Patent Publication Number: US-8976192-B2

Title: Contrast control for display device

Description:
This application claims priority from U.S. Provisional Patent Application Ser. No. 61/362,693, filed on Jul. 9, 2010, the content of which is incorporated herein by reference. 
    
    
     BACKGROUND 
     (a) Technical Field 
     The invention relates to an image display system, particularly to a contrast control device and method thereof. 
     (b) Description of the Related Art 
       FIG. 1  shows a block diagram of a conventional image display system. Referring to  FIG. 1 , a conventional image display device includes a display control portion  102 , an average brightness detecting portion  104 , a peak detecting portion  106 , a liquid crystal display (LCD) portion  108 , a backlight control portion  112  and a backlight  110 . 
     The display control portion  102  performs control operations for display on an input image signal (PIC) (e.g. a frame or a field) and supplies an output signal (DRV) to the LCD portion  108 . The average brightness detecting portion  104  and the peak detecting portion  106  respectively calculates an average brightness (AVE) value and detects a peak (PEK) value of the PIC signal. According to the AVE value and the PEK value, the backlight control portion  112  adjusts the control signal CTL to control the brightness of the backlight  110 . The backlight  110  serves as a light source for providing the brightness to the LCD portion  108 . 
     The conventional image display system only adjusts the brightness of the backlight  110  to reduce power consumption. Therefore, viewers may perceive distorted display images sometimes. Accordingly, what is needed is a device to address the above-identified problems. The invention addresses such a need. 
     BRIEF SUMMARY 
     One objective of the invention is to provide a contrast control device that can solve the above problems in the prior art. 
     One embodiment of the invention provides a contrast control device. The contrast control device comprises: a detecting unit for receiving a current image, calculating an average value and detecting a maximum value of a plurality of pixels of the current image; a judging unit for generating a gain index and a brightness index according to the average value; a backlight control unit for generating a backlight control signal according to the brightness index; a gain generating unit being coupled to the detecting unit for generating a first contrast gain value and a soft-clamping gain value according to the gain index, the average value and the maximum value and for selecting one from the first contrast gain value and the soft-clamping gain value for output as a second contrast gain value according to an input pixel value of the current image; and, a contrast gain processing unit for generating an output pixel value according to the second contrast gain value and the input pixel value. 
     One embodiment of the invention provides a contrast control method. The contrast control method, applied to an image display system, comprises: calculating an average value and detecting a maximum value according to a plurality of pixels of a current image; determining a gain index and a brightness index according to the average value; generating a backlight control signal according to the brightness index; obtaining a first contrast gain value and a soft-clamping gain value according to the gain index, the average value and the maximum value; selecting one from the first contrast gain value and the soft-clamping gain value as a second contrast gain value according to an input pixel value of the current image; and, obtaining an output pixel value according to the second contrast gain value and the input pixel value. 
     Further scope of the applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus are not limitative of the present invention, and wherein: 
         FIG. 1  shows a block diagram of a conventional image display device. 
         FIG. 2  shows a block diagram of a contrast control device according to one embodiment of the invention. 
         FIG. 3  shows output pixel values saturate at high luminance levels or/and high chrominance levels. 
         FIG. 4  shows a block diagram of a contrast control device according to another embodiment of the invention. 
         FIG. 5  shows a soft clamping mechanism of the invention. 
         FIG. 6  shows an adaptive clamping gain adjustment mechanism of the invention. 
         FIG. 7  shows a flow chart of a contrast control method according to an embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION 
     In the present disclosure, numerous specific details are provided, such as examples of electrical circuits, components, and methods, to provide a thorough understanding of embodiments of the invention. Persons of ordinary skill in the art will recognize, however, that the invention can be practiced without one or more of the specific details. In other instances, well-known details are not shown or described to avoid obscuring aspects of the invention. 
     According to the invention, in order to reduce power consumption, a backlight brightness is reduced and a contrast gain is compensated (or a display brightness is enhanced) at the same time. Accordingly, viewers are capable of enjoying the same image quality without perceiving distorted images. Since the power consumption of a backlight occupies the better part in a whole image display system, the total power consumption of the image display system is reduced according to the invention. 
       FIG. 2  shows a block diagram of a contrast control device according to one embodiment of the invention. Referring to  FIG. 2 , a contrast control device  200  of the invention, applied to an image display system, includes an average/maximum detecting unit  202 , a gain judging unit  204 , a brightness judging unit  208 , a gain mapping unit  210 , a backlight source control unit  212  and a contrast gain processing unit  216 . 
     After receiving an input image (e.g. a frame or a field), the average/maximum detecting unit  202  calculates an average (AVE) value and detects a maximum (PEK) value according to luminance values and/or chrominance values of the pixels (I) in the input image. According to the AVE value, the gain judging unit  204  generates a corresponding gain index (GI) value while the brightness judging unit  208  generates a corresponding brightness index (BI) value. According to the BI value, the backlight control unit  212  provides a control signal (CTL) to a backlight (not shown, embedded in the liquid crystal display device  218 ). Preferably, according to the BI index, the backlight control unit  212  reduces the duty cycles of a pulse width modulation (PWM) signal to reduce the brightness of the backlight. Those skilled in the art can calculate the power consumption of the backlight in view of the PWM duty cycle. 
