Patent Publication Number: US-6993200-B2

Title: System and method for effectively rendering high dynamic range images

Description:
BACKGROUND SECTION 
   1. Field of the Invention 
   This invention relates generally to techniques for manipulating data, and relates more particularly to a system and method for effectively rendering high dynamic range images. 
   2. Description of the Background Art 
   Implementing effective methods for manipulating data is a significant consideration for designers and manufacturers of contemporary electronic devices. However, effectively manipulating data with electronic devices may create substantial challenges for system designers. For example, enhanced demands for increased device functionality and performance may require more system processing power and require additional hardware resources. An increase in processing or hardware requirements may also result in a corresponding detrimental economic impact due to increased production costs and operational inefficiencies. 
   Furthermore, enhanced device capability to perform various advanced operations may provide additional benefits to a system user, but may also place increased demands on the control and management of various device components. For example, an enhanced electronic device that effectively captures and manipulates digital image data may benefit from an effective implementation because of the large amount and complexity of the digital data involved. 
   Due to growing demands on system resources and substantially increasing data magnitudes, it is apparent that developing new techniques for manipulating data is a matter of concern for related electronic technologies. Therefore, for all the foregoing reasons, developing effective systems for manipulating data remains a significant consideration for designers, manufacturers, and users of contemporary electronic devices. 
   SUMMARY 
   In accordance with the present invention, a system and method are disclosed for effectively rendering high dynamic range images. In one embodiment, a rendering manager may initially separate original image data into an original luminance image and chrominance information. Then, the rendering manager may preferably divide the original luminance image into original subband images using any appropriate technique or method. 
   Next, the rendering manager may preferably generate original contrast images that each correspond to one of the original subband images by utilizing any appropriate means. The rendering manager may then preferably calculate original contrast thresholds that correspond to the original subband images. Finally, the rendering manager may preferably generate original perceived contrast images that each correspond to one of the original contrast images by utilizing any appropriate and effective means. 
   In accordance with the present invention, the rendering manager may then preferably perform a compression procedure upon the original perceived contrast images to produce corresponding compressed perceived contrast images by utilizing any appropriate techniques to reduce the dynamic range of the original perceived contrast images. 
   The rendering manager may next preferably calculate display contrast thresholds corresponding to the particular type of display device upon which the rendered image data is ultimately viewed. Then, the rendering manager may preferably generate compressed contrast images corresponding to the compressed perceived contrast images by utilizing any appropriate technique. Next, the rendering manager may preferably generate compressed subband images corresponding to the compressed contrast images by utilizing any effective means. 
   The rendering manager may then preferably scale the pixel luminance values of a lowest-frequency subband image from the original subband images so that a mean pixel luminance value of the lowest-frequency subband image matches a mean luminance value of the particular type of display device upon which the rendered image data is viewed. 
   The rendering manager may next preferably perform a subband combination procedure to combine the lowest-frequency subband image with the compressed subband images to thereby produce a single rendered luminance image. In certain embodiments, the rendering manager may preferably begin the subband combination procedure by adding an initial compressed subband image (the next-to-lowest frequency compressed subband image) to the lowest-frequency subband image (upscaled by a factor of 2) to thereby generate a current combined subband image. The rendering manager may then continue the subband combination procedure by sequentially adding each of the remaining compressed subband images to the current combined subband image in an order of ascending subband frequencies to finally produce the rendered luminance image. 
   Finally, the rendering manager may preferably combine the rendered luminance image with the corresponding chrominance information to generate a rendered composite image. A system user may then access and utilize the rendered composite image in any desired manner. The present invention thus provides an improved system and method for effectively rendering high dynamic range images. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a block diagram for one embodiment of a camera device, in accordance with the present invention; 
       FIG. 2  is a block diagram for one embodiment of the capture subsystem of  FIG. 1 , in accordance with the present invention; 
       FIG. 3  is a block diagram for one embodiment of the control module of  FIG. 1 , in accordance with the present invention; 
       FIG. 4  is a block diagram for one embodiment of the memory of  FIG. 3 , in accordance with the present invention; 
       FIG. 5  is a block diagram illustrating an image rendering procedure, in accordance with one embodiment of the present invention; 
       FIG. 6  is a flowchart of initial method steps for performing an image rendering procedure, in accordance with one embodiment of the present invention; 
       FIG. 7  is a flowchart of method steps for performing a compression procedure, in accordance with one embodiment of the present invention; and 
       FIG. 8  is a flowchart of final method steps for performing an image rendering procedure, in accordance with one embodiment of the present invention. 
   

