG. 39  is a power/distortion curve for exemplary images; 
         FIG. 40  is a power plot showing fixed distortion; 
         FIG. 41  is a distortion plot showing fixed distortion; 
         FIG. 42  is an exemplary tone scale adjustment curve; 
         FIG. 43  is a zoomed-in view of the dark region of the tone scale adjustment curve shown in  FIG. 42 ; 
         FIG. 44  is another exemplary tone scale adjustment curve; 
         FIG. 45  is a zoomed-in view of the dark region of the tone scale adjustment curve shown in  FIG. 44 ; 
         FIG. 46  is a chart showing image code value adjustment based on a maximum color channel value; 
         FIG. 47  is a chart showing image code value adjustment of multiple color channels based on maximum color channel code value; 
         FIG. 48  is a chart showing image code value adjustment of multiple color channels based on a code value characteristic of one of the color channels; 
         FIG. 49  is a diagram showing embodiments of the present invention comprising a tone scale generator that receives a maximum color channel code value as input; 
         FIG. 50  is a diagram showing embodiments of the present invention comprising frequency decomposition and color channel code distinctions with tone scale adjustment; 
         FIG. 51  is a diagram showing embodiments of the present invention comprising frequency decomposition, color channel distinction and color-preserving clipping; 
         FIG. 52  is a diagram showing embodiments of the present invention comprising color-preserving clipping based on color channel code value characteristics; 
         FIG. 53  is a diagram showing embodiments of the present invention comprising a low-pass/high-pass frequency split and selection of a maximum color channel code value 
         FIG. 54  is a diagram showing embodiments of the present invention comprising gain image smoothing; 
         FIG. 55  is a diagram showing embodiments of the present invention comprising gain image smoothing and a HP/HF gain process; 
         FIG. 56  is a diagram showing embodiments of the present invention comprising gain image smoothing and an image-specific gain process; 
         FIG. 57  is a diagram showing embodiments of the present invention comprising gain image smoothing and a gain process based on color channel analysis; 
         FIG. 58  is a diagram showing embodiments of the present invention comprising gain image smoothing and color channel cove value characteristic analysis; and 
         FIG. 59  is a diagram showing embodiments of the present invention comprising gain image smoothing and color-preserving clipping. 
     
    
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     Embodiments of the present invention will be best understood by reference to the drawings, wherein like parts are designated by like numerals throughout. The figures listed above are expressly incorporated as part of this detailed description. 
     It will be readily understood that the components of the present invention, as generally described and illustrated in the figures herein, could be arranged and designed in a wide variety of different configurations. Thus, the following more detailed description of the embodiments of the methods and systems of the present invention is not intended to limit the scope of the invention but it is merely representative of the presently preferred embodiments of the invention. 
     All of the patent applications listed above in the section entitled “Related References” are hereby incorporated herein by reference. 
     Elements of embodiments of the present invention may be embodied in hardware, firmware and/or software. While exemplary embodiments revealed herein may only describe one of these forms, it is to be understood that one skilled in the art would be able to effectuate these elements in any of these forms while resting within the scope of the present invention. 
     Display devices using light valve modulators, such as LC modulators and other modulators may be reflective, wherein light is radiated onto the front surface (facing a viewer) and reflected back toward the viewer after passing through the modulation panel layer. Display devices may also be transmissive, wherein light is radiated onto the back of the modulation panel layer and allowed to pass through the modulation layer toward the viewer. Some display devices may also be transflexive, a combination of reflective and transmissive, wherein light may pass through the modulation layer from back to front while light from another source is reflected after entering from the front of the modulation layer. In any of these cases, the elements in the modulation layer, such as the individual LC elements, may control the perceived brightness of a pixel. 
     In backlit, front-lit and side-lit displays, the light source may be a series of fluorescent tubes, an LED array or some other source. Once the display is larger than a typical size of about 18″, the majority of the power consumption for the device is due to the light source. For certain applications, and in certain markets, a reduction in power consumption is important. However, a reduction in power means a reduction in the light flux of the light source, and thus a reduction in the maximum brightness of the display. 
     A basic equation relating the current gamma-corrected light valve modulator&#39;s gray-level code values, CV, light source level, L source , and output light level, L out , is:
 
 L   out   =L   source   *g ( CV +dark) γ +ambient  Equation 1
 
     Where g is a calibration gain, dark is the light valve&#39;s dark level, and ambient is the light hitting the display from the room conditions. From this equation, it can be seen that reducing the backlight light source by x % also reduces the light output by x %. 
     The reduction in the light source level can be compensated by changing the light valve&#39;s modulation values; in particular, boosting them. In fact, any light level less than (1−x %) can be reproduced exactly while any light level above (1−x %) cannot be reproduced without an additional light source or an increase in source intensity. 
     Setting the light output from the original and reduced sources gives a basic code value correction that may be used to correct code values for an x % reduction (assuming dark and ambient are 0) is:
 
 L   out   =L   source   *g ( CV ) γ   =L   reduced   *g ( CV   boost ) γ   Equation 2
 
 CV   boost   =CV *( L   source   /L   reduced ) 1/γ   =CV *(1 /x  %) 1/γ   Equation 3
 
       FIG. 2A  illustrates this adjustment. In  FIGS. 2A and 2B , the original display values correspond to points along line  12 . When the backlight or light source is placed in power-save mode and the light source illumination is reduced, the display code values need to be boosted to allow the light valves to counteract the reduction in light source illumination. These boosted values coincide with points along line  14 . However, this adjustment results in code values  18  higher than the display is capable of producing (e.g., 255 for an 8 bit display). Consequently, these values end up being clipped  20  as illustrated in  FIG. 2B . Images adjusted in this way may suffer from washed out highlights, an artificial look, and generally low quality. 
     Using this simple adjustment model, code values below the clipping point  15  (input code value  230  in this exemplary embodiment) will be displayed at a luminance level equal to the level produced with a full power light source while in a reduced source light illumination mode. The same luminance is produced with a lower power resulting in power savings. If the set of code values of an image are confined to the range below the clipping point  15  the power savings mode can be operated transparently to the user. Unfortunately, when values exceed the clipping point  15 , luminance is reduced and detail is lost. Embodiments of the present invention provide an algorithm that can alter the LCD or light valve code values to provide increased brightness (or a lack of brightness reduction in power save mode) while reducing clipping artifacts that may occur at the high end of the luminance range. 
     Some embodiments of the present invention may eliminate the reduction in brightness associated with reducing display light source power by matching the image luminance displayed with low power to that displayed with full power for a significant range of values. In these embodiments, the reduction in source light or backlight power which divides the output luminance by a specific factor is compensated for by a boost in the image data by a reciprocal factor. 
     Ignoring dynamic range constraints, the images displayed under full power and reduced power may be identical because the division (for reduced light source illumination) and multiplication (for boosted code values) essentially cancel across a significant range. Dynamic range limits may cause clipping artifacts whenever the multiplication (for code value boost) of the image data exceeds the maximum of the display. Clipping artifacts caused by dynamic range constraints may be eliminated or reduced by rolling off the boost at the upper end of code values. This roll-off may start at a maximum fidelity point (MFP) above which the luminance is no longer matched to the original luminance. 
     In some embodiments of the present invention, the following steps may be executed to compensate for a light source illumination reduction or a virtual reduction for image enhancement:
         1) A source light (backlight) reduction level is determined in terms of a percentage of luminance reduction;   2) A Maximum Fidelity Point (MFP) is determined at which a roll-off from matching reduced-power output to full-power output occurs;   3) Determine a compensating tone scale operator;
           a. Below the MFP, boost the tone scale to compensate for a reduction in display luminance;   b. Above the MFP, roll off the tone scale gradually (in some embodiments, keeping continuous derivatives);   
           4) Apply tone scale mapping operator to image; and   5) Send to the display.       

     The primary advantage of these embodiments is that power savings can be achieved with only small changes to a narrow category of images. (Differences only occur above the MFP and consist of a reduction in peak brightness and some loss of bright detail). Image values below the MFP can be displayed in the power savings mode with the same luminance as the full power mode making these areas of an image indistinguishable from the full power mode. 
     Some embodiments of the present invention may use a tone scale map that is dependent upon the power reduction and display gamma and which is independent of image data. These embodiments may provide two advantages. Firstly, flicker artifacts which may arise due to processing frames differently do not arise, and, secondly, the algorithm has a very low implementation complexity. In some embodiments, an off-line tone scale design and on-line tone scale mapping may be used. Clipping in highlights may be controlled by the specification of the MFP. 
     Some aspects of embodiments of the present invention may be described in relation to  FIG. 3 .  FIG. 3  is a graph showing image code values plotted against luminance for several situations. A first curve  32 , shown as dotted, represents the original code values for a light source operating at 100% power. A second curve  30 , shown as a dash-dot curve, represents the luminance of the original code values when the light source operates at 80% of full power. A third curve  36 , shown as a dashed curve, represents the luminance when code values are boosted to match the luminance provided at 100% light source illumination while the light source operates at 80% of full power. A fourth curve  34 , shown as a solid line, represents the boosted data, but with a roll-off curve to reduce the effects of clipping at the high end of the data. 
     In this exemplary embodiment, shown in  FIG. 3 , an MFP  35  at code value  180  was used. Note that below code value  180 , the boosted curve  34  matches the luminance output  32  by the original 100% power display. Above  180 , the boosted curve smoothly transitions to the maximum output allowed on the 80% display. This smoothness reduces clipping and quantization artifacts. In some embodiments, the tone scale function may be defined piecewise to match smoothly at the transition point given by the MFP  35 . Below the MFP  35 , the boosted tone scale function may be used. Above the MFP  35 , a curve is fit smoothly to the end point of boosted tone scale curve at the MFP and fit to the end point  37  at the maximum code value 
     In some embodiments, the slope of the curve may be matched to the slope of the boosted tone scale curve/line at the MFP  35 . This may be achieved by matching the slope of the line below the MFP to the slope of the curve above the MFP by equating the derivatives of the line and curve functions at the MFP and by matching the values of the line and curve functions at that point. Another constraint on the curve function may be that it be forced to pass through the maximum value point [255,255]  37 . In some embodiments the slope of the curve may be set to 0 at the maximum value point  37 . In some embodiments, an MFP value of 180 may correspond to a light source power reduction of 20%. 
     In some embodiments of the present invention, the tone scale curve may be defined by a linear relation with gain, g, below the Maximum Fidelity Point (MFP). The tone scale may be further defined above the MFP so that the curve and its first derivative are continuous at the MFP. This continuity implies the following form on the tone scale function: 
     
       
         
           
             
               
                 
                   
                     
                       
                         y 
                         = 
                         
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                         B 
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                         = 
                         
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                                     · 
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                                   + 
                                   
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                                       ) 
                                     
                                     · 
                                     
                                       
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                   Equation 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   4 
                 
               
             
           
         
       
     
     The gain may be determined by display gamma and brightness reduction ratio as follows: 
     
       
         
           
             
               
                 
                   g 
                   = 
                   
                     
                       ( 
                       
                         FullPower 
                         ReducedPower 
                       
                       ) 
                     
                     
                       1 
                       γ 
                     
                   
                 
               
               
                 
                   Equation 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   5 
                 
               
             
           
         
       
     
     In some embodiments, the MFP value may be tuned by hand balancing highlight detail preservation with absolute brightness preservation. 
     The MFP can be determined by imposing the constraint that the slope be zero at the maximum point. This implies: 
     
       
         
           
             
               
                 
                   
                     
                       
                         slope 
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                   Equation 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   6 
                 
               
             
           
         
       
     
     In some exemplary embodiments, the following equations may be used to calculate the code values for simple boosted data, boosted data with clipping and corrected data, respectively, according to an exemplary embodiment. 
                         ToneScale   boost     ⁡     (   cv   )       =         (     1   /   x     )       1   /   γ       ·   cv       ⁢     
     ⁢         ToneScale   clipped     ⁡     (   cv   )       =     {                   (     1   /   x     )       1   /   γ       ·   cv           cv   ≤     255   ·       (   x   )       1   /   γ                   255       otherwise         ⁢     
     ⁢       ToneScale   corrected     ⁡     (   cv   )         =     {               (     1   /   x     )       1   /   γ       ·   cv           cv   ≤   MFP                 A   ·     cv   2       +     B   ·   cv     +   C         otherwise                         Equation   ⁢           ⁢   7               
The constants A, B, and C may be chosen to give a smooth fit at the MFP and so that the curve passes through the point [255,255]. Plots of these functions are shown in  FIG. 4 .
 
       FIG. 4  is a plot of original code values vs. adjusted code values. Original code values are shown as points along original data line  40 , which shows a 1:1 relationship between adjusted and original values as these values are original without adjustment. According to embodiments of the present invention, these values may be boosted or adjusted to represent higher luminance levels. A simple boost procedure according to the “tonescale boost” equation above, may result in values along boost line  42 . Since display of these values will result in clipping, as shown graphically at line  46  and mathematically in the “tonescale clipped” equation above, the adjustment may taper off from a maximum fidelity point  45  along curve  44  to the maximum value point  47 . In some embodiments, this relationship may be described mathematically in the “tonescale corrected” equation above. 
     Using these concepts, luminance values represented by the display with a light source operating at 100% power may be represented by the display with a light source operating at a lower power level. This is achieved through a boost of the tone scale, which essentially opens the light valves further to compensate for the loss of light source illumination. However, a simple application of this boosting across the entire code value range results in clipping artifacts at the high end of the range. To prevent or reduce these artifacts, the tone scale function may be rolled-off smoothly. This roll-off may be controlled by the MFP parameter. Large values of MFP give luminance matches over a wide interval but increase the visible quantization/clipping artifacts at the high end of code values. 
     Embodiments of the present invention may operate by adjusting code values. In a simple gamma display model, the scaling of code values gives a scaling of luminance values, with a different scale factor. To determine whether this relation holds under more realistic display models, we may consider the Gamma Offset Gain-Flair (GOG-F) model. Scaling the backlight power corresponds to linear reduced equations where a percentage, p, is applied to the output of the display, not the ambient. It has been observed that reducing the gain by a factor p is equivalent to leaving the gain unmodified and scaling the data, code values and offset, by a factor determined by the display gamma. Mathematically, the multiplicative factor can be pulled into the power function if suitably modified. This modified factor may scale both the code values and the offset.
 