   DETAILED DESCRIPTION 
   The present invention relates to an improvement in data manipulation techniques. The following description is presented to enable one of ordinary skill in the art to make and use the invention and is provided in the context of a patent application and its requirements. Various modifications to the disclosed embodiments will be readily apparent to those skilled in the art, and the generic principles herein may be applied to other embodiments. Thus, the present invention is not intended to be limited to the embodiments shown, but is to be accorded the widest scope consistent with the principles and features described herein. 
   The present invention comprises a system and method for effectively rendering high dynamic range images, and may include a rendering manager that initially divides an original luminance image into a plurality of original subband images. The rendering manager may then convert the original subband images into original contrast images which may then advantageously be converted into original perceived contrast images. 
   The rendering manager may then perform a compression procedure upon the original perceived contrast images to produce compressed perceived contrast images. The rendering manager may next convert the compressed perceived contrast images into compressed contrast images which may then be converted into compressed subband images. The rendering manager may finally perform a subband combination procedure for combining the compressed subband images together with a lowest-frequency subband image to thereby generate a rendered luminance image. 
   Referring now to  FIG. 1 , a block diagram for one embodiment of a camera device  110  is shown, in accordance with the present invention. In the  FIG. 1  embodiment, camera device  110  may include, but is not limited to, a capture subsystem  114 , a system bus  116 , and a control module  118 . In the  FIG. 1  embodiment, capture subsystem  114  may be optically coupled to a target object  112 , and may also be electrically coupled via system bus  116  to control module  118 . 
   In alternate embodiments, camera device  110  may readily include various other components in addition to, or instead of, those components discussed in conjunction with the  FIG. 1  embodiment. In addition, in certain embodiments, the present invention may alternately be embodied in any appropriate type of electronic device other than the camera device  110  of  FIG. 1 . For example, camera device  110  may readily be implemented as a scanner device or a video camera device. Furthermore, the present invention may readily be implemented in any effective manner. For example, the present invention may be implemented as software instructions executed by an electronic device, and/or as appropriate hardware devices. 
   In the  FIG. 1  embodiment, once a system user has focused capture subsystem  114  on target object  112  and requested camera device  110  to capture image data corresponding to target object  112 , then control module  118  may preferably instruct capture subsystem  114  via system bus  116  to capture image data representing target object  112 . The captured image data may then be transferred over system bus  116  to control module  118 , which may responsively perform various processes and functions with the image data. System bus  116  may also bi-directionally pass various status and control signals between capture subsystem  114  and control module  118 . 
   Referring now to  FIG. 2 , a block diagram for one embodiment of the  FIG. 1  capture subsystem  114  is shown, in accordance with the present invention. In the  FIG. 2  embodiment, imaging device  114  preferably comprises, but is not limited to, a lens  220  having an iris (not shown), a filter  222 , an image sensor  224 , a timing generator  226 , red, green, and blue amplifiers  228 , an analog-to-digital (A/D) converter  230 , an interface  232 , and one or more motors  234  to adjust the focus of lens  220 . In alternate embodiments, capture subsystem  114  may readily include various other components in addition to, or instead of, those components discussed in conjunction with the  FIG. 2  embodiment. 
   In the  FIG. 2  embodiment, capture subsystem  114  may preferably capture image data corresponding to target object  112  via reflected light impacting image sensor  224  along optical path  236 . Image sensor  224 , which may preferably include a charged-coupled device (CCD), may responsively generate a set of image data representing the target object  112 . The image data may then be routed through red, green, and blue amplifiers  228 , A/D converter  230 , and interface  232 . Interface  232  may preferably include separate interfaces for controlling ASP  228 , motors  234 , timing generator  226 , and red, green, and blue amplifiers  228 . From interface  232 , the image data passes over system bus  116  to control module  118  for appropriate processing and storage. Other types of image capture sensors, such as CMOS or linear arrays are also contemplated for capturing image data in conjunction with the present invention. 
   Referring now to  FIG. 3 , a block diagram for one embodiment of the  FIG. 1  control module  118  is shown, in accordance with the present invention. In the  FIG. 3  embodiment, control module  118  preferably includes, but is not limited to, a viewfinder  308 , a central processing unit (CPU)  344 , a memory  346 , and one or more input/output interface(s) (I/O)  348 . Viewfinder  308 , CPU  344 , memory  346 , and I/O  348  preferably are each coupled to, and communicate, via common system bus  116  that also communicates with capture subsystem  114 . In alternate embodiments, control module  118  may readily include various other components in addition to, or instead of, those components discussed in conjunction with the  FIG. 3  embodiment. 
   In the  FIG. 3  embodiment, CPU  344  may preferably be implemented to include any appropriate microprocessor device. Alternately, CPU  344  may be implemented using any other appropriate technology. For example, CPU  344  may be implemented to include certain application-specific integrated circuits (ASICS) or other appropriate electronic devices. Memory  346  may preferably be implemented as one or more appropriate storage devices, including, but not limited to, read-only memory, random-access memory, and various types of non-volatile memory, such as floppy disc devices, hard disc devices, or flash memory. I/O  348  preferably may provide one or more effective interfaces for facilitating bi-directional communications between camera device  110  and any external entity, including a system user or another electronic device. I/O  348  may be implemented using any appropriate input and/or output devices. The operation and utilization of control module  118  is further discussed below in conjunction with  FIGS. 4 through 11 . 
   Referring now to  FIG. 4 , a block diagram for one embodiment of the  FIG. 3  memory  346  is shown, in accordance with the present invention. In the  FIG. 4  embodiment, memory  346  may preferably include, but is not limited to, a camera application  412 , an operating system  414 , a rendering manager  416 , original image data  418 , rendered image data  420 , temporary image data  422 , and miscellaneous information  424 . In alternate embodiments, memory  346  may readily include various other components in addition to, or instead of, those components discussed in conjunction with the  FIG. 4  embodiment. 
   In the  FIG. 4  embodiment, camera application  412  may include program instructions that are preferably executed by CPU  344  ( FIG. 3 ) to perform various functions and operations for camera device  110 . The particular nature and functionality of camera application  412  preferably varies depending upon factors such as the type and particular use of the corresponding camera device  110 . 
   In the  FIG. 4  embodiment, operating system  414  preferably controls and coordinates low-level functionality of camera device  110 . In accordance with the present invention, rendering manager  416  may preferably control and coordinate an image rendering procedure to convert original image data  418  into rendered image data  420 . In alternate embodiments, rendering manager  416  may be implemented using one or more hardware devices that perform the same or similar functions as those described herein for operation of the software instructions forming rendering manager  416 . 
   In the  FIG. 4  embodiment, temporary image data  422  may include any type of image information that is temporarily required for performing the foregoing image rendering procedure. Miscellaneous routines  424  may include any desired software instructions or types of data to facilitate functions performed by camera device  110 . The operation and utilization of rendering manager  416  is further discussed below in conjunction with  FIGS. 5 through 8 . 
   Referring now to  FIG. 5 , a block diagram illustrating an image rendering procedure  510  is shown, in accordance with one embodiment of the present invention. In alternate embodiments of the present invention, image rendering procedures may readily be performed with various other configurations, and may also include various elements or steps that are different from those discussed in conjunction with the  FIG. 5  embodiment. 
   Imaging technology typically strives to accurately reproduce the impression an observer receives when looking at a particular scene or object. However, a substantial difference still remains in how a camera captures a scene, as compared to what the human visual system is capable of achieving. One of the primary restrictions in consumer digital still cameras (DSC) is the limited dynamic range. At least three distinctive factors within the camera architecture can be identified as being responsible. 
   The first factor is the transformation of the analog signal from a charge-coupled device (CCD) into a digital signal. Currently, 12 bits are typically used. Naturally, that number limits the capability of the camera to simultaneously capture details in very light and very dark areas. The second factor is the number of bits available for the final output image of the camera. That number is typically 8 bits per channel, and is closely related to capabilities of different output devices like computer screens and TV monitors. The last factor concerns the specific transformation that is used to convert a twelve bits/channel image into an eight bits/channel image. The present invention addresses transforming a high dynamic range image into a low dynamic range image. The high dynamic range image could, for example, be a 12 bit per channel image from an A/D converter, or it could be an image that has been calculated from several corresponding images at different exposures. 