GOG-F model   Equation 8
 
 L=G ·( CV +dark) γ +ambient
 
Linear Luminance Reduction   Equation 9
 
 L   Linear reduced   =p·G ·( CV +dark) γ +ambient
 
 L   Linear reduced   =G ·( p   1/γ ·( CV +dark)) γ +ambient
 
 L   Linear reduced   =G ·( p   1/γ   ·CV+p   1/γ ·dark) γ +ambient
 
Code Value Reduction   Equation 10
 
 L   CV reduced   =G ·( p   1/γ   ·CV +dark) γ +ambient
 
     Some embodiments of the present invention may be described with reference to  FIG. 5 . In these embodiments, a tone scale adjustment may be designed or calculated off-line, prior to image processing, or the adjustment may be designed or calculated on-line as the image is being processed. Regardless of the timing of the operation, the tone scale adjustment  56  may be designed or calculated based on at least one of a display gamma  50 , an efficiency factor  52  and a maximum fidelity point (MFP)  54 . These factors may be processed in the tone scale design process  56  to produce a tone scale adjustment model  58 . The tone scale adjustment model may take the form of an algorithm, a look-up table (LUT) or some other model that may be applied to image data. 
     Once the adjustment model  58  has been created, it may be applied to the image data. The application of the adjustment model may be described with reference to  FIG. 6 . In these embodiments, an image is input  62  and the tone scale adjustment model  58  is applied  64  to the image to adjust the image code values. This process results in an output image  66  that may be sent to a display. Application  64  of the tone scale adjustment is typically an on-line process, but may be performed in advance of image display when conditions allow. 
     Some embodiments of the present invention comprise systems and methods for enhancing images displayed on displays using light-emitting pixel modulators, such as LED displays, plasma displays and other types of displays. These same systems and methods may be used to enhance images displayed on displays using light-valve pixel modulators with light sources operating in full power mode or otherwise. 
     These embodiments work similarly to the previously-described embodiments, however, rather than compensating for a reduced light source illumination, these embodiments simply increase the luminance of a range of pixels as if the light source had been reduced. In this manner, the overall brightness of the image is improved. 
     In these embodiments, the original code values are boosted across a significant range of values. This code value adjustment may be carried out as explained above for other embodiments, except that no actual light source illumination reduction occurs. Therefore, the image brightness is increased significantly over a wide range of code values. 
     Some of these embodiments may be explained with reference to  FIG. 3  as well. In these embodiments, code values for an original image are shown as points along curve  30 . These values may be boosted or adjusted to values with a higher luminance level. These boosted values may be represented as points along curve  34 , which extends from the zero point  33  to the maximum fidelity point  35  and then tapers off to the maximum value point  37 . 
     Some embodiments of the present invention comprise an unsharp masking process. In some of these embodiments the unsharp masking may use a spatially varying gain. This gain may be determined by the image value and the slope of the modified tone scale curve. In some embodiments, the use of a gain array enables matching the image contrast even when the image brightness cannot be duplicated due to limitations on the display power. 
     Some embodiments of the present invention may take the following process steps:
         1. Compute a tone scale adjustment model;   2. Compute a High Pass image;   3. Compute a Gain array;   4. Weight High Pass Image by Gain;   5. Sum Low Pass Image and Weighted High Pass Image; and   6. Send to the display       

     Other embodiments of the present invention may take the following process steps:
         1. Compute a tone scale adjustment model;   2. Compute Low Pass image;   3. Compute High Pass image as difference between Image and Low Pass image;   4. Compute Gain array using image value and slope of modified Tone Scale Curve;   5. Weight High Pass Image by Gain;   6. Sum Low Pass Image and Weighted High Pass Image; and   7. Send to the reduced power display.       

     Using some embodiments of the present invention, power savings can be achieved with only small changes on a narrow category of images. (Differences only occur above the MFP and consist of a reduction in peak brightness and some loss of bright detail). Image values below the MFP can be displayed in the power savings mode with the same luminance as the full power mode making these areas of an image indistinguishable from the full power mode. Other embodiments of the present invention improve this performance by reducing the loss of bright detail. 
     These embodiments may comprise spatially varying unsharp masking to preserve bright detail. As with other embodiments, both an on-line and an off-line component may be used. In some embodiments, an off-line component may be extended by computing a gain map in addition to the Tone Scale function. The gain map may specify an unsharp filter gain to apply based on an image value. A gain map value may be determined using the slope of the Tone Scale function. In some embodiments, the gain map value at a particular point “P” may be calculated as the ratio of the slope of the Tone Scale function below the MFP to the slope of the Tone Scale function at point “P.” In some embodiments, the Tone Scale function is linear below the MFP, therefore, the gain is unity below the MFP. 
     Some embodiments of the present invention may be described with reference to  FIG. 7 . In these embodiments, a tone scale adjustment may be designed or calculated off-line, prior to image processing, or the adjustment may be designed or calculated on-line as the image is being processed. Regardless of the timing of the operation, the tone scale adjustment  76  may be designed or calculated based on at least one of a display gamma  70 , an efficiency factor  72  and a maximum fidelity point (MFP)  74 . These factors may be processed in the tone scale design process  76  to produce a tone scale adjustment model  78 . The tone scale adjustment model may take the form of an algorithm, a look-up table (LUT) or some other model that may be applied to image data as described in relation to other embodiments above. In these embodiments, a separate gain map  77  is also computed  75 . This gain map  77  may be applied to specific image subdivisions, such as frequency ranges. In some embodiments, the gain map may be applied to frequency-divided portions of an image. In some embodiments, the gain map may be applied to a high-pass image subdivision. It may also be applied to specific image frequency ranges or other image subdivisions. 
     An exemplary tone scale adjustment model may be described in relation to  FIG. 8 . In these exemplary embodiments, a Function Transition Point (FTP)  84  (similar to the MFP used in light source reduction compensation embodiments) is selected and a gain function is selected to provide a first gain relationship  82  for values below the FTP  84 . In some embodiments, the first gain relationship may be a linear relationship, but other relationships and functions may be used to convert code values to enhanced code values. Above the FTP  84 , a second gain relationship  86  may be used. This second gain relationship  86  may be a function that joins the FTP  84  with a maximum value point  88 . In some embodiments, the second gain relationship  86  may match the value and slope of the first gain relationship  82  at the FTP  84  and pass through the maximum value point  88 . Other relationships, as described above in relation to other embodiments, and still other relationships may also serve as a second gain relationship  86 . 
     In some embodiments, a gain map  77  may be calculated in relation to the tone scale adjustment model, as shown in  FIG. 8 . An exemplary gain map  77 , may be described in relation to  FIG. 9 . In these embodiments, a gain map function relates to the tone scale adjustment model  78  as a function of the slope of the tone scale adjustment model. In some embodiments, the value of the gain map function at a specific code value is determined by the ratio of the slope of the tone scale adjustment model at any code value below the FTP to the slope of the tone scale adjustment model at that specific code value. In some embodiments, this relationship may be expressed mathematically in equation 11: 
     
       
         
           
             
               
                 
                   
                     Gain 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     
                       ( 
                       cv 
                       ) 
                     
                   
                   = 
                   
                     
                       ToneScaleSlope 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       
                         ( 
                         1 
                         ) 
                       
                     
                     
                       ToneScaleSlope 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       
                         ( 
                         cv 
                         ) 
                       
                     
                   
                 
               
               
                 
                   Equation 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   11 
                 
               
             
           
         
       
     
     In these embodiments, the gain map function is equal to one below the FTP where the tone scale adjustment model results in a linear boost. For code values above the FTP, the gain map function increases quickly as the slope of the tone scale adjustment model tapers off. This sharp increase in the gain map function enhances the contrast of the image portions to which it is applied. 
     The exemplary tone scale adjustment factor illustrated in  FIG. 8  and the exemplary gain map function illustrated in  FIG. 9  were calculated using a display percentage (source light reduction) of 80%, a display gamma of 2.2 and a Maximum Fidelity Point of 180. 
     In some embodiments of the present invention, an unsharp masking operation may be applied following the application of the tone scale adjustment model. In these embodiments, artifacts are reduced with the unsharp masking technique. 
     Some embodiments of the present invention may be described in relation to  FIG. 10 . In these embodiments, an original image  102  is input and a tone scale adjustment model  103  is applied to the image. The original image  102  is also used as input to a gain mapping process  105  which results in a gain map. The tone scale adjusted image is then processed through a low pass filter  104  resulting in a low-pass adjusted image. The low pass adjusted image is then subtracted  106  from the tone scale adjusted image to yield a high-pass adjusted image. This high-pass adjusted image is then multiplied  107  by the appropriate value in the gain map to provide a gain-adjusted high-pass image which is then added  108  to the low-pass adjusted image, which has already been adjusted with the tone scale adjustment model. This addition results in an output image  109  with increased brightness and improved high-frequency contrast. 
     In some of these embodiments, for each component of each pixel of the image, a gain value is determined from the Gain map and the image value at that pixel. The original image  102 , prior to application of the tone scale adjustment model, may be used to determine the Gain. Each component of each pixel of the high-pass image may also be scaled by the corresponding gain value before being added back to the low pass image. At points where the gain map function is one, the unsharp masking operation does not modify the image values. At points where the gain map function exceeds one, the contrast is increased. 
     Some embodiments of the present invention address the loss of contrast in high-end code values, when increasing code value brightness, by decomposing an image into multiple frequency bands. In some embodiments, a Tone Scale Function may be applied to a low-pass band increasing the brightness of the image data to compensate for source-light luminance reduction on a low power setting or simply to increase the brightness of a displayed image. In parallel, a constant gain may be applied to a high-pass band preserving the image contrast even in areas where the mean absolute brightness is reduced due to the lower display power. The operation of an exemplary algorithm is given by:
         1. Perform frequency decomposition of original image   2. Apply brightness preservation, Tone Scale Map, to a Low Pass Image   3. Apply constant multiplier to High Pass Image   4. Sum Low Pass and High Pass Images   5. Send result to the display       

     The Tone Scale Function and the constant gain may be determined off-line by creating a photometric match between the full power display of the original image and the low power display of the process image for source-light illumination reduction applications. The Tone Scale Function may also be determined off-line for brightness enhancement applications. 
     For modest MFP values, these constant-high-pass gain embodiments and the unsharp masking embodiments are nearly indistinguishable in their performance. These constant-high-pass gain embodiments have three main advantages compared to the unsharp masking embodiments: reduced noise sensitivity, ability to use larger MFP/FTP and use of processing steps currently in the display system. The unsharp masking embodiments use a gain which is the inverse of the slope of the Tone Scale Curve. When the slope of this curve is small, this gain incurs a large amplifying noise. This noise amplification may also place a practical limit on the size of the MFP/FTP. The second advantage is the ability to extend to arbitrary MFP/FTP values. The third advantage comes from examining the placement of the algorithm within a system. Both the constant-high-pass gain embodiments and the unsharp masking embodiments use frequency decomposition. The constant-high-pass gain embodiments perform this operation first while some unsharp masking embodiments first apply a Tone Scale Function before the frequency decomposition. Some system processing such as de-contouring will perform frequency decomposition prior to the brightness preservation algorithm. In these cases, that frequency decomposition can be used by some constant-high-pass embodiments thereby eliminating a conversion step while some unsharp masking embodiments must invert the frequency decomposition, apply the Tone Scale Function and perform additional frequency decomposition. 
     Some embodiments of the present invention prevent the loss of contrast in high-end code values by splitting the image based on spatial frequency prior to application of the tone scale function. In these embodiments, the tone scale function with roll-off may be applied to the low pass (LP) component of the image. In light-source illumination reduction compensation applications, this will provide an overall luminance match of the low pass image components. In these embodiments, the high pass (HP) component is uniformly boosted (constant gain). The frequency-decomposed signals may be recombined and clipped as needed. Detail is preserved since the high pass component is not passed through the roll-off of the tone scale function. The smooth roll-off of the low pass tone scale function preserves head room for adding the boosted high pass contrast. Clipping that may occur in this final combination has not been found to reduce detail significantly. 
     Some embodiments of the present invention may be described with reference to  FIG. 11 . These embodiments comprise frequency splitting or decomposition  111 , low-pass tone scale mapping  112 , constant high-pass gain or boost  116  and summation or re-combination  115  of the enhanced image components. 
     In these embodiments, an input image  110  is decomposed into spatial frequency bands  111 . In an exemplary embodiment, in which two bands are used, this may be performed using a low-pass (LP) filter  111 . The frequency division is performed by computing the LP signal via a filter  111  and subtracting  113  the LP signal from the original to form a high-pass (HP) signal  118 . In an exemplary embodiment, spatial 5×5 rect filter may be used for this decomposition though another filter may be used. 
     The LP signal may then be processed by application of tone scale mapping as discussed for previously described embodiments. In an exemplary embodiment, this may be achieved with a Photometric matching LUT. In these embodiments, a higher value of MFP/FTP can be used compared to some previously described unsharp masking embodiment since most detail has already been extracted in filtering  111 . Clipping should not generally be used since some head room should typically be preserved in which to add contrast. 
     In some embodiments, the MFP/FTP may be determined automatically and may be set so that the slope of the Tone Scale Curve is zero at the upper limit. A series of tone scale functions determined in this manner are illustrated in  FIG. 12 . In these embodiments, the maximum value of MFP/FTP may be determined such that the tone scale function has slope zero at 255. This is the largest MFP/FTP value that does not cause clipping. 
     In some embodiments of the present invention, described with reference to  FIG. 11 , processing the HP signal  118  is independent of the choice of MFP/FTP used in processing the low pass signal. The HP signal  118  is processed with a constant gain  116  which will preserve the contrast when the power/light-source illumination is reduced or when the image code values are otherwise boosted to improve brightness. The formula for the HP signal gain  116  in terms of the full and reduced backlight powers (BL) and display gamma is given immediately below as a high pass gain equation. The HP contrast boost is robust against noise since the gain is typically small (e.g. gain is 1.1 for 80% power reduction and gamma 2.2). 
     