   Currently, a typical digital still camera may have difficulty in capturing a scene with a high dynamic range, i.e. a scene which contains very bright and very dark areas. Either the scene may be accurately reproduced in the very bright areas, and the dark areas may lose details and may appear uniform (black), or the dark areas are accurate, and the bright areas may lose detail and may appear saturated (usually white). 
   An ideal system would “measure” the perception that a human observer has of a scene and produce an image that would be perceived by the observer in the same way as the original scene. Perception consists of a collection of attributes, but certain embodiments of the present invention focus on the attribute of perceived contrast. The present invention provides a transformation from a high dynamic range image into a low dynamic range image, which performs the necessary compression, but which results in an image that is perceptually as close as possible to the original scene. 
   With regard to capturing original image data, the present invention may operate on raw image data from a single source. However, in alternate embodiments, if a scene is not properly captured due to the limited dynamic range of the camera, several differently exposed images may be used to record every detail of interest in the scene. To merge the source images into a single image, the capture algorithm first typically assumes that the images are aligned with respect to each other. If this is not the case, then the source images may be aligned using a motion estimation algorithm. 
   It is further assumed that the capture algorithm has access to camera raw data, i.e. the data that is delivered by the camera sensor (CCD). By using the aperture and shutter speed values of the image, as well as the OECF function of the camera, a radiance image may be built. This radiance image preferably contains a luminance value for each pixel of the scene. The images may be merged together in a Bayesian framework in which each pixel is considered as a Gaussian Random Variable with a mean value equal to the pixel luminance value, and a variance derived from a prior knowledge of the camera characteristics. The pixel value of the final image (original image data  418 ) may be computed by taking the most efficient estimate of pixels in all the source images corresponding to the same object in the scene. 
   In accordance with the present invention, a rendering manager  416  ( FIG. 4 ) may initially decompose the original image data  418  into one achromatic channel (herein referred to as original luminance image  514 ) and two chromatic channels. Rendering manager  416  may then preferably process original luminance image  514  while the chrominance information is kept unchanged during the image rendering procedure. 
   In general terms, rendering manager  416  may preferably perform the image rendering procedure  510  by initially transforming the original luminance image  514  into a perceptual contrast space, taking the luminance, the spatial frequency, and the respective contrast threshold into account. Rendering manager  416  may then perform a compression procedure on the data in the perceived contrast space in order to comply with the limited dynamic range available for final output of the digital still camera. Finally, rendering manager may preferably transform the compressed image data back into an image space for display or other use by a system user. 
   In the  FIG. 5  embodiment, in order to bring original luminance image  514  into perceptual contrast space, a contrast sensitivity function may preferably be computed. Several relevant parameters may preferably include luminance and spatial frequency. Spatial frequency may be defined as the frequency of contrast changes in an image within a given physical distance in the same image. To introduce dependence on spatial frequency, a multi-resolution approach may preferably be utilized. 
   Rendering manager  416  may therefore preferably divide original luminance image  514  into a plurality of original subband images  516  of differing frequencies by using a third-order spline wavelet filter bank or other appropriate means. Each original subband image  516  may then preferably be treated separately. The original subband image  516  containing the highest frequency component may preferably be centered around 16 cycles/degree, and the number of original subband images  516  may be such that the lowest-frequency subband image  546  in the multi-resolution pyramid contains less than 10 pixel values. 
   For each original subband image  516  (with the exception of lowest-frequency subband image  546 ), rendering manager  416  may preferably compute a corresponding original contrast image  518  by calculating individual pixel values M p  according to the following equation: 
               M   p     =         L   p     -     L   A                L   A          +   s               (3.1)             
 