       
         
           
             
               
                 
                   HighPassGain 
                   ⁢ 
                   
                       
                   
                   = 
                   
                     
                       ( 
                       
                         
                           BL 
                           Full 
                         
                         
                           BL 
                           Reduced 
                         
                       
                       ) 
                     
                     
                       1 
                       / 
                       γ 
                     
                   
                 
               
               
                 
                   Equation 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   12 
                 
               
             
           
         
       
     
     In some embodiments, once the tone scale mapping  112  has been applied to the LP signal, through LUT processing or otherwise, and the constant gain  116  has been applied to the HP signal, these frequency components may be summed  115  and, in some cases, clipped. Clipping may be necessary when the boosted HP value added to the LP value exceeds 255. This will typically only be relevant for bright signals with high contrast. In some embodiments, the LP signal is guaranteed not to exceed the upper limit by the tone scale LUT construction. The HP signal may cause clipping in the sum, but the negative values of the HP signal will never clip maintaining some contrast even when clipping does occur. 
     Image-Dependent Source Light Embodiments 
     In some embodiments of the present invention a display light source illumination level may be adjusted according to characteristics of the displayed image, previously-displayed images, images to be displayed subsequently to the displayed image or combinations thereof. In these embodiments, a display light source illumination level may be varied according to image characteristics. In some embodiments, these image characteristics may comprise image luminance levels, image chrominance levels, image histogram characteristics and other image characteristics. 
     Once image characteristics have been ascertained, the light source (backlight) illumination level may be varied to enhance one or more image attributes. In some embodiments, the light source level may be decreased or increased to enhance contrast in darker or lighter image regions. A light source illumination level may also be increased or decreased to increase the dynamic range of the image. In some embodiments, the light source level may be adjusted to optimize power consumption for each image frame. 
     When a light source level has been modified, for whatever reason, the code values of the image pixels can be adjusted using a tone-scale adjustment to further improve the image. If the light source level has been reduced to conserve power, the pixel values may be increased to regain lost brightness. If the light source level has been changed to enhance contrast in a specific luminance range, the pixel values may be adjusted to compensate for decreased contrast in another range or to further enhance the specific range. 
     In some embodiments of the present invention, as illustrated in  FIG. 13 , image tone scale adjustments may be dependent upon image content. In these embodiments, an image may be analyzed  130  to determine image characteristics. Image characteristics may comprise luminance channel characteristics, such as an Average Picture Level (APL), which is the average luminance of an image; a maximum luminance value; a minimum luminance value; luminance histogram data, such as a mean histogram value, a most frequent histogram value and others; and other luminance characteristics. Image characteristics may also comprise color characteristics, such as characteristic of individual color channels (e.g., R, G &amp; B in an RGB signal). Each color channel can be analyzed independently to determine color channel specific image characteristics. In some embodiments, a separate histogram may be used for each color channel. In other embodiments, blob histogram data which incorporates information about the spatial distribution of image data, may be used as an image characteristic. Image characteristics may also comprise temporal changes between video frames. 
     Once an image has been analyzed  130  and characteristics have been determined, a tone scale map may be calculated or selected  132  from a set of pre-calculated maps based on the value of the image characteristic. This map may then be applied  134  to the image to compensate for backlight adjustment or otherwise enhance the image. 
     Some embodiments of the present invention may be described in relation to  FIG. 14 . In these embodiments, an image analyzer  142  receives an image  140  and determines image characteristics that may be used to select a tone scale map. These characteristics are then sent to a tone scale map selector  143 , which determines an appropriate map based on the image characteristics. This map selection may then be sent to an image processor  145  for application of the map to the image  140 . The image processor  145  will receive the map selection and the original image data and process the original image with the selected tone scale map  144  thereby generating an adjusted image that is sent to a display  146  for display to a user. In these embodiments, one or more tone scale maps  144  are stored for selection based on image characteristics. These tone scale maps  144  may be pre-calculated and stored as tables or some other data format. These tone scale maps  144  may comprise simple gamma conversion tables, enhancement maps created using the methods described above in relation to  FIGS. 5 ,  7 ,  10  &amp;  11  or other maps. 
     Some embodiments of the present invention may be described in relation to  FIG. 15 . In these embodiments, an image analyzer  152  receives an image  150  and determines image characteristics that may be used to calculate a tone scale map. These characteristics are then sent to a tone scale map calculator  153 , which may calculate an appropriate map based on the image characteristics. The calculated map may then be sent to an image processor  155  for application of the map to the image  150 . The image processor  155  will receive the calculated map  154  and the original image data and process the original image with the tone scale map  154  thereby generating an adjusted image that is sent to a display  156  for display to a user. In these embodiments, a tone scale map  154  is calculated, essentially in real-time based on image characteristics. A calculated tone scale map  154  may comprise a simple gamma conversion table, an enhancement map created using the methods described above in relation to  FIGS. 5 ,  7 ,  10  &amp;  11  or another map. 
     Further embodiments of the present invention may be described in relation to  FIG. 16 . In these embodiments a source light illumination level may be dependent on image content while the tone scale map is also dependent on image content. However, there may not necessarily be any communication between the source light calculation channel and the tone scale map channel. 
     In these embodiments, an image is analyzed  160  to determine image characteristics required for source light or tone scale map calculations. This information is then used to calculate a source light illumination level  161  appropriate for the image. This source light data is then sent  162  to the display for variation of the source light (e.g. backlight) when the image is displayed. Image characteristic data is also sent to a tone scale map channel where a tone scale map is selected or calculated  163  based on the image characteristic information. The map is then applied  164  to the image to produce an enhanced image that is sent to the display  165 . The source light signal calculated for the image is synchronized with the enhanced image data so that the source light signal coincides with the display of the enhanced image data. 
     Some of these embodiments, illustrated in  FIG. 17  employ stored tone scale maps which may comprise a simple gamma conversion table, an enhancement map created using the methods described above in relation to  FIGS. 5 ,  7 ,  10  &amp;  11  or another map. In these embodiments, an image  170  is sent to an image analyzer  172  to determine image characteristics relevant to tone scale map and source light calculations. These characteristics are then sent to a source light calculator  177  for determination of an appropriate source light illumination level. Some characteristics may also be sent to a tone scale map selector  173  for use in determining an appropriate tone scale map  174 . The original image  170  and the map selection data are then sent to an image processor  175  which retrieves the selected map  174  and applies the map  174  to the image  170  to create an enhanced image. This enhanced image is then sent to a display  176 , which also receives the source light level signal from the source light calculator  177  and uses this signal to modulate the source light  179  while the enhanced image is being displayed. 
     Some of these embodiments, illustrated in  FIG. 18  may calculate a tone scale map on-the-fly. These maps may comprise a simple gamma conversion table, an enhancement map created using the methods described above in relation to  FIGS. 5 ,  7 ,  10  &amp;  11  or another map. In these embodiments, an image  180  is sent to an image analyzer  182  to determine image characteristics relevant to tone scale map and source light calculations. These characteristics are then sent to a source light calculator  187  for determination of an appropriate source light illumination level. Some characteristics may also be sent to a tone scale map calculator  183  for use in calculating an appropriate tone scale map  184 . The original image  180  and the calculated map  184  are then sent to an image processor  185  which applies the map  184  to the image  180  to create an enhanced image. This enhanced image is then sent to a display  186 , which also receives the source light level signal from the source light calculator  187  and uses this signal to modulate the source light  189  while the enhanced image is being displayed. 
     Some embodiments of the present invention may be described with reference to  FIG. 19 . In these embodiments, an image is analyzed  190  to determine image characteristics relative to source light and tone scale map calculation and selection. These characteristics are then used to calculate  192  a source light illumination level. The source light illumination level is then used to calculate or select a tone scale adjustment map  194 . This map is then applied  196  to the image to create an enhanced image. The enhanced image and the source light level data are then sent  198  to a display. 
     An apparatus used for the methods described in relation to  FIG. 19  may be described with reference to  FIG. 20 . In these embodiments, an image  200  is received at an image analyzer  202 , where image characteristics are determined. The image analyzer  202  may then send image characteristic data to a source light calculator  203  for determination of a source light level. Source light level data may then be sent to a tone scale map selector or calculator  204 , which may calculate or select a tone scale map based on the light source level. The selected map  207  or a calculated map may then be sent to an image processor  205  along with the original image for application of the map to the original image. This process will yield an enhanced image that is sent to a display  206  with a source light level signal that is used to modulate the display source light while the image is displayed. 
     In some embodiments of the present invention, a source light control unit is responsible for selecting a source light reduction which will maintain image quality. Knowledge of the ability to preserve image quality in the adaptation stage is used to guide the selection of source light level. In some embodiments, it is important to realize that a high source light level is needed when either the image is bright or the image contains highly saturated colors i.e. blue with code value 255. Use of only luminance to determine the backlight level may cause artifacts with images having low luminance but large code values i.e. saturated blue or red. In some embodiments each color plane may be examined and a decision may be made based on the maximum of all color planes. In some embodiments, the backlight setting may be based upon a single specified percentage of pixels which are clipped. In other embodiments, illustrated in  FIG. 22 , a backlight modulation algorithm may use two percentages: the percentage of pixels clipped  236  and the percentage of pixels distorted  235 . Selecting a backlight setting with these differing values allows room for the tone scale calculator to smoothly roll-off the tone scale function rather than imposing a hard clip. Given an input image, the histogram of code values for each color plane is determined. Given the two percentages P Clipped    236  and P Distored 235 , the histogram of each color plane  221 - 223  is examined to determine the code values corresponding to these percentages  224 - 226 . This gives C Clipped (color)  228  and C Distorted (color)  227 . The maximum clipped code value  234  and the maximum distorted code value  233  among the different color planes may be used to determine the backlight setting  229 . This setting ensures that for each color plane at most the specified percentage of code values will be clipped or distorted.
 
 Cv   Clipped =max( C   Clipped   color )
 
 Cv   Distorted =max( C   Distorted   color )  Equation 13
 
     The backlight (BL) percentage is determined by examining a tone scale (TS) function which will be used for compensation and choosing the BL percentage so that the tone scale function will clip at 255 at code value Cv Clipped    234 . The tone scale function will be linear below the value Cv Distorted  (the value of this slope will compensate for the BL reduction), constant at 255 for code values above Cv Clipped , and have a continuous derivative. Examining the derivative illustrates how to select the lower slope and hence the backlight power which gives no image distortion for code values below Cv Distorted . 
     In the plot of the TS derivative, shown in  FIG. 21 , the value H is unknown. For the TS to map Cv Clipped  to 255, the area under the TS derivative must be 255. This constraint allows us to determine the value of H as below. 
     
       
         
           
             
               
                 
                   
                     Area 
                     = 
                     
                       
                         H 
                         · 
                         
                           Cv 
                           clipped 
                         
                       
                       + 
                       
                         
                           1 
                           2 
                         
                         · 
                         H 
                         · 
                         
                           ( 
                           
                             
                               Cv 
                               Distorted 
                             
                             - 
                             
                               Cv 
                               Clipped 
                             
                           
                           ) 
                         
                       
                     
                   
                   ⁢ 
                   
                     
 
                   
                   ⁢ 
                   
                     Area 
                     = 
                     
                       
                         1 
                         2 
                       
                       · 
                       H 
                       · 
                       
                         ( 
                         
                           
                             Cv 
                             Distorted 
                           
                           + 
                           
                             Cv 
                             Clipped 
                           
                         
                         ) 
                       
                     
                   
                   ⁢ 
                   
                     
 
                   
                   ⁢ 
                   
                     H 
                     = 
                     
                       
                         2 
                         · 
                         Area 
                       
                       
                         ( 
                         
                           
                             Cv 
                             Distorted 
                           
                           + 
                           
                             Cv 
                             Clipped 
                           
                         
                         ) 
                       
                     
                   
                   ⁢ 
                   
                     
 
                   
                   ⁢ 
                   
                     H 
                     = 
                     
                       
                         2 
                         · 
                         255 
                       
                       
                         ( 
                         
                           
                             Cv 
                             Distorted 
                           
                           + 
                           
                             Cv 
                             Clipped 
                           
                         
                         ) 
                       
                     
                   
                 
               
               
                 
                   Equation 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   14 
                 
               
             
           
         
       
     
     The BL percentage is determined from the code value boost and display gamma and the criteria of exact compensation for code values below the Distortion point. The BL ratio which will clip at Cv Clipped  and allow a smooth transition from no distortion below Cv Distorted  is given by: 
     
       
         
           
             
               
                 
                   BacklightRatio 
                   = 
                   
                     
                       ( 
                       
                         
                           ( 
                           
                             CvDistorted 
                             + 
                             CvClipped 
                           
                           ) 
                         
                         
                           2 
                           · 
                           255 
                         
                       
                       ) 
                     
                     γ 
                   
                 
               
               
                 
                   Equation 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   15 
                 
               
             
           
         
       
     
     Additionally to address the issue of BL variation, an upper limit is placed on the BL ratio. 
     
       
         
           
             
               
                 
                   BacklightRatio 
                   = 
                   
                     Min 
                     ⁡ 
                     
                       ( 
                       
                         
                           
                             ( 
                             
                               
                                 ( 
                                 
                                   CvDistorted 
                                   + 
                                   CvClipped 
                                 
                                 ) 
                               
                               
                                 2 
                                 · 
                                 255 
                               
                             
                             ) 
                           
                           γ 
                         
                         , 
                         MaxBacklightRatio 
                       
                       ) 
                     
                   
                 
               
               
                 
                   Equation 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   16 
                 
               
             
           
         
       
     