where L p  is a luminance value at a particular location p in an original subband image  516 , L A  is a luminance average around the location p (which may be equal to a pixel value at a corresponding location in a next-lower frequency original subband image  516 ), and s is a saturation term to avoid divisions by zero. In the  FIG. 5  embodiment, s should be set using a maximum allowable contrast. For example, in certain embodiments, s may be approximately equal to 0.01.
 
   Rendering manager  416  may then transform the original contrast images into perceptual space to produce original perceived contrast images  522 . Rendering manage  416  may preferably begin by computing a diameter of a pupil of the human eye d according to the following equation:
 
 d =5−3 tan  h (0.4 log 10    L ) [mm]  (3.2)
 
where L is a mean luminance of a captured scene expressed in cd/m 2  (candelas per square meter).
 
   Then, for each pixel in each original subband image  516  (with the exception of lowest-frequency subband image  546 ), rendering manager  416  may compute an original contrast threshold m t  by utilizing the following equation: 
               m   t     =       k       M   opt     ⁡     (   u   )         ⁢         2   T     ⁢     (       1     X   o   2       +     1     X   MAX   2       +       u   2       N   MAX   2         )     ⁢     (       1     η   ⁢           ⁢   pE       +       ϕ   0       1   -     e     -       (     u     u   0       )     2               )                   (3.3)             
 
where u is a spatial frequency value for a corresponding pixel and the surrounding pixels, X 0  is an object size for the captured scene expressed in angular degrees for the human eye, N max  is a maximum number of cycles that a human eye can integrate, X max  is a maximum object size that a human eye can integrate expressed in angular degrees for the human eye,  η  is the quantum efficiency of cones in a human eye defined as an average number of photons causing an excitation of photo-receptors divided by a number of photons entering the human eye, p is a photon conversion factor for converting light units in units for flux density of photons, and may be defined as a number of photons per unit of time per unit of angular area per unit of luminous flux per angular area of light entering the human eye, E is a retinal illuminance value for the captured scene expressed in Trolands, φ 0  is a spectral density of neural noise caused by statistical fluctuations in a signal transport to the human brain, u o  is a lateral inhibition frequency limit, k is a signal-to-noise ratio of the captured scene, M opt (u) is an optical modulation transfer function that describes filtering of modulation by an image forming system, such as the human eye, as a function of spatial frequency, and T is an integration time of the human eye.
 
   In the  FIG. 5  embodiment, rendering manager  416  may preferably compute the foregoing retinal illuminance E in accordance with the following equation: 
             E   =         π   ⁢           ⁢     d   2       4     ⁢       L   A     ⁡     [     1   -       (     d   9.7     )     2     +       (     d   12.4     )     4       ]                 (3.4)             
 
where d is a pupil diameter of the human eye in mm, and L A  is a luminance average around a location p of a considered pixel, expressed in cd/m 2 . The result is expressed in Trolands [Td].
 
   In the  FIG. 5  embodiment, rendering manager  416  may preferably compute the optical modulation function M opt (u) according to the following equation:
 
 M   opt ( u )= e   −2π     2     σ     2     u     2   
 
σ=√{square root over (σ 0   2 +( c   ab   d ) 2 )}  (3.5)
 
where d is a pupil diameter of the human eye, u is a spatial frequency value, σ 0  is a point spread function basic width expressed in degrees, and c ab  describes an increase of the point spread function width at an increasing pupil size.
 
   In accordance with certain embodiments of the present invention, a set of exemplary values for the parameters involved in foregoing equations 3.3, 3.4, and 3.5 is given in Table 1. In alternate embodiments, the present invention may readily utilize other values than those shown in the following Table 1. 
                                      k = 3   T = 0.1 sec   n = 0.03       σ 0  = 0.0083 deg   X max  = 12 deg   φ 0  = 3 × 10 −8  sec deg 2         C ab  = 0.0013 deg/mm   X 0  = 60 deg   u 0  = 7 cycles/deg       p = 1.24 × 10 6     N max  = 15 cycles       photons/(sec. deg 2 . Td)                    
Table 1. Exemplary Parameters for Contrast Sensitivity Function.
 
   In accordance with the  FIG. 5  embodiment, rendering manager  416  may then transform original contrast images  518  into original perceived contrast images  522  by calculating individual pixel values C p  with the following equation: 
               C   p     =         M   p       m   t                 (   3.6   )             
 
where M p  is an original pixel contrast value from original contrast images  518  and m t  is a corresponding original contrast threshold.
 
   In the  FIG. 5  embodiment, rendering manager  416  may then perform a compression procedure on original perceived contrast images  522  using any effective means. In the  FIG. 5  embodiment, the effect of the compression procedure is preferably to keep small pixel amplitudes constant (or amplify them slightly), and to reduce large pixel amplitudes. 
   An example of one such a compression procedure is given in Table 2 below, where C cp  is the output of the compression procedure which is referred to herein as compressed perceived contrast images  530 . 
                                           Cp   Cp                                                    −∞   −10           −15    −10           −8   −7           −5   −5           −1   −1.2             1   1.2             5   5             8   7           15   10           +∞   10                        
Table 2. Compression Function Applied to Perceptual Contrast Space.
 