     Temporal low pass filtering  231  may be applied to the image dependant BL signal derived above to compensate for the lack of synchronization between LCD and BL. A diagram of an exemplary backlight modulation algorithm is shown in  FIG. 22 , differing percentages and values may be used in other embodiments. 
     Tone scale mapping may compensate for the selected backlight setting while minimizing image distortion. As described above, the backlight selection algorithm is designed based on the ability of the corresponding tone scale mapping operations. The selected BL level allows for a tone scale function which compensates for the backlight level without distortion for code values below a first specified percentile and clips code values above a second specified percentile. The two specified percentiles allow a tone scale function which translates smoothly between the distortion free and clipping ranges. 
     Ambient-Light-Sensing Embodiments 
     Some embodiments of the present invention comprise an ambient illumination sensor, which may provide input to an image processing module and/or a source light control module. In these embodiments, the image processing, including tone scale adjustment, gain mapping and other modifications, may be related to ambient illumination characteristics. These embodiments may also comprise source light or backlight adjustment that is related to the ambient illumination characteristics. In some embodiments, the source light and image processing may be combined in a single processing unit. In other embodiments, these functions may be performed by separate units. 
     Some embodiments of the present invention may be described with reference to  FIG. 23 . In these embodiments, an ambient illumination sensor  270  may be used as input for image processing methods. In some exemplary embodiments, an input image  260  may be processed based on input from an ambient illumination sensor  270  and a source light  268  level. A source light  268 , such as a back light for illuminating an LCD display panel  266  may be modulated or adjusted to save power or for other reasons. In these embodiments, an image processor  262  may receive input from an ambient illumination sensor  270  and a source light  268 . Based on these inputs, the image processor  262  may modify the input image to account for ambient conditions and source light  268  illumination levels. An input image  260  may be modified according to any of the methods described above for other embodiments or by other methods. In an exemplary embodiment, a tone scale map may be applied to the image to increase image pixel values in relation to decreased source light illumination and ambient illumination variations. The modified image  264  may then be registered on a display panel  266 , such as an LCD panel. In some embodiments, the source light illumination level may be decreased when ambient light is low and may be further decreased when a tone scale adjustment or other pixel value manipulation technique is used to compensate for the source light illumination decrease. In some embodiments, a source light illumination level may be decreased when ambient illumination decreases. In some embodiments, a source light illumination level may be increased when ambient illumination reaches an upper threshold value and/or a lower threshold value. 
     Further embodiments of the present invention may be described with reference to  FIG. 24 . In these embodiments, an input image  280  is received at an image processing unit  282 . Processing of input image  280  may be dependent on input from an ambient illumination sensor  290 . This processing may also be dependent on output from a source light processing unit  294 . In some embodiments, a source light processing unit  294  may receive input from an ambient illumination sensor  290 . Some embodiments may also receive input from a device mode indicator  292 , such as a power mode indicator that may indicate a device power consumption mode, a device battery condition or some other device condition. A source light processing unit  294  may use an ambient light condition and/or a device condition to determine a source light illumination level, which is used to control a source light  288  that will illuminate a display, such as an LCD display  286 . The source light processing unit may also pass the source light illumination level and/or other information to the image processing unit  282 . 
     The image processing unit  282  may use source light information from the source light processing unit  294  to determine processing parameters for processing the input image  280 . The image processing unit  282  may apply a tone-scale adjustment, gain map or other procedure to adjust image pixel values. In some exemplary embodiments, this procedure will improve image brightness and contrast and partially or wholly compensate for a light source illumination reduction. The result of processing by image processing unit  282  is an adjusted image  284 , which may be sent to the display  286  where it may be illuminated by source light  288 . 
     Other embodiments of the present invention may be described with reference to  FIG. 25 . In these embodiments, an input image  300  is received at an image processing unit  302 . Processing of input image  300  may be dependent on input from an ambient illumination sensor  310 . This processing may also be dependent on output from a source light processing unit  314 . In some embodiments, a source light processing unit  314  may receive input from an ambient illumination sensor  310 . Some embodiments may also receive input from a device mode indicator  312 , such as a power mode indicator that may indicate a device power consumption mode, a device battery condition or some other device condition. A source light processing unit  314  may use an ambient light condition and/or a device condition to determine a source light illumination level, which is used to control a source light  308  that will illuminate a display, such as an LCD display  306 . The source light processing unit may also pass the source light illumination level and/or other information to the image processing unit  302 . 
     The image processing unit  302  may use source light information from the source light processing unit  314  to determine processing parameters for processing the input image  300 . The image processing unit  302  may also use ambient illumination information from the ambient illumination sensor  310  to determine processing parameters for processing the input image  300 . The image processing unit  302  may apply a tone-scale adjustment, gain map or other procedure to adjust image pixel values. In some exemplary embodiments, this procedure will improve image brightness and contrast and partially or wholly compensate for a light source illumination reduction. The result of processing by image processing unit  302  is an adjusted image  304 , which may be sent to the display  306  where it may be illuminated by source light  308 . 
     Further embodiments of the present invention may be described with reference to  FIG. 26 . In these embodiments, an input image  320  is received at an image processing unit  322 . Processing of input image  320  may be dependent on input from an ambient illumination sensor  330 . This processing may also be dependent on output from a source light processing unit  334 . In some embodiments, a source light processing unit  334  may receive input from an ambient illumination sensor  330 . In other embodiments, ambient information may be received from an image processing unit  322 . A source light processing unit  334  may use an ambient light condition and/or a device condition to determine an intermediate source light illumination level. This intermediate source light illumination level may be sent to a source light post-processor  332 , which may take the form of a quantizer, a timing processor or some other module that may tailor the intermediate light source illumination level to the needs of a specific device. In some embodiments, the source light post-processor  332  may tailor the light source control signal for timing constraints imposed by the light source  328  type and/or by an imaging application, such as a video application. The post-processed signal may then be used to control a source light  328  that will illuminate a display, such as an LCD display  326 . The source light processing unit may also pass the post-processed source light illumination level and/or other information to the image processing unit  322 . 
     The image processing unit  322  may use source light information from the source light post-processor  332  to determine processing parameters for processing the input image  320 . The image processing unit  322  may also use ambient illumination information from the ambient illumination sensor  330  to determine processing parameters for processing the input image  320 . The image processing unit  322  may apply a tone-scale adjustment, gain map or other procedure to adjust image pixel values. In some exemplary embodiments, this procedure will improve image brightness and contrast and partially or wholly compensate for a light source illumination reduction. The result of processing by image processing unit  322  is an adjusted image  344 , which may be sent to the display  326  where it may be illuminated by source light  328 . 
     Some embodiments of the present invention may comprise separate image analysis  342 ,  362  and image processing  343 ,  363  modules. While these units may be integrated in a single component or on a single chip, they are illustrated and described as separate modules to better describe their interaction. 
     Some of these embodiments of the present invention may be described with reference to  FIG. 27 . In these embodiments, an input image  340  is received at an image analysis module  342 . The image analysis module may analyze an image to determine image characteristics, which may be passed to an image processing module  343  and/or a source light processing module  354 . Processing of input image  340  may be dependent on input from an ambient illumination sensor  330 . In some embodiments, a source light processing module  354  may receive input from an ambient illumination sensor  350 . A source light processing unit  354  may also receive input from a device condition or mode sensor  352 . A source light processing unit  354  may use an ambient light condition, an image characteristic and/or a device condition to determine a source light illumination level. This source light illumination level may be sent to a source light  348  that will illuminate a display, such as an LCD display  346 . The source light processing module  354  may also pass the post-processed source light illumination level and/or other information to the image processing module  343 . 
     The image processing module  322  may use source light information from the source light processing module  354  to determine processing parameters for processing the input image  340 . The image processing module  343  may also use ambient illumination information that is passed from the ambient illumination sensor  350  through the source light processing module  354 . This ambient illumination information may be used to determine processing parameters for processing the input image  340 . The image processing module  343  may apply a tone-scale adjustment, gain map or other procedure to adjust image pixel values. In some exemplary embodiments, this procedure will improve image brightness and contrast and partially or wholly compensate for a light source illumination reduction. The result of processing by image processing module  343  is an adjusted image  344 , which may be sent to the display  346  where it may be illuminated by source light  348 . 
     Some embodiments of the present invention may be described with reference to  FIG. 28 . In these embodiments, an input image  360  is received at an image analysis module  362 . The image analysis module may analyze an image to determine image characteristics, which may be passed to an image processing module  363  and/or a source light processing module  374 . Processing of input image  360  may be dependent on input from an ambient illumination sensor  370 . This processing may also be dependent on output from a source light processing module  374 . In some embodiments, ambient information may be received from an image processing module  363 , which may receive the ambient information from an ambient sensor  370 . This ambient information may be passed through and/or processed by the image processing module  363  on the way to the source light processing module  374 . A device condition or mode may also be passed to the source light processing module  374  from a device module  372 . 
     A source light processing module  374  may use an ambient light condition and/or a device condition to determine a source light illumination level. This source light illumination level may be used to control a source light  368  that will illuminate a display, such as an LCD display  366 . The source light processing unit  374  may also pass the source light illumination level and/or other information to the image processing unit  363 . 
     The image processing module  363  may use source light information from the source light processing module  374  to determine processing parameters for processing the input image  360 . The image processing module  363  may also use ambient illumination information from the ambient illumination sensor  370  to determine processing parameters for processing the input image  360 . The image processing module  363  may apply a tone-scale adjustment, gain map or other procedure to adjust image pixel values. In some exemplary embodiments, this procedure will improve image brightness and contrast and partially or wholly compensate for a light source illumination reduction. The result of processing by image processing module  363  is an adjusted image  364 , which may be sent to the display  366  where it may be illuminated by source light  368 . 
     Distortion-Adaptive Power Management Embodiments 
     Some embodiments of the present invention comprise methods and systems for addressing the power needs, display characteristics, ambient environment and battery limitations of display devices including mobile devices and applications. In some embodiments, three families of algorithms may be used: Display Power Management Algorithms, Backlight Modulation Algorithms, and Brightness Preservation (BP) Algorithms. While power management has a higher priority in mobile, battery-powered devices, these systems and methods may be applied to other devices that may benefit from power management for energy conservation, heat management and other purposes. In these embodiments, these algorithms may interact, but their individual functionality may comprise:
         Power Management—these algorithms manage backlight power across a series of frames exploiting variations in the video content to optimize power consumption.   Backlight Modulation—these algorithms select backlight power levels to use for an individual frame and exploit statistics within an image to optimize power consumption.   Brightness Preservation—these algorithms process each image to compensate for reduced backlight power and preserve image brightness while avoiding artifacts.       

     Some embodiments of the present invention may be described with reference to  FIG. 29 , which comprises a simplified block diagram indicating the interaction of components of these embodiments. In some embodiments, the power management algorithm  406  may manage the fixed battery resource  402  over a video, image sequence or other display task and may guarantee a specified average power consumption while preserving quality and/or other characteristics. The backlight modulation algorithm  410  may receive instructions from the power management algorithm  406  and select a power level subject to the limits defined by the power management algorithm  406  to efficiently represent each image. The brightness preservation algorithm  414  may use the selected backlight level  415 , and possible clipping value  413 , to process the image compensating for the reduced backlight. 
     Display Power Management 
     In some embodiments, the display power management algorithm  406  may manage the distribution of power use over a video, image sequence or other display task. In some embodiments, the display power management algorithm  406  may allocate the fixed energy of the battery to provide a guaranteed operational lifetime while preserving image quality. In some embodiments, one goal of a Power Management algorithm is to provide guaranteed lower limits on the battery lifetime to enhance usability of the mobile device. 
     Constant Power Management 
     One form of power control which meets an arbitrary target is to select a fixed power which will meet the desired lifetime. A system block diagram showing a system based on constant power management is shown in  FIG. 30 . The essential point being that the power management algorithm  436  selects a constant backlight power based solely on initial battery fullness  432  and desired lifetime  434 . Compensation  442  for this backlight level  444  is performed on each image  446 . 
     
       
         
           
             
               
                 
                   Constant 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   Power 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   management 
                 
               
               
                 
                   Equation 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   17 
                 
               
             
             
               
                 
                   
                     
                       P 
                       Selected 
                     
                     ⁡ 
                     
                       ( 
                       t 
                       ) 
                     
                   
                   = 
                   
                     InitialCharge 
                     DesiredLifetime 
                   
                 
               
               
                 
                     
                 
               
             
           
         
       
     
     The backlight level  444  and hence power consumption are independent of image data  440 . Some embodiments may support multiple constant power modes allowing the selection of power level to be made based on the power mode. In some embodiments, image-dependent backlight modulation may not be used to simplify the system implementation. In other embodiments, a few constant power levels may be set and selected based on operating mode or user preference. Some embodiments may use this concept with a single reduced power level, i.e. 75% of maximum power. 
     Simple Adaptive Power Management 
     Some embodiments of the present invention may be described with reference to  FIG. 31 . These embodiments comprise an adaptive Power Management algorithm  456 . The power reduction  455  due to backlight modulation  460  is fed back to the Power Management algorithm  456  allowing improved image quality while still providing the desired system lifetime. 
     In some embodiments, the power savings with image-dependant backlight modulation may be included in the power management algorithm by updating the static maximum power calculation over time as in Equation 18. Adaptive power management may comprise computing the ratio of remaining battery fullness (mA-Hrs) to remaining desired lifetime (Hrs) to give an upper power limit (mA) to the backlight modulation algorithm  460 . In general, backlight modulation  460  may select an actual power below this maximum giving further power savings. In some embodiments, power savings due to backlight modulation may be reflected in the form of feedback through the changing values of remaining battery charge or running average selected power and hence influence subsequent power management decisions. 
     
       
         
           
             
               
                 
                   
                     Adaptive 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     Power 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     Management 
                   
                   ⁢ 
                   
                     
 
                   
                   ⁢ 
                   
                     
                       
                         P 
                         Maximum 
                       
                       ⁡ 
                       
                         ( 
                         t 
                         ) 
                       
                     
                     = 
                     
                       
                         Remaining 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         
                           Charge 
                           ⁡ 
                           
                             ( 
                             t 
                             ) 
                           
                         
                       
                       
                         RemainingLifetime 
                         ⁡ 
                         
                           ( 
                           t 
                           ) 
                         
                       
                     
                   
                 
               
               
                 
                   Equation 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   18 
                 
               
             
           
         
       
     
     In some embodiments, if battery status information is unavailable or inaccurate, the remaining battery charge can be estimated by computing the energy used by the display, average selected power times operating time, and subtracting this from the initial battery charge.
 
Estimating Remaining Battery Charge   Equation 19
 
DisplayEnergyUsed( t )=AverageSelectedPower· t  
 
RemainingCharge( t )=InitialCharge−DisplayEnergyUsed( t 
 
This latter technique has the advantage of being done without interaction with the battery.
 
Power-Distortion Management
 
     The inventor has observed, in a study of distortion versus power, that many images exhibit vastly different distortion at the same power. Dim images, those with poor contrast such a underexposed photographs, can actually be displayed better at a low power due to the elevation of the black level that results from high power use. A power control algorithm may trade off image distortion for battery capacity rather than direct power settings. In some embodiments of the present invention, illustrated in  FIG. 29 , power management techniques may comprise a distortion parameter  403 , such as a maximum distortion value, in addition to a maximum power  401  given to the Backlight Control algorithm  410 . In these embodiments, the power management algorithm  406  may use feedback from the backlight modulation algorithm  410  in the form of power/distortion characteristics  405  of the current image. In some embodiments, the maximum image distortion may be modified based upon the target power and the power-distortion property of the current frame. In these embodiments, in addition to feedback on the actual selected power, the power management algorithm may select and provide distortion targets  403  and may receive feedback on the corresponding image distortion  405  in addition to feedback on the battery fullness  402 . In some embodiments, additional inputs could be used in the power control algorithm such as: ambient level  408 , user preference, and operating mode (i.e., Video/Graphics). 
     Some embodiments of the present invention may attempt to optimally allocate power across a video sequence while preserving display quality. In some embodiments, for a given video sequence, two criteria may be used for selecting a trade-off between total power used and image distortion. Maximum image distortion and average image distortion may be used. In some embodiments, these terms may be minimized. In some embodiments, minimizing maximum distortion over an image sequence may be achieved by using the same distortion for each image in the sequence. In these embodiments, the power management algorithm  406  may select this distortion  403  allowing the backlight modulation algorithm  410  to select the backlight level which meets this distortion target  403 . In some embodiments, minimizing the average distortion may be achieved when power selected for each image is such that the slopes of the power distortion curves are equal. In this case, the power management algorithm  406  may select the slope of the power distortion curve relying on the backlight modulation algorithm  410  to select the appropriate backlight level. 
       FIGS. 32A and 32B  may be used to illustrate power savings when considering distortion in the power management process.  FIG. 32A  is a plot of source light power level for sequential frames of an image sequence.  FIG. 32A  shows the source light power levels needed to maintain constant distortion  480  between frames and the average power  482  of the constant distortion graph.  FIG. 32B  is a plot of image distortion for the same sequential frames of the image sequence.  FIG. 32B  shows the constant power distortion  484  resulting from maintaining a constant power setting, the constant distortion level  488  resulting from maintaining constant distortion throughout the sequence and the average constant power distortion  486  when maintaining constant power. The constant power level has been chosen to equal the average power of the constant distortion result. Thus both methods use the same average power. Examining distortion we find that the constant power  484  gives significant variation in image distortion. Note also that the average distortion  486  of the constant power control is more than 10 times the distortion  488  of the constant distortion algorithm despite both using the same average power. 
     In practice, optimizing to minimize either the maximum or average distortion across a video sequence may prove too complex for some applications as the distortion between the original and reduced power images must be calculated at each point of the power distortion function to evaluate the power-distortion trade-off. Each distortion evaluation may require that the backlight reduction and corresponding compensating image brightening be calculated and compared with the original image. Consequently, some embodiments may comprise simpler methods for calculating or estimating distortion characteristics. 
     In some embodiments, some approximations may be used. First we observe that a point-wise distortion metric such as a Mean-Square-Error (MSE) can be computed from the histogram of image code values rather than the image itself, as expressed in Equation 20. In this case, the histogram is a one dimensional signal with only 256 values as opposed to an image which at 320×240 resolution has 7680 samples. This could be further reduced by subsampling the histograms if desired. 
     In some embodiments, an approximation may be made by assuming the image is simply scaled with clipping in the compensation stage rather than applying the actual compensation algorithm. In some embodiments, inclusion of a black level elevation term in the distortion metric may also be valuable. In some embodiments, use of this term may imply that a minimum distortion for an entirely black frame occurs at zero backlight. 
     
       
         
           
             
               
                 
                   
                     Simplifying 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     Distortion 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     Calculation 
                   
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   
                      
                   
                   ⁢ 
                   
                     
                       Distortion 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       
                         ( 
                         Power 
                         ) 
                       
                     
                     = 
                     
                       
                         ∑ 
                         pixels 
                       
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       
                         
                            
                           
                             
                               Image 
                               Original 
                             
                             - 
                             
                               Power 
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               
                                 Image 
                                 Brightened 
                               
                             
                           
                            
                         
                         2 
                       
                     
                   
                   ⁢ 
                   
                     
 
                   
                   ⁢ 
                   
                     
                       Distortion 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       
                         ( 
                         Power 
                         ) 
                       
                     
                     = 
                     
                       
                         ∑ 
                         
                           cv 
                           ∈ 
                           CodeValues 
                         
                       
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       
                         
                           Histogram 
                           ⁢ 
                           
                             ⁠  
                             ⁢ 
                             
                                 
                             
                           
                           ( 
                           cv 
                           ) 
                         
                         · 
                         
                           
                              
                             
                               
                                 Display 
                                 ⁢ 
                                 
                                     
                                 
                                 ⁢ 
                                 
                                   ( 
                                   cv 
                                   ) 
                                 
                               
                               - 
                               
                                 
                                   Power 
                                   · 
                                   Display 
                                 
                                 ⁢ 
                                 
                                     
                                 
                                 ⁢ 
                                 
                                   ( 
                                   
                                     Brightened 
                                     ⁢ 
                                     
                                         
                                     
                                     ⁢ 
                                     
                                       ( 
                                       cv 
                                       ) 
                                     
                                   
                                   ) 
                                 
                               
                             
                              
                           
                           2 
                         
                       
                     
                   
                 
               
               
                 
                   Equation 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   20 
                 
               
             
           
         
       
     
     In some embodiments, to compute the distortion at a given power level, for each code value, the distortion caused by a linear boost with clipping may be determined. The distortion may then be weighted by the frequency of the code value and summed to give a mean image distortion at the specified power level. In these embodiments, the simple linear boost for brightness compensation does not give acceptable quality for image display, but serves as a simple source for computing an estimate of the image distortion caused by a change in backlight. 
     In some embodiments, illustrated in  FIG. 33 , to control both power consumption and image distortion, the power management algorithm  500  may track not only the battery fullness  506  and remaining lifetime  508 , but image distortion  510  as well. In some embodiments, both an upper limit on power consumption  512  and a distortion target  511  may be supplied to the backlight modulation algorithm  502 . The backlight Modulation algorithm  502  may then select a backlight level  512  consistent with both the power limit and the distortion target. 
     Backlight Modulation Algorithms (BMA) 
     The backlight modulation algorithm  502  is responsible for selecting the backlight level used for each image. This selection may be based upon the image to be displayed and the signals from the power management algorithm  500 . By respecting the limit on the maximum power supplied  512  by the power management algorithm  500 , the battery  506  may be managed over the desired lifetime. In some embodiments, the backlight modulation algorithm  502  may select a lower power depending upon the statistics of the current image. This may be a source of power savings on a particular image. 
     Once a suitable backlight level  415  is selected, the backlight  416  is set to the selected level and this level  415  is given to the brightness preservation algorithm  414  to determine the necessary compensation. For some images and sequences, allowing a small amount of image distortion can greatly reduce the required backlight power. Therefore, some embodiments comprise algorithms that allow a controlled amount of image distortion. 
       FIG. 34  is a graph showing the amount of power savings on a sample DVD clip as a function of frame number for several tolerances of distortion. The percentage of pixels with zero distortion was varied from 100% to 97% to 95% and the average power across the video clip was determined. The average power ranged from 95% to 60% respectively. Thus allowing distortion in 5% of the pixels gave an additional 35% power savings. This demonstrates significant power savings possible by allowing small image distortion. If the brightness preservation algorithm can preserve subjective quality while introducing a small distortion, significant power savings can be achieved. 
     Some embodiments of the present invention may be described with reference to  FIG. 30 . These embodiments may also comprise information from an ambient light sensor  438  and may be reduced in complexity for a mobile application. These embodiments comprise a static histogram percentile limit and a dynamic maximum power limit supplied by the power management algorithm  436 . Some embodiments may comprise a constant power target while other embodiments may comprise a more sophisticated algorithm. In some embodiments, the image may be analyzed by computing histograms of each of the color components. The code value in the histogram at which the specified percentile occurs may be computed for each color plane. In some embodiments, a target backlight level may be selected so that a linear boost in code values will just cause clipping of the code value selected from the histograms. The actual backlight level may be selected as the minimum of this target level and the backlight level limit provided by the power management algorithm  436 . These embodiments may provide guaranteed power control and may allow a limited amount of image distortion in cases where the power control limit can be reached 
                     Histogram   ⁢           ⁢   Percentile   ⁢           ⁢   Based   ⁢           ⁢   Power   ⁢           ⁢   Selection     ⁢     
     ⁢             P   target     =       (       CodeValue   Percenile     255     )     γ                   P   Selected     =     min   ⁡     (       P   target     ,     P   Maximum       )                       Equation   ⁢           ⁢   21               
Image-Distortion-Based Embodiments
 
     Some embodiments of the present invention may comprise a distortion limit and a maximum power limit supplied by the power management algorithm.  FIGS. 32B and 34  demonstrate that the amount of distortion at a given backlight power level varies greatly depending upon image content. The properties of the power-distortion behavior of each image may be exploited in the backlight selection process. In some embodiments, the current image may be analyzed by computing histograms for each color component. A power distortion curve defining the distortion (e.g., MSE) may be computed by calculating the distortion at a range of power values using the second expression of Equation 20. The backlight modulation algorithm may select the smallest power with distortion at, or below, the specified distortion limit as a target level. The backlight level may then be selected as the minimum of the target level and the backlight level limit supplied by the power management algorithm. Additionally, the image distortion at the selected level may be provided to the power management algorithm to guide the distortion feedback. The sampling frequency of the power distortion curve and the image histogram can be reduced to control complexity. 
     Brightness Preservation (BP) 
     In some embodiments, the BP algorithm brightens an image based upon the selected backlight level to compensate for the reduced illumination. The BP algorithm may control the distortion introduced into the display and the ability of the BP algorithm to preserve quality dictates how much power the backlight modulation algorithm can attempt to save. Some embodiments may compensate for the backlight reduction by scaling the image clipping values which exceed 255. In these embodiments, the backlight modulation algorithm must be conservative in reducing power or annoying clipping artifacts are introduced thus limiting the possible power savings. Some embodiments are designed to preserve quality on the most demanding frames at a fixed power reduction. Some of these embodiments compensate for a single backlight level (i.e., 75%). Other embodiments may be generalized to work with backlight modulation. 
     Some embodiments of the brightness preservation (BP) algorithm may utilitize a description of the luminance output from a display as a function of the backlight and image data. Using this model, BP may determine the modifications to an image to compensate for a reduction in backlight. With a transflective display, the BP model may be modified to include a description of the reflective aspect of the display. The luminance output from a display becomes a function of the backlight, image data, and ambient. In some embodiments, the BP algorithm may determine the modifications to an image to compensate for a reduction in backlight in a given ambient environment. 
     Ambient Influence 
     Due to implementation constraints, some embodiments may comprise limited complexity algorithms for determining BP parameters. For example, developing an algorithm running entirely on an LCD module limits the processing and memory available to the algorithm. In this example, generating alternate gamma curves for different backlight/ambient combinations may be used for some BP embodiments. In some embodiments, limits on the number and resolution of the gamma curves may be needed. 
     Power/Distortion Curves 
     Some embodiments of the present invention may obtain, estimate, calculate or otherwise determine power/distortion characteristics for images including, but not limited to, video sequence frames.  FIG. 35  is a graph showing power/distortion characteristics for four exemplary images. In  FIG. 35 , the curve  520  for image C maintains a negative slope for the entire source light power band. The curves  522 ,  524  &amp;  526  for images A, B and D fall on a negative slope until they reach a minimum, then rise on a positive slope. For images A, B and D, increasing source light power will actually increase distortion at specific ranges of the curves where the curves have a positive slope  528 . This may be due to display characteristics such as, but not limited to, LCD leakage or other display irregularities that cause the displayed image, as seen by a viewer, to consistently differ from code values. 
     Some embodiments of the present invention may use these characteristics to determine appropriate source light power levels for specific images or image types. Display characteristics (e.g., LCD leakage) may be considered in the distortion parameter calculations, which are used to determine the appropriate source light power level for an image. 
     Exemplary Methods 
     Some embodiments of the present invention may be described in relation to  FIG. 36 . In these embodiments, a power budget is established  530 . This may be performed using simple power management, adaptive power management and other methods described above or by other methods. Typically, establishing the power budget may comprise estimating a backlight or source light power level that will allow completion of a display task, such as display of a video file, while using a fixed power resource, such as a portion of a battery charge. In some embodiments, establishing a power budget may comprise determining an average power level that will allow completion of a display task with a fixed amount of power. 
     In these embodiments, an initial distortion criterion  532  may also be established. This initial distortion criterion may be determined by estimating a reduced source light power level that will meet a power budget and measuring image distortion at that power level. The distortion may be measured on an uncorrected image, on an image that has been modified using a brightness preservation (BP) technique as described above or on an image that has been modified with a simplified BP process. 
     Once the initial distortion criterion is established, a first portion of the display task may be displayed  534  using source light power levels that cause a distortion characteristic of the displayed image or images to comply with the distortion criterion. In some embodiments, light source power levels may be selected for each frame of a video sequence such that each frame meets the distortion requirement. In some embodiments, the light source values may be selected to maintain a constant distortion or distortion range, keep distortion below a specified level or otherwise meet a distortion criterion. 
     Power consumption may then be evaluated  536  to determine whether the power used to display the first portion of the display task met power budget management parameters. Power may be allocated using a fixed amount for each image, video frame or other display task element. Power may also be allocated such that the average power consumed over a series of display task elements meets a requirement while the power consumed for each display task element may vary. Other power allocation schemes may also be used. 
     When the power consumption evaluation  536  shows that power consumption for the first portion of the display task did not meet power budget requirements, the distortion criterion may be modified  538 . In some embodiments, in which a power/distortion curve can be estimated, assumed, calculated or otherwise determined, the distortion criterion may be modified to allow more or less distortion as needed to conform to a power budget requirement. While power/distortion curves are image specific, a power/distortion curve for a first frame of a sequence, for an exemplary image in a sequence or for a synthesized image representative of the display task may be used. 
     In some embodiments, when more that the budgeted amount of power was used for the first portion of the display task and the slope of the power/distortion curve is positive, the distortion criterion may be modified to allow less distortion. In some embodiments, when more that the budgeted amount of power was used for the first portion of the display task and the slope of the power/distortion curve is negative, the distortion criterion may be modified to allow more distortion. In some embodiments, when less that the budgeted amount of power was used for the first portion of the display task and the slope of the power/distortion curve is negative or positive, the distortion criterion may be modified to allow less distortion. 
     Some embodiments of the present invention may be described with reference to  FIG. 37 . These embodiments typically comprise a battery-powered device with limited power. In these embodiments, battery fullness or charge is estimated or measured  540 . A display task power requirement may also be estimated or calculated  542 . An initial light source power level may also be estimated or otherwise determined  544 . This initial light source power level may be determined using the battery fullness and display task power requirement as described for constant power management above or by other methods. 
     A distortion criterion that corresponds to the initial light source power level may also be determined  546 . This criterion may be the distortion value that occurs for an exemplary image at the initial light source power level. In some embodiments, the distortion value may be based on an uncorrected image, an image modified with an actual or estimated BP algorithm or another exemplary image. 
     Once the distortion criterion is determined  546 , the first portion of the display task is evaluated and a source light power level that will cause the distortion of the first portion of the display task to conform to the distortion criterion is selected  548 . The first portion of the display task is then displayed  550  using the selected source light power level and the power consumed during display of the portion is estimated or measured  552 . When this power consumption does not meet a power requirement, the distortion criterion may be modified  554  to bring power consumption into compliance with the power requirement. 
     Some embodiments of the present invention may be described with reference to  FIGS. 38A &amp; 38B . In these embodiments, a power budget is established  560  and a distortion criterion is also established  562 . These are both typically established with reference to a particular display task, such as a video sequence. An image is then selected  564 , such as a frame or set of frames of a video sequence. A reduced source light power level is then estimated  566  for the selected image, such that the distortion resulting from the reduced light power level meets the distortion criterion. This distortion calculation may comprise application of estimated or actual brightness preservation (BP) methods to image values for the selected image. 
     The selected image may then be modified with BP methods  568  to compensate for the reduced light source power level. Actual distortion of the BP modified image may then be measured  570  and a determination may be made as to whether this actual distortion meets the distortion criterion  572 . If the actual distortion does not meet the distortion criterion, the estimation process  574  may be adjusted and the reduced light source power level may be re-estimated  566 . If the actual distortion does meet the distortion criterion, the selected image may be displayed  576 . Power consumption during image display be then be measured  578  and compared to a power budget constraint  580 . If the power consumption meets the power budget constraint, the next image, such as a subsequent set of video frames may be selected  584  unless the display task is finished  582 , at which point the process will end. If a next image is selected  584 , the process will return to point “B” where a reduced light source power level will be estimated  566  for that image and the process will continue as for the first image. 
     If the power consumption for the selected image does not meet a power budget constraint  580 , the distortion criterion may be modified  586  as described for other embodiments above and a next image will be selected  584 . 
     Improved Black-Level Embodiments 
     Some embodiments of the present invention comprise systems and methods for display black level improvement. Some embodiments use a specified backlight level and generate a luminance matching tone scale which both preserves brightness and improves black level. Other embodiments comprise a backlight modulation algorithm which includes black level improvement in its design. Some embodiments may be implemented as an extension or modification of embodiments described above. 
     Improved Luminance Matching (Target Matching Ideal Display) 
     The luminance matching formulation presented above, Equation 7, is used to determine a linear scaling of code values which compensates for a reduction in backlight. This has proven effective in experiments with power reduction to as low as 75%. In some embodiments with image dependant backlight modulation, the backlight can be significantly reduced, e.g. below 10%, for dark frames. For these embodiments, the linear scaling of code values derived in Equation 7 may not be appropriate since it can boost dark values excessively. While embodiments employing these methods may duplicate the full power output on a reduced power display, this may not serve to optimize output. Since the full power display has an elevated black level, reproducing this output for dark scenes does not achieve the benefit of a reduced black level made possible with a lower backlight power setting. In these embodiments, the matching criteria may be modified and a replacement for the result given in Equation 7 may be derived. In some embodiments, the output of an ideal display is matched. The ideal display may comprise a zero black level and the same maximum output, white level=W, as the full power display. The response of this exemplary ideal display to a code value, cv, may be expressed in Equation 22 in terms of the maximum output, W, display gamma and maximum code value. 
     