   In the  FIG. 5  embodiment, rendering manage  416  may then transform compressed perceived contrast images  530  from perceptual space back into an image space where they can be displayed or otherwise utilized. In the  FIG. 5  embodiment, the luminance range of an intended output display is assumed to be known. For instance, if the output display is a CRT monitor, the luminance range may typically range from 0 cd/m 2  to 100 cd/m 2 . To compute the adaptation of a human eye, rendering manager  416  may specify a mean value of the output display, Lout, which should be within the display range. By default, a middle point of the range may be taken, such as 50 cd/m 2 . Then, rendering manager  416  may compute a pupil diameter d for viewing the display by utilizing foregoing equation 3.2, and setting L=L out . 
   In the  FIG. 5  embodiment, for each pixel location p, rendering manager  416  may then compute a display contrast threshold by utilizing a average surrounding pixel value L A  at location p. Rendering manager  416  may calculate the display contrast threshold by utilizing foregoing equations 3.3 and 3.4. Then, rendering manager  416  may utilize the inverse of foregoing equation 3.6 to obtain a series of compressed contrast images  534 . Rendering manager  416  may next utilize the inverse of equation 3.1 to retrieve a corresponding series of compressed subband images  538 . 
   In the  FIG. 5  embodiment, rendering manager  416  may then process the only original subband image  516  that has not gone through the contrast and perceptual mapping of equations 3.1 and 3.6, namely the lowest-frequency subband image  546 . Ideally, lowest-frequency subband image  546  should be composed of a single pixel, but for practical reasons, if the original image size if not a power of two, it may be preferable to keep lowest-frequency subband image  546  to a size of approximately 10 pixels. 
   Rendering manager  416  may linearly rescale lowest-frequency subband image  546  such that its mean value is equal to the mean output luminance L out . Then, iteratively, rendering manager  416  may add each compressed subband image  538  to lowest-frequency subband image  546  to eventually produce rendered luminance image  550  for display after recombination with the corresponding chrominance information. In the  FIG. 5  embodiment, to add an initial compressed subband image  538  to lowest-frequency subband image  546 , rendering manager  416  may preferably upscale lowest-frequency subband image  546  by a factor of two to match the size of the initial compressed subband image  538 . 
   In the  FIG. 5  embodiment, rendering manager  416  may begin a subband combination procedure  542  by adding an initial compressed subband image  538  (the next-to-lowest compressed subband image) to lowest-frequency subband image  546  (upscaled by a factor of 2) to thereby generate a current combined subband image. Rendering manager  416  may then continue the subband combination procedure  542  by sequentially adding each of the remaining compressed subband images  538  to the current combined subband image in an order of ascending subband frequencies to finally produce rendered luminance image  550 . 
   The present invention thus utilizes a rather complete model for the description of perceived contrast, and makes extensive use of the contrast sensitivity function of the human visual system, also taking into account the dependence on a set of parameters. In addition, the present invention performs an explicit compression of the perceived contrast, and treats lowest-frequency subband image  546  in a manner that is an alliance with treatment of the higher frequency subband images. Therefore, as compared to other approaches, the present invention provides a strong connection to the way in which human beings perceive high contrast scenes. 
   The present invention may be implemented in hardware inside a digital still camera or other device to covert image data from, for example, 12 bits per channel down to 8 bits per channel. Another possibility would be to apply the algorithm in the form of a software module to high dynamic images gained either directly from a digital still camera, or to a high dynamic range image that has been calculated from several images with different exposures. 
   The foregoing approach may be utilized in digital still cameras, but may also be readily extended for video cameras. In that case, the time parameter should also be considered in calculating the contrast threshold. Another possibility is to apply the current algorithms in the field of Computer Graphics. The current algorithm may not only be used to optimize perceived contrast, but may also increase perceived contrast by changing the compression function mentioned above. Another natural extension is to apply the algorithm not only to the luminance channel, but also to the two chromatic channels. 
   Referring now to  FIG. 6 , a flowchart of initial method steps for performing an image rendering procedure  510  is shown, in accordance with one embodiment of the present invention. The  FIG. 6  embodiment is presented for purposes of illustration, and in alternate embodiments, the present invention may readily utilize various other steps and sequences than those discussed in conjunction with the  FIG. 6  embodiment. 
   In the  FIG. 6  embodiment, in step  612 , rendering manager  416  may initially separate a composite image (from original image data  418  of  FIG. 4 ) into an original luminance image  514  ( FIG. 5 ) and chrominance information. Then, in step  614 , rendering manager  416  may preferably divide original luminance image  514  into original subband images  516  using any appropriate technique or method. For example, rendering manager  416  may utilize techniques similar to those discussed in conjunction with foregoing  FIG. 5 . 
   In step  618 , rendering manager  416  may preferably generate original contrast images  518  that each correspond to one of the original subband images  516  by utilizing any appropriate means. In certain embodiments, rendering manager  416  may preferably generate original contrast images  518  in accordance with equation 3.1, as discussed above in conjunction with foregoing  FIG. 5 . 