       
         
           
             
               
                 
                   
                     Ideal 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     Display 
                   
                   ⁢ 
                   
                     
 
                   
                   ⁢ 
                   
                     
                       
                         L 
                         ideal 
                       
                       ⁡ 
                       
                         ( 
                         cv 
                         ) 
                       
                     
                     = 
                     
                       W 
                       · 
                       
                         
                           ( 
                           
                             cv 
                             
                               cv 
                               Max 
                             
                           
                           ) 
                         
                         γ 
                       
                     
                   
                 
               
               
                 
                   Equation 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   22 
                 
               
             
           
         
       
     
     In some embodiments, and exemplary LCD may have the same maximum output, W, and gamma, but a nonzero black level, B. This exemplary LCD may be modeled using the GOG model described above for full power output. The output scales with the relative backlight power for power less than 100%. The gain and offset model parameters may be determined by the maximum output, W, and black level, B, of the full power display, as shown in Equation 23. 
                     Full   ⁢           ⁢   Power   ⁢           ⁢   GOG   ⁢           ⁢   model     ⁢     
     ⁢               L   fullpower     ⁡     (   cv   )       =       (       Gain   ·     (     cv     cv   ⁢           ⁢   Max       )       +   offset     )     γ                 offset   =     B     1   γ                   Gain   =       W     1   γ       -     B     1   γ                         Equation   ⁢           ⁢   23               
The output of the reduced power display with relative backlight power P may be determined by scaling the full power results by the relative power.
 
     
       
         
           
             
               
                 
                   
                     Actual 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     LCD 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     output 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     vs 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     Power 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     and 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     code 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     value 
                   
                   ⁢ 
                   
                     
 
                   
                   ⁢ 
                   
                     
                       L 
                       actual 
                     
                     ⁡ 
                     
                       ( 
                       
                         P 
                         , 
                         cv 
                       
                       ) 
                     
                   
                   = 
                   
                     P 
                     · 
                     
                       
                         ( 
                         
                           
                             
                               ( 
                               
                                 
                                   W 
                                   
                                     1 
                                     γ 
                                   
                                 
                                 - 
                                 
                                   B 
                                   
                                     1 
                                     γ 
                                   
                                 
                               
                               ) 
                             
                             · 
                             
                               ( 
                               
                                 cv 
                                 
                                   cv 
                                   ⁢ 
                                   
                                       
                                   
                                   ⁢ 
                                   Max 
                                 
                               
                               ) 
                             
                           
                           + 
                           
                             B 
                             
                               1 
                               γ 
                             
                           
                         
                         ) 
                       
                       γ 
                     
                   
                 
               
               
                 
                   Equation 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   24 
                 
               
             
           
         
       
     
     In these embodiments, the code values may be modified so that the outputs of the ideal and actual displays are equal, where possible. (If the ideal output is not less than or greater than that possible with a given power on the actual display) 
                     Criteria   ⁢           ⁢   for   ⁢           ⁢   matching   ⁢           ⁢   outputs     ⁢     
     ⁢         L   ideal     ⁡     (   x   )       =       L   actual     ⁡     (     P   ,     x   ~       )         ⁢     
     ⁢       W   ·       (     x     cv   Max       )     γ       =     P   ·       (         (       W     1   γ       ⁢           -           ⁢     B     1   γ         )     ·     (       x   ~       cv   ⁢           ⁢   Max       )       +     B     1   γ         )     γ                 Equation   ⁢           ⁢   25               
Some calculation solves for {tilde over (x)} in terms of x, P, W, B.
 
     
       
         
           
             
               
                 
                   
                     Code 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     Value 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     relation 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     for 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     matching 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     output 
                   
                   ⁢ 
                   
                     
 
                   
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                                 ⁢ 
                                 
                                     
                                 
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                                   ⁢ 
                                   
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                                 cv 
                                 ⁢ 
                                 
                                     
                                 
                                 ⁢ 
                                 Max 
                               
                               
                                 ( 
                                 
                                   
                                     
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                                         B 
                                       
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                                     ( 
                                     
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                                       P 
                                     
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                                 ⁢ 
                                 
                                     
                                 
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                                 ( 
                                 
                                   
                                     
                                       ( 
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                                       ) 
                                     
                                     
                                       1 
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                                   - 
                                   1 
                                 
                                 ) 
                               
                             
                           
                         
                       
                     
                   
                 
               
               
                 
                   Equation 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   26 
                 
               
             
           
         
       
     
     These embodiments demonstrate a few properties of the code value relation for matching the ideal output on an actual display with non-zero black level. In this case, there is clipping at both the upper ({tilde over (x)}=cvMax) and lower ({tilde over (x)}=0) ends. These correspond to clipping input at x low  and x high  given by Equation 27 
                   Clipping   ⁢           ⁢   points           Equation   ⁢           ⁢   27                           x   lower     ⁡     (   P   )       =     cv   ⁢           ⁢     Max   ·       (     P   CR     )       1   γ                           x   high     ⁡     (   P   )       =     cv   ⁢           ⁢     Max   ·       (   P   )       1   γ                   ⁢                                   
These results agree with our prior development for other embodiments in which the display is assumed to have zero black level i.e. contrast ratio is infinite.
 
Backlight Modulation Algorithm
 
     In these embodiments, a luminance matching theory that incorporates black level considerations, by doing a match between the display at a given power and a reference display with zero black level, to determine a backlight modulation algorithm. These embodiments use a luminance matching theory to determine the distortion an image must have when displayed with power P compared to being displayed on the ideal display. The backlight modulation algorithm may use a maximum power limit and a maximum distortion limit to select the least power that results in distortion below the specified maximum distortion. 
     Power Distortion 
     In some embodiments, given a target display specified by black level and maximum brightness at full power and an image to display, the distortion in displaying the image at a given power P may be calculated. The limited power and nonzero black level of the display may be measured as clipping applied when using the ideal reference. The distortion of an image may be defined as the MSE between the original image code values and the clipped code values, however, other distortion measures may be used in some embodiments. 
     The image with clipping is defined by the power dependant code value clipping limits introduced in Equation 27 is given in Equation 28. 
                     Clipped   ⁢           ⁢   image     ⁢     
     ⁢         I   ~     ⁡     (     x   ,   y   ,   c   ,   P     )       =     {             x   low     ⁡     (   P   )               I   ⁡     (     x   ,   y   ,   c     )       ≤       x   low     ⁡     (   P   )                   I   ⁡     (     x   ,   y   ,   c     )                 x   low     ⁡     (   P   )       &lt;     I   ⁡     (     x   ,   y   ,   c     )       &lt;       x   high     ⁡     (   P   )                     x   high     ⁡     (   P   )                 x   high     ⁡     (   P   )       ≤     I   ⁡     (     x   ,   y   ,   c     )                           Equation   ⁢           ⁢   28               
The distortion between the image on the ideal display and on the display with power P in the pixel domain becomes
 
               D   ⁡     (     I   ,   P     )       =       1   N     ·       ∑     x   ,   y   ,   c       ⁢       max   c     ⁢              I   ⁡     (     x   ,   y   ,   c     )       -       I   ~     ⁡     (     x   ,   v   ,   c   ,   P     )              2                 
Observe that this can be computed using the histogram of image code values.
 
     
       
         
           
             
               D 
               ⁡ 
               
                 ( 
                 
                   I 
                   , 
                   P 
                 
                 ) 
               
             
             = 
             
               
                 ∑ 
                 
                   n 
                   , 
                   c 
                 
               
               ⁢ 
               
                 
                   
                     h 
                     ~ 
                   
                   ⁡ 
                   
                     ( 
                     
                       n 
                       , 
                       c 
                     
                     ) 
                   
                 
                 · 
                 
                   
                     max 
                     c 
                   
                   ⁢ 
                   
                     
                        
                       
                         ( 
                         
                           n 
                           - 
                           
                             
                               I 
                               ~ 
                             
                             ⁡ 
                             
                               ( 
                               
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                                 , 
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                               ) 
                             
                           
                         
                         ) 
                       
                        
                     
                     2 
                   
                 
               
             
           
         
       
     
     The definition of the tone scale function can be used to derive an equivalent form of this distortion measure, shown as Equation 29. 
                   Distortion   ⁢           ⁢   measure           Equation   ⁢           ⁢   29                 D   ⁡     (     I   ,   P     )       =       ∑     n   &lt;     cv   low         ⁢           ⁢         h   ~     ⁡     (     n   ,   c     )       ·       max   c     ⁢              (     n   -     cv   low            2     +       ∑     n   &gt;     cv   high         ⁢           ⁢         h   ~     ⁡     (     n   ,   c     )       ·       max   c     ⁢            (     n   -     cv   high       )          2                                             
This measure comprises a weighted sum of the clipping error at the high and low code values. A power/distortion curve may be constructed for an image using the expression of Equation 29.  FIG. 39  is a graph showing power/distortion curves for various exemplary images.  FIG. 39  shows a power/distortion plot  590  for a solid white image, a power/distortion plot  592  for a bright close-up of a yellow flower, a power/distortion plot  594  for a dark, low contrast image of a group of people, a power/distortion plot  596  for a solid black image and a power/distortion plot  598  for a bright image of a surfer on a wave.
 
     As can be seen from  FIG. 39 , different images can have quite different/power-distortion relations. At the extremes, a black frame  596  has minimum distortion at zero backlight power with distortion rising sharply as power increases to 10%. Conversely, a white frame  590  has maximum distortion at zero backlight with distortion declining steadily until rapidly dropping to zero at 100% power. The bright surfing image  598  shows a steady decrease in distortion as power increases. The two other images  592  and  594  show minimum distortion at intermediate power levels. 
     Some embodiments of the present invention may comprise a backlight modulation algorithm that operates as follows:
         1. Compute image histogram   2. Compute power distortion function for image   3. Calculate least power with distortion below distortion limit.   4. (Optional) limit selected power based on supplied power upper and lower limits   5. Select computed power for backlight       

     In some embodiments, described in relation to  FIGS. 40 and 41 , the backlight value  604  selected by the BL modulation algorithm may be provided to the BP algorithm and used for tone scale design. Average power  602  and distortion  606  are shown. An upper bound on the average power  600  used in this experiment is also shown. Since the average power use is significantly below this upper bound better power allocation could be used. 
     Development of a Smooth Tone Scale Function. 
     In some embodiments of the present invention, the smooth tone scale function comprises two design aspects. The first assumes parameters for the tone scale are given and determines a smooth tone scale function meeting those parameters. The second comprises an algorithm for selecting the design parameters. 
     Tone Scale Design Assuming Parameters 
     The code value relation defined by Equation 26 has slope discontinuities when clipped to the valid range [cvMin, cvMax]. In some embodiments of the present invention, smooth roll-off at the dark end may be defined analogously to that done at the bright end in Equation 7. These embodiments assume both a Maximum Fidelity Point (MFP) and a Least Fidelity Point (LFP) between which the tone scale agrees with Equation 26. In some embodiments, the tone scale may be constructed to be continuous and have a continuous first derivative at both the MFP and the LFP. In some embodiments, the tone scale may pass through the extreme points (ImageMinCV, cvMin) and (ImageMaxCV, cvMax). In some embodiments, the tone scale may be modified from an affine boost at both the upper and lower ends. Additionally, the limits of the image code values may be used to determine the extreme points rather than using fixed limits. It is possible to used fixed limits in this construction but problems may arise with large power reduction. In some embodiments, these conditions uniquely define a piecewise quadratic tone scale which as derived below. 
     Conditions: 
     
       
         
           
             
               
                 
                   Tone 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   scale 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   definition 
                 
               
               
                 
                   
                     Equation 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     30 
                   
                   ⁢ 
                   
                       
                   
                 
               
             
             
               
                 
                   
                     TS 
                     ⁡ 
                     
                       ( 
                       x 
                       ) 
                     
                   
                   = 
                   
                     { 
                     
                       
                         
                           
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                             ⁢ 
                             
                                 
                             
                             ⁢ 
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                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               Min 
                             
                             ≤ 
                             x 
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                                 Image 
                                 ⁢ 
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                               A 
                               · 
                               
                                 
                                   ( 
                                   
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                               ⁢ 
                               
                                   
                               
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                   Tone 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   scale 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   slope 
                 
               
               
                 
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                   ⁢ 
                   
                       
                   
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                   31 
                 
               
             
             
               
                 
                   
                     
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                               2 
                               · 
                               D 
                               · 
                               
                                 ( 
                                 
                                   x 
                                   - 
                                   MFP 
                                 
                                 ) 
                               
                             
                             + 
                             E 
                           
                         
                         
                           
                             x 
                             &gt; 
                             MFP 
                           
                         
                       
                     
                   
                 
               
               
                 
                     
                 
               
             
           
         
       
     
     Quick observation of continuity of the tone scale and first derivative at LFP and MFP yields.
 
Solution for tone scale parameters B, C, E, F   Equation 32
 
 B=α 
 
 C=α·LFP+β 
 
 E=α 
 
 F=α·MFP+β 
 
     The end points determine the constants A and D as: 
     
       
         
           
             
               
                 
                   Solution 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   for 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   tone 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   scale 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   parameters 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   A 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   and 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   D 
                 
               
               
                 
                   Equation 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   33 
                 
               
             
             
               
                 
                   A 
                   = 
                   
                     
                       
                         cv 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         Min 
                       
                       - 
                       
                         B 
                         · 
                         
                           ( 
                           
                             
                               Image 
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               Min 
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               CV 
                             
                             - 
                             LFP 
                           
                           ) 
                         
                       
                       - 
                       C 
                     
                     
                       
                         ( 
                         
                           
                             Image 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             Min 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             CV 
                           
                           - 
                           LFP 
                         
                         ) 
                       
                       2 
                     
                   
                 
               
               
                 
                     
                 
               
             
             
               
                 
                   D 
                   = 
                   
                     
                       
                         cv 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         Max 
                       
                       - 
                       
                         E 
                         · 
                         
                           ( 
                           
                             
                               Image 
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               Max 
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               CV 
                             
                             - 
                             MFP 
                           
                           ) 
                         
                       
                       - 
                       F 
                     
                     
                       
                         ( 
                         
                           
                             Image 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             Max 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             CV 
                           
                           - 
                           MFP 
                         
                         ) 
                       
                       2 
                     
                   
                 
               
               
                 
                     
                 
               
             
           
         
       
     
     In some embodiments, these relations define the smooth extension of the tone scale assuming MFP/LFP and ImageMaxCV/ImageMinCV are available. This leaves open the need to select these parameters. Further embodiments comprise methods and systems for selection of these design parameters. 
     Parameter Selection (MFP/LFP) 
     Some embodiments of the present invention described above and in related applications address only the MFP with ImageMaxCV equal to 255, cvMax was used in place of ImageMaxCV introduced in these embodiments. Those previously described embodiments had a linear tone scale at the lower end due to the matching based on the full power display rather than the ideal display. In some embodiments, the MFP was selected so that the smooth tone scale had slope zero at the upper limit, ImageMaxCV. Mathematically, the MFP was defined by:
 
MFP selection criterion   Equation 34
 
 TS ′(ImageMax CV )=0
 
2 ·D ·(ImageMax CV−MFP )+ E= 0
 
     The solution to this criterion relates the MFP to the upper clipping point and the maximum code value: 
     
       
         
           
             
               
                 
                   Prior 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   MFP 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   selection 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   criteria 
                 
               
               
                 
                   Equation 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   35 
                 
               
             
             
               
                 
                   MFP 
                   = 
                   
                     
                       2 
                       · 
                       
                         x 
                         high 
                       
                     
                     - 
                     
                       Image 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       Max 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       CV 
                     
                   
                 
               
               
                 
                     
                 
               
             
             
               
                 
                   MFP 
                   = 
                   
                     
                       
                         2 
                         · 
                         cv 
                       
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       
                         Max 
                         · 
                         
                           
                             ( 
                             P 
                             ) 
                           
                           
                             1 
                             γ 
                           
                         
                       
                     
                     - 
                     
                       Image 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       Max 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       CV 
                     
                   
                 
               
               
                 
                     
                 
               
             
           
         
       
     
     For modest power reduction such as P=80% this prior MFP selection criteria works well. For large power reduction, these embodiments may improve upon the results of previously described embodiments. 
     In some embodiments, we select an MFP selection criterion appropriate for large power reduction. Using the value ImageMaxCV directly in Equation 35 may cause problems. In images where power is low we expect a low maximum code value. If the maximum code value in an image, ImageMaxCV, is known to be small Equation 35 gives a reasonable value for the MFP but in some cases ImageMaxCV is either unknown or large, which can result in unreasonable i.e. negative MFP values. In some embodiments, if the maximum code value is unknown or too high, an alternate value may be selected for ImageMaxCV and applied in the result above. 
     In some embodiments, k may be defined as a parameter defining the smallest fraction of the clipped value x high  the MFP can have. Then, k may be used to determine if the MFP calculated by Equation 35 is reasonable i.e.
 
“Reasonable” MFP criteria   Equation 36
 
 MFP≧k·x   high  
 
If the calculated MFP is not reasonable, the MFP may be defined to be the smallest reasonable value and the necessary value of ImageMaxCV may be determined, Equation 37. The values of MFP and ImageMaxCV may then be used to determine the tone scale via as discussed below.
 
     
       
         
           
             
               
                 
                   Correcting 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   ImageMaxCV 
                 
               
               
                 
                   Equation 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   37 
                 
               
             
             
               
                 
                   MFP 
                   = 
                   
                     k 
                     · 
                     
                       x 
                       high 
                     
                   
                 
               
               
                 
                     
                 
               
             
             
               
                 
                   
                     k 
                     · 
                     
                       x 
                       high 
                     
                   
                   = 
                   
                     
                       
                         2 
                         · 
                         cv 
                       
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       
                         Max 
                         · 
                         
                           
                             ( 
                             P 
                             ) 
                           
                           
                             1 
                             γ 
                           
                         
                       
                     
                     - 
                     
                       Image 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       Max 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       CV 
                     
                   
                 
               
               
                 
                     
                 
               
             
             
               
                 
                   
                     Image 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     Max 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     CV 
                   
                   = 
                   
                     
                       ( 
                       
                         2 
                         - 
                         k 
                       
                       ) 
                     
                     · 
                     
                       x 
                       high 
                     
                   
                 
               
               
                 
                     
                 
               
             
           
         
       
     
     Steps for the MFP selection, of some embodiments, are summarized below:
         1. Compute candidate MFP using ImageMaxCV (or CVMax if unavailable)   2. Test reasonableness using Equation 36   3. If unreasonable, define MFP based on fraction k of clipping code value   4. Calculate new ImageMaxCV using Equation 37.   5. Compute smooth tone scale function using MFP, ImageMaxCV and power.
 
Similar techniques may be applied to select the LFP at the dark end using ImageMinCV and x low .
       

     Exemplary tone scale designs based on smooth tone scale design algorithms and automatic parameter selection are shown in  FIGS. 42-45 .  FIGS. 42 and 43  show an exemplary tone scale design where a backlight power level of 11% has been selected. A line  616  corresponding to the linear section of the tone scale design between the MFP  610  and the LFP  612  is shown. The tone scale design  614  curves away from line  616  above the MFP  610  and below the LFP  612 , but is coincident with the line  616  between the LFP  612  and the MFP  610 .  FIG. 41  is zoomed-in image of the lark region of the tone scale design of  FIG. 42 . The LFP  612  is clearly visible and the lower curve  620  of the tone scale design can be seen curving away from the linear extension  622 . 
       FIGS. 44 and 45  show an exemplary tone scale design wherein the backlight level has been selected at 89% of maximum power.  FIG. 44  shows a line  634  coinciding with the linear portion of the tone scale design. Line  634  represents an ideal display response. The tone scale design  636  curves away  636 ,  638  from the ideal linear display representation  634  above the MFP  630  and below the LFP  632 .  FIG. 45  shows a zoomed-in view of the dark end of the tone scale design  636  below the LFP  640  where the tone scale design  642  curves away from the ideal display extension  644 . 
     In some embodiments of the present invention, the distortion calculation can be modified by changing the error calculation between the ideal and actual display images. In some embodiments, the MSE may be replaced with a sum of distorted pixels. In some embodiments, the clipping error at upper and lower regions may be weighed differently. 
     Some embodiments of the present invention may comprise an ambient light sensor. If an ambient light sensor is available, the sensor can be used to modify the distortion metric including the effects of surround illumination and screen reflection. This can be used to modify the distortion metric and hence the backlight modulation algorithm. The ambient information can be used to control the tone scale design also by indicating the relevant perceptual clipping point at the black end. 
     Color Preservation Embodiments 
     Some embodiments of the present invention comprise systems and methods for preserving color characteristics while enhancing image brightness. In some embodiments, brightness preservation comprises mapping the full power gamut solid into the smaller gamut solid of a reduced power display. In some embodiments different methods are used for color preservation. Some embodiments preserve the hue/saturation of a color in exchange for a reduction in luminance boost. 
     Some non-color-preserving embodiments described above process each color channel independently operating to give a luminance match on each color channel. In those non-color-preserving embodiments, highly saturated or highlight colors can be become desaturated and/or change in hue following processing. Color-preserving embodiments address these color artifacts, but, in some case, may slightly reduce the luminance boost. 
     Some color-preserving embodiments may also employ a clipping operation when the low pass and high pass channels are recombined. Clipping each color channel independently can again result in a change in color. In embodiments employing color-preserving clipping, a clipping operation may be used to maintain hue/saturation. In some cases, this color-preserving clipping may reduce the luminance of clipped values below that of other non-color-preserving embodiments. 
     Some embodiments of the present invention may be described with reference to  FIG. 46 . In these embodiments, an input image  650  is read and code values corresponding to different color channels for a specified pixel location are determined  652 . In some embodiments, the input image may be in a format that has separate color channel information recorded in the image file. In an exemplary embodiment the image may be recorded with red, green and blue (RGB) color channels. In other embodiments, an image file may be recorded in a cyan, magenta, yellow and black (CMYK) format, an Lab, YUV or another format. An input image may be in a format comprising a separate luminance channel, such as Lab, or a format without a separate luminance channel, such as RGB. When an image file does not have separate color channel data readily available, the image file may be converted to format with color channel data. 
     Once code values for each color channel are determined  652 , the maximum code value among the color channel code values is then determined  654 . This maximum code value may then be used to determine parameters of a code value adjustment model  656 . The code value adjustment model may be generated in many ways. A tone-scale adjustment curve, gain function or other adjustment models may be used in some embodiments. In an exemplary embodiment, a tone scale adjustment curve that enhances the brightness of the image in response to a reduced backlight power setting may be used. In some embodiments, the code value adjustment model may comprise a tone-scale adjustment curve as described above in relation to other embodiments. The code value adjustment curve may then be applied  658  to each of the color channel code values. In these embodiments, application of the code value adjustment curve will result in the same gain value being applied to each color channel. Once the adjustments are performed, the process will continue for each pixel  660  in the image. 
     Some embodiments of the present invention may be described with reference to  FIG. 47 . In these embodiments, an input image is read  670  and a first pixel location is selected  672 . The code values for a first color channel are determined  674  for the selected pixel location and the code values for a second color channel are determined  676  for the selected pixel location. These code values are then analyzed and one of them is selected  678  based on a code value selection criterion. In some embodiments, the maximum code value may be selected. This selected code value may then be used as input for a code value adjustment model generator  680 , which will generate a model. The model may then be applied  682  to both the first and second color channel code values with substantially equal gain being applied to each channel. In some embodiments, a gain value obtained from the adjustment model may be applied to all color channels. Processing may then proceed to the next pixel  684  until the entire image is processed. 
     Some embodiments of the present invention may be described with reference to  FIG. 48 . In these embodiments, an input image  690  is input to the system. The image is then filtered  692  to create a first frequency range image. In some embodiments, this may be a low-pass image or some other frequency range image. A second frequency range image  694  may also be generated. In some embodiments, the second frequency range image may be created by subtracting the first frequency range image from the input image. In some embodiments, where the first frequency range image is a low-pass (LP) image, the second frequency range image may be a high-pass (HP) image. A code value for a first color channel in the first frequency range image may then be determined  696  for a pixel location and a code value for a second color channel in the first frequency range image may also be determined  698  at the pixel location. One of the color channel code values is then selected  700  by comparison of the code values or their characteristics. In some embodiments, a maximum code value may be selected. An adjustment model may then be generated or accessed  702  using the selected code value as input. This may result in a gain multiplier that may be applied  704  to the first color channel code value and the second color channel code value. 
     Some embodiments of the present invention may be described with reference to  FIG. 49 . In these embodiments, an input image  710  may be input to a pixel selector  712  that may identify a pixel to be adjusted. A first color channel code value reader  714  may read a code value for the selected pixel for a first color channel. A second color channel code value reader  716  may also read a code value for a second color channel at the selected pixel location. These code values may be analyzed in a analysis module  718 , where one of the code values will be selected based on a code value characteristic. In some embodiments, a maximum code value may be selected. This selected code value may then be input to a model generator  720  or model selector that may determine a gain value or model. This gain value or model may then be applied  722  to both color channel code values regardless of whether the code value was selected by the analysis module  718 . In some embodiments, the input image may be accessed  728  in applying the model. Control may then be passed  726  back to the pixel selector  712  to iterate through other pixels in the image. 
     Some embodiments of the present invention may be described with reference to  FIG. 50 . In these embodiments, an input image  710  may be input to a filter  730  to obtain a first frequency range image  732  and a second frequency range image  734 . The first frequency range image may be converted to allow access to separate color channel code values  736 . In some embodiments, the input image may allow access to color channel code values without any conversion. A code value for a first color channel of the first frequency range  738  may be determined and a code value for a second color channel of the first frequency range  740  may be determined. 
     These code values may be input to a code value characteristic analyzer  742 , which may determine code value characteristics. A code value selector  744  may then select one of the code values based on the code value analysis. This selection may then be input to an adjustment model selector or generator  746  that will generate or select a gain value or gain map based on the code value selection. The gain value or map may then be applied  748  to the first frequency range code values for both color channels at the pixel being adjusted. This process may be repeated until the entire first frequency range image has been adjusted  750 . A gain map may also be applied  753  to the second frequency range image  734 . In some embodiments, a constant gain factor may be applied to all pixels in the second frequency range image. In some embodiments, the second frequency range image may be a high-pass version of the input image  710 . The adjusted first frequency range image  750  and the adjusted second frequency range image  753  may be added or otherwise combined  754  to create an adjusted output image  756 . 
     Some embodiments of the present invention may be described with reference to  FIG. 51 . In these embodiments, an input image  710  may be sent to a filter  760  or other some other processor for dividing the image into multiple frequency range images. In some embodiments, filter  760  may comprise a low-pass (LP) filter and a processor for subtracting an LP image created with the LP filter from the input image to create a high-pass (HP) image. The filter module  760  may output two or more frequency-specific images  762 ,  764 , each having a specific frequency range. A first frequency range image  762  may have color channel data for a first color channel  766  and a second color channel  768 . The code values for these color channels may be sent to a code value characteristic evaluator  770  and/or code value selector  772 . This process will result in the selection of one of the color channel code values. In some embodiments, the maximum code value from the color channel data for a specific pixel location will be selected. This selected code value may be passed to an adjustment mode generator  774 , which will generate a code value adjustment model. In some embodiments, this adjustment model may comprise a gain map or gain value. This adjustment model may then be applied  776  to each of the color channel code values for the pixel under analysis. This process may be repeated for each pixel in the image resulting in a first frequency range adjusted image  778 . 
     A second frequency range image  764  may optionally be adjusted with a separate gain function  765  to boost its code values. In some embodiments no adjustment may be applied. In other embodiments, a constant gain factor may be applied to all code values in the second frequency range image. This second frequency range image may be combined with the adjusted first frequency range image  778  to form an adjusted combined image  781 . 
     In some embodiments, the application of the adjustment model to the first frequency range image and/or the application of the gain function to the second frequency range image may cause some image code values to exceed the range of a display device or image format. In these cases, the code values may need to be “clipped” to the required range. In some embodiments, a color-preserving clipping process  782  may be used. In these embodiments, code values that fall outside a specified range may be clipped in a manner that preserves the relationship between the color values. In some embodiments, a multiplier may be calculated that is no greater than the maximum required range value divide by the maximum color channel code value for the pixel under analysis. This will result in a “gain” factor that is less than one and that will reduce the “oversize” code value to the maximum value of the required range. This “gain” or clipping value may be applied to all of the color channel code values to preserve the color of the pixel while reducing all code values to value that are less than or equal to the maximum value or the specified range. Applying this clipping process results in an adjusted output image  784  that has all code values within a specified range and that maintains the color relationship of the code values. 
     Some embodiments of the present invention may be described in relation to  FIG. 52 . In these embodiments, color-preserving clipping is used to maintain color relationships while limiting code values to a specified range. In some embodiments, a combined adjusted image  792  may correspond to the combined adjusted image  781  described in relation to  FIG. 51 . In other embodiments the combined adjusted image  792  may be any other image that has code values that need to be clipped to a specified range. 
     In these embodiments, a first color channel code value is determined  794  and a second color channel code value is determined  796  for a specified pixel location. These color channel code values  794 ,  796  are evaluated in a code value characteristic evaluator  798  to determine selective code value characteristic and select a color channel code value. In some embodiments, the selective characteristic will be a maximum value and the higher code value will be selected as input for the adjustment generator  800 . The selected code value may be used as input to generate a clipping adjustment  800 . In some embodiments, this adjustment will reduce the maximum code value to a value within the specified range. This clipping adjustment may then be applied to all color channel code values. In an exemplary embodiment, the code values of the first color channel and the second color channel will be reduced  802  by the same factor thereby preserving the ratio of the two code values. The application of this process to all pixel in an image will result in an output image  804  with code values that fall within a specified range. 
     Some embodiments of the present invention may be described with reference to  FIG. 53 . In these embodiments, methods are implemented in the RGB domain by manipulating the gain applied to all three color components based on the maximum color component. In these embodiments, an input image  810  is processed by frequency decomposition  812 . In an exemplary embodiment, a low-pass (LP) filter  814  is applied to the image to create an LP image  820  that is then subtracted from the input image  810  to create a high-pass (HP) image  826 . In some embodiments, a spatial 5×5 rect filter may be used for the LP filter. At each pixel in the LP image  820 , the maximum value or the three color channels (R, G &amp; B) is selected  816  and input to an LP gain map  818 , which selects an appropriate gain function to be applied to all color channel values for that particular pixel. In some embodiments, the gain at a pixel with values [r, g, b] may be determined by a 1-D LUT indexed by max(r, g, b). The gain at value x may be derived from value of a Photometric matching tone scale curve, described above, at the value x divided by x. 
     A gain function  834  may also be applied to the HP image  826 . In some embodiments, the gain function  834  may be a constant gain factor. This modified HP image is combined  830  with the adjusted LP image to form an output image  832 . In some embodiments, the output image  832  may comprise code values that are out-of-range for an application. In these embodiments, a clipping process may be applied as explained above in relation to  FIGS. 51 and 52 . 
     In some embodiments of the present invention described above, the code value adjustment model for the LP image may be designed so that for pixels whose maximum color component is below a parameter, e.g. Maximum Fidelity Point, the gain compensates for a reduction in backlight power level. The Low Pass gain smoothly rolls off to 1 at the boundary of the color gamut in such a way that the processed Low Pass signal remains within Gamut. 
     In some embodiments, processing the HP signal may be independent of the choice of processing the low pass signal. In embodiments which compensate for reduced backlight power, the HP signal may be processed with a constant gain which will preserve the contrast when the power is reduced. The formula for the HP signal gain in terms of the full and reduced backlight powers and display gamma is given in 5. In these embodiments, the HP contrast boost is robust against noise since the gain is typically small e.g. gain is 1.1 for 80% power reduction and gamma 2.2. 
     In some embodiments, the result of processing the LP signal and the HP signal is summed and clipped. Clipping may be applied to the entire vector of RGB samples at each pixel scaling all three components equally so that the largest component is scaled to 255. Clipping occurs when the boosted HP value added to the LP value exceed 255 and is typically relevant for bright signals with high contrast only. Generally, the LP signal is guaranteed not to exceed the upper limit by the LUT construction. The HP signal may cause clipping in the sum but the negative values of the HP signal will never clip thereby maintaining some contrast even when clipping does occur. 
     Embodiments of the present invention may attempt to optimize the brightness of an image or they may attempt to optimize color preservation or matching while increasing brightness. Typically there is a tradeoff of a color shift when maximizing luminance or brightness. When the color shift is prevented, typically the brightness will suffer. Some embodiments of the present invention may attempt to balance the tradeoff between color shift and brightness by forming a weighted gain applied to each color component as shown in Equation 38.
 