   In step  632 , rendering manager  416  may preferably calculate original contrast thresholds that correspond to the original subband images  516 . In certain embodiments, rendering manager  416  may preferably calculate original contrast thresholds in accordance with equation 3.3, as discussed above in conjunction with  FIG. 5 . Finally, in step  636 , rendering manager  416  may preferably generate original perceived contrast images  522  that each correspond to one of the original contrast images  518  by utilizing any appropriate and effective means. In the  FIG. 6  embodiment, rendering manager  416  may preferably generate perceived contrast images  522  in accordance with equation 3.3, as discussed above in conjunction with  FIG. 5 . The  FIG. 6  process may then preferably advance to letter “A” of  FIG. 7 . 
   Referring now to  FIG. 7 , a flowchart of method steps for performing a compression procedure  526  is shown, in accordance with one embodiment of the present invention. The  FIG. 7  embodiment is presented for purposes of illustration, and in alternate embodiments, the present invention may readily utilize various other steps and sequences than those discussed in conjunction with the  FIG. 7  embodiment. 
   In the  FIG. 7  embodiment, in step  812 , rendering manager  416  may preferably access the original perceived contrast images  522  that were discussed above in conjunction with  FIG. 6 . Then, in step  816 , rendering manager  416  may preferably perform a compression procedure  526  upon the original perceived contrast images  522  to produce corresponding compressed perceived contrast images  820 . In the  FIG. 7  embodiment, the compression procedure  526  may utilize any appropriate techniques to reduce the dynamic range of the original perceived contrast images  522 . For example, the compression procedure  526  may utilize techniques similar to those discussed above in conjunction with  FIG. 5 . Finally, in step  820 , the compression procedure  816  may preferably output the compressed perceived contrast images  820  for further processing by rendering manager  416 . The  FIG. 7  process may then preferably advance to letter “B” of  FIG. 8 . 
   Referring now to  FIG. 8 , a flowchart of final method steps for performing an image rendering procedure  510  is shown, in accordance with one embodiment of the present invention. The  FIG. 8  embodiment is presented for purposes of illustration, and in alternate embodiments, the present invention may readily utilize various other steps and sequences than those discussed in conjunction with the  FIG. 8  embodiment. 
   In the  FIG. 8  embodiment, in step  812 , rendering manager  416  may preferably calculate display contrast thresholds corresponding to the particular type of display upon which rendered image data  420  is ultimately viewed. In certain embodiments, rendering manager  416  may preferably calculate display contrast thresholds m t  in accordance with equation 3.3, as discussed above in conjunction with  FIG. 5 . 
   Then, in step  816 , rendering manager  416  may preferably generate compressed contrast images  534  corresponding to the compressed perceived contrast images  530  by utilizing any appropriate technique. In certain embodiments, rendering manager  416  may preferably generate pixel values M p  for compressed contrast images  534  in accordance with equation 3.6 by utilizing pixel values C p  from compressed perceived contrast images  530  and the foregoing calculated display contrast thresholds m t , as discussed above in conjunction with  FIG. 5 . 
   Next, in step  820 , rendering manager  416  may preferably generate compressed subband images  538  corresponding to the compressed contrast images  534  by utilizing any appropriate technique. In certain embodiments, rendering manager  416  may preferably generate pixel values L p  for compressed subband images  538  in accordance with equation 3.1 by utilizing pixel values M p  from compressed contrast images  534  and appropriate surrounding luminance average values L A , as discussed above in conjunction with  FIG. 5 . 
   In step  824 , rendering manager  416  may preferably scale pixel luminance values of the lowest-frequency subband image  546  from original subband images  516  so that a mean pixel luminance value of lowest-frequency subband image  546  matches a mean luminance value of the particular type of display upon which rendered image data  420  is ultimately viewed. 
   In step  828 , rendering manager  416  may then preferably perform a subband combination procedure  542  to combine lowest-frequency subband image  546  and compressed subband images  538  to thereby produce a single rendered luminance image  550 . As discussed above in conjunction with  FIG. 5 , in certain embodiments, rendering manager  416  may preferably begin subband combination procedure  542  by adding an initial compressed subband image  538  (the next-to-lowest compressed subband image) to lowest-frequency subband image  546  (upscaled by a factor of 2) to thereby generate a current combined subband image. Rendering manager  416  may then continue the subband combination procedure  542  by sequentially adding each of the remaining compressed subband images  538  to the current combined subband image in an order of ascending subband frequencies to finally produce rendered luminance image  550 . 
   Finally, in step  832 , rendering manager  416  may preferably combine rendered luminance image  550  with corresponding chrominance information to generate a rendered composite image that may be stored as rendered image data  420  in memory  346 . A system user may then access and utilize rendered image data  420  in any appropriate manner. 
   The invention has been explained above with reference to certain embodiments. Other embodiments will be apparent to those skilled in the art in light of this disclosure. For example, the present invention may readily be implemented using configurations and techniques other than those described in the embodiments above. Additionally, the present invention may effectively be used in conjunction with systems other than those described above. Therefore, these and other variations upon the discussed embodiments are intended to be covered by the present invention, which is limited only by the appended claims.