WeightedGain( cv   x ,α)=α·Gain( cv   x )+(1−α)·Gain(max( cv   R   ,cv   G   ,cv   B )  Equation 38 Weighted Gain
 
This weighted gain varies between maximal luminance match at, alpha 0, to minimal color artifacts, at alpha 1. Note that when all code values are below the MFP parameter all three gains are equal.
 
Low Frequency Gain Map Smoothing
 
     Many embodiments of the present invention described above and other embodiments may be improved through the use of a spatially-smoothed low frequency gain map. In some situations, when a tone map increases pixel values in relation to the original pixel intensity, the values of spatial neighbors can be increased disproportionately. This can result in loss of detail in the adjusted image. However, this problem can be alleviated, in some embodiments, by the application of a spatially-smoothed, low-frequency gain map, which reduces the disproportionate gain between spatial neighbors. 
     Some embodiments of the present invention may be described in relation to  FIG. 54 . In these embodiments, an image may be split into two or more frequency ranges resulting in a low-pass or low-frequency (LF/LP) image  840  and a high-pass or high-frequency (HF/HP) image  841 . In some embodiments, this process may be performed by low-pass filtering the original image  847  to produce the low-pass image  840  and by subtracting the low-pass image from the original image to produce the high-pass image  841 . In other embodiments, other methods may be used to produce a low-pass image  840  and a high-pass image  841  from an original image  847 . 
     In these embodiments, a gain function or process  842  may also be generated. The gain process  842  may comprise generation of a gain map or another mathematical or logical process for manipulation of image values. The gain process  842  may be based on one or more characteristics of the original image  847 , one or more characteristics of the low-pass image  840  and/or other information. Once a gain process  842  is generated, a gain image  843  may be produced. A gain image  843  may comprise gain values for each pixel or sub-pixel in an image. The gain image  843  may then be spatially-smoothed  844  by one or more of many methods known in the art. This smoothed gain image may then be combined  845  with the low-pass image  840  to produce an enhanced low-pass image. In some embodiments, this combination step may comprise multiplication of smoothed gain image values by low-pass image  840  values. The combination  845  of the smoothed gain image with the low-pass image  840  may produce an enhanced low-pass image that may then be combined  847  with the high-pass image  841 . The combination  847  of the enhanced low-pass image and the high-pass image may result in an enhanced output image  846 . 
     Some embodiments of the present invention may be described in relation to  FIG. 55 . In these embodiments, an original input image  850  may be split into two or more frequency ranges resulting in a low-pass or low-frequency image  851  and a high-pass or high-frequency image  852 . In some embodiments, this process may be performed by low-pass filtering the original image  850  to produce the low-pass image  851  and by subtracting the low-pass image from the original image to produce the high-pass image  852 . In other embodiments, other methods may be used to produce a low-pass image  851  and a high-pass image  852  from an original image  850 . 
     In these embodiments, a gain process  853  may also be generated. The gain process  853  may be based on one or more characteristics of the original image  850 , one or more characteristics of the low-pass image  851  and/or other information. A tone map  853  may also be generated by any of the methods described above. Once a tone map  853  is generated, a gain image  854  may be produced. A gain image  854  may comprise gain values for each pixel or sub-pixel in an image. The gain image  854  may then be spatially-smoothed  855  by one or more of many methods known in the art. This smoothed gain image  856  may then be combined  857  with the low-pass image  851  to produce a enhanced low-pass image. In some embodiments, this combination step  857  may comprise multiplication of smoothed gain image values by low-pass image values. The combination  857  of the smoothed gain image with the low-pass image may produce an enhanced low-pass image. 
     In these embodiments, the high-pass image  852  may also be modified. A high-pass gain process  860  may be applied  858  to the high pass image  852  to produce an enhanced high-pass image. In some embodiments, the high-pass gain process may comprise a constant gain factor for all high-pass image elements. In other embodiments the high-pass gain process may result in a variable gain function. In other embodiments, other variations of gain functions and applications may be applied. 
     The enhanced low-pass image and the enhanced high-pass image may then be combined  859  to produce an enhanced output image  861 . In some embodiments, this process may comprise addition of the two images. 
     Some embodiments of the present invention may be described in relation to  FIG. 56 . In these embodiments, an original input image  870  serves as input to a LF/LP gain process  871 . In these embodiments, a LF/LP gain process may be created or modified in relation to the characteristics of the input image  871 . The LF/LP gain process may then be applied to the input image  870 , an LF/LP version of the input image or another variation of the input image to produce a LF/LP gain image  874 . This LF/LP gain image  874  may then be spatially smoothed  875  to produce a smoothed LF/LP gain image. 
     The input image  870  may also serve as input to a filter module  873 . This may be done by passing the input image  870  through the LF/LP gain process module  871  or the input image  870  may be sent directly  881  to the filter module  873 . In some embodiments, the filter module may comprise a low-pass filter, which, when applied to the input image  870 , creates a low-frequency or low-pass (LF/LP) image. The LF/LP image may then be combined  876  with the smoothed gain image to create an enhanced LF/LP image. 
     The input image  870  may also serve as input to a HF/HP gain process  872  whereby a high-frequency or high-pass (HF/HP) gain process is created. A HF/HP image may also be created by subtracting or otherwise processing the original input image  870  with the LF/LP image. In some embodiments the HF/HP image may be created independently of the LF/LP image. The HF/HP gain process may then be applied  878  to the HF/HP image to create an enhanced HF/HP image. In some embodiments, application of the HF/HP gain map to the HF/HP image may comprise multiplication of gain map values by the corresponding image values. 
     The enhanced HF/HP image may then be combined  879  with the enhanced LF/LP image to produce an output image  880 . 
     Some embodiments of the present invention may be described in relation to  FIG. 57 . In these embodiments, an original input image  890  serves as input to a frequency decomposition process  891 . In some embodiments, a low-pass filter  892  may be used to create a LF/LP image. This LF/LP image may then be used to create  893  a HF/HP image by subtraction from the input image  890  or by other methods. 
     In some embodiments, a color analysis process  896  may also be used. This process may comprise analysis of individual color channels of the input image or of the LF/LP image. Characteristics of one or more color channels may be used to determine a gain process, which may be applied to the LF/LP image to create an LF/LP gain image  894 . This LF/LP gain image  894  may then be smoothed to create a smoothed LF/LP gain image  895 . The smoothed LF/LP gain image may then be applied  897  to the LF/LP image to create an enhanced LF/LP image. 
     An HF/HP gain process  900  may also be used. This process may be independent of image characteristics or may analyze the image and adapt thereto. The HF/HP gain may be applied  899  to the HF/HP image to create an enhanced HF/HP image. Once the enhanced, frequency-specific images are created, they may be combined  898  to form an enhanced output image  901 . This combination may comprise addition of the two enhanced images. 
     Some embodiments of the present invention may be described with reference to  FIG. 58 . In these embodiments, an input image  910  may be input to a filter module  930 , comprising one or more filters or other elements for image frequency decomposition. The filter module  930  process may result in a first frequency range image  932  and a second frequency range image  934 . In some embodiments, the first frequency range image may be converted to allow access to separate color channel code values  936 . In some embodiments, the input image may allow access to color channel code values without any conversion. A code value for a first color channel of the first frequency range  938  may be determined and a code value for a second color channel of the first frequency range  940  may be determined. 
     These code values may be input to a code value characteristic analyzer  942 , which may determine code value characteristics. A code value selector  944  may then select one of the code values based on the code value analysis. This selection may then be input to an adjustment model selector or generator  945  that will generate or select a gain value or gain process based on the code value selection. 
     In these embodiments, the selected gain value or process may then be applied to the input image  910  or the first frequency range image  932  to obtain a first frequency range gain image  946 . The first frequency range gain image  946  may represent gain values that are to be multiplied by image values to effect a gain process. This first frequency range gain image  946  may then be spatially smoothed  947  to create a first frequency range smoothed gain image. 
     The smoothed first frequency range gain image may then be applied  948  to the first frequency range code values. A gain map may also be applied  953  to the second frequency range image  934 . In some embodiments, a constant gain factor may be applied to all pixels in the second frequency range image. In some embodiments, the second frequency range image may be a high-pass version of the input image  910 . The adjusted first frequency range image  950  and the adjusted second frequency range image  953  may be added or otherwise combined  954  to create an adjusted output image  956 . 
     Some embodiments of the present invention may be described with reference to  FIG. 59 . In these embodiments, an input image  910  may be sent to a filter  960  or some other processor for dividing the image into multiple frequency range images. In some embodiments, filter  960  may comprise a low-pass (LP) filter and a processor for subtracting an LP image created with the LP filter from the input image to create a high-pass (HP) image. The filter module  960  may output two or more frequency-specific images  962 ,  964 , each having a specific frequency range. A first frequency range image  962  may have color channel data for a first color channel  966  and a second color channel  968 . The code values for these color channels may be sent to a code value characteristic evaluator  970  and/or code value selector  972 . This process will result in the selection of one of the color channel code values. In some embodiments, the maximum code value from the color channel data for a specific pixel location will be selected. This selected code value may be passed to an adjustment mode generator  973 , which will generate a code value adjustment model. In some embodiments, this adjustment model may comprise a gain map or gain value. 
     This gain map or gain value may then be applied to the input image  910  or to the first frequency range image  962  to produce a first frequency range gain image  974 . The first frequency range gain image  974  may represent values for each pixel location by which corresponding image values may be multiplied to effect a gain process. This gain image  974  may then be spatially smoothed to produce a first frequency range smoothed image  975 . 
     The first frequency range smoothed gain image  975  may then be applied  976  to the input image  910  or the first frequency range image  962  to produce a first frequency range adjusted image  978 . 
     A second frequency range image  964  may optionally be adjusted with a separate gain function  965  to boost its code values. In some embodiments no adjustment may be applied. In other embodiments, a constant gain factor may be applied to all code values in the second frequency range image. This second frequency range image may be combined with the adjusted first frequency range image  978  to form an adjusted combined image  981 . 
     In some embodiments, the application of the adjustment model to the first frequency range image and/or the application of the gain function to the second frequency range image may cause some image code values to exceed the range of a display device or image format. In these cases, the code values may need to be “clipped” to the required range. In some embodiments, a color-preserving clipping process  982  may be used. In these embodiments, code values that fall outside a specified range may be clipped in a manner that preserves the relationship between the color values. In some embodiments, a multiplier may be calculated that is no greater than the maximum required range value divided by the maximum color channel code value for the pixel under analysis. This will result in a “gain” factor that is less than one and that will reduce the “oversize” code value to the maximum value of the required range. This “gain” or clipping value may be applied to all of the color channel code values to preserve the color of the pixel while reducing all code values to values that are less than or equal to the maximum value or the specified range. Applying this clipping process results in an adjusted output image  984  that has all code values within a specified range and that maintains the color relationship of the code values. 
     The terms and expressions which have been employed in the foregoing specification are used therein as terms of description and not of limitation, and there is no intention in the use of such terms and expressions of excluding equivalence of the features shown and described or portions thereof, it being recognized that the scope of the invention is defined and limited only by the claims which follow.