     On the other hand, the gain mapping unit  210  generates a corresponding contrast gain (CG) value (i.e, the slope g in  FIG. 3 ) according to the GI value. Then, the contrast gain processing unit  216  generates a corresponding output pixel value (P) according to an input pixel value (I) (e.g. its luminance value and/or chrominance value) and the CG value. In this embodiment, the contrast gain processing unit  216  is implemented using a multiplier. However, the specific details are intended to be illustrative, and not limitations of the invention. The multiplier  216  multiplies the input pixel value (I) (such as a luminance value or a chrominance value) by the CG value to obtain the output pixel value (P). 
     In this embodiment, based on different AVE values, the brightness judging unit  208  generates a corresponding BI value to reduce different backlight brightness and the gain judging unit  204  generates a corresponding GI value to enhance display brightness at the same time. Therefore, the power consumption can be reduced and the viewers do not perceive degraded image quality. In view of hardware cost, a limited number of sets of parameters (AVE/PEK/GI/BI), such as 20 sets of parameters, are provided for the system. 
     However, the output pixel value (P), generated by the multiplier  216 , may be greater than a threshold value and saturated at high levels (including high luminance levels or/and high chrominance levels). In a case of 8-bit-per-pixel luminance data, the output pixel value (P) may be greater than level 255 and saturated at the white level (level 255). As shown in  FIG. 3 , when the CG value equals g (i.e. slope=g and g&gt;1), the output pixel values (P), corresponding to the input pixel values (I) ranging from Max/g to Max, reach its saturation level (i.e. Max on Y axis), making the output pixel values (P) not distinguishable at high luminance levels. 
     In view of the problem, an alternative embodiment is provided and describes as follows. 
       FIG. 4  shows a block diagram of a contrast control device according to another embodiment of the invention. Referring to  FIG. 4 , in this embodiment, modification is found in the addition of a step change judging unit  406  and a saturation compensation unit  416 . 
     The step change judging unit  406  is provided for fast scene change applications. When there are fast-changing scenes in displayed images, a flicker phenomenon may occur since the BI and GI values vary rapidly. The step change judging unit  406  receives the AVE values from the average/maximum detecting unit  202 , determines the AVE difference between the current image and the previous image, and finally supplies a corresponding reference index (RI) to the gain judging unit  204  and the brightness judging unit  208 . It should be noted that the backlight brightness is normally adjusted once for each image. When there is a big change of scene from the input images (e.g. a big AVE difference between the current image and the previous image), instead of being precisely changed to a target value at a time, the RI value is adjusted in a step-by-step manner such that the BI and GI values are also adjusted step-by-step. For example, when there is a big change of scene from the input images, two approaches may be adopted as follows. First, in a series of consecutive images, the RI value is adjusted in a small-step manner for each image until the RI value reaches its target values. Secondly, in a series of consecutive images, the RI value is adjusted in a big-step manner for every other n (n is a positive integer) images until the RI value reaches its target values. Since the step change judging unit  406  adjusts the backlight brightness gradually and smoothly, the viewers do not perceive screen flicker. 
     The saturation compensation unit  416  of  FIG. 4  includes a soft-clamping mechanism for reducing the CG values at high levels (including high luminance levels or/and high chrominance levels). In this embodiment, a resultant contrast gain value (CG′) is determined by the AVE value and clamped by the saturation compensation unit  416 .  FIG. 5  shows a soft clamping mechanism of the invention. When an input pixel value (I) is greater than N, the CG′ value is clamped to k (i.e., a soft clamping gain, (SG) equal to k, and 0&lt;k&lt;1) by the saturation compensation unit  416 . Accordingly, after contrast processing (performed by the contrast gain processing unit  216 ), the output pixel values (P) at the high levels (their corresponding input pixel values (I) greater than N) are distinguishable and retain linearity. Please be noted that both of the g and k values are not fixed, but depend on the AVE value for each input image. Specifically, the g value is firstly determined by the AVE value. According to the g value and PEK value, the saturation compensation unit  416  can determine whether saturation will occur at the high levels (in other words, determine whether the output pixel values (P) will saturate at the high levels). If it is determined that saturation will occur at the high levels (for example, the PEK value is greater than Max/g), the saturation compensation unit  416  will apply a soft clamping gain (SG) to the high levels and determine an appropriate k value according to the AVE value. In this regard, the N and M positions in  FIG. 5  are determined after the slopes g and k are settled. 
     On the other hand, the soft-clamping mechanism embedded in the saturation compensation unit  416  further includes an adaptive clamping gain adjustment mechanism. According to the AVE and PEK values, the saturation compensation unit  416  adaptively adjusts the soft clamping gain (SG). For image data with bright colors (e.g. sunshine), the soft clamping gain (SG) will be further adjusted and therefore the image contrast at the high levels will be further enhanced. According to one embodiment, the following equations are provided for the saturation compensation unit  416  to determine whether to conduct the adaptive clamping gain adjustment. 
     
       
         
           
               
               
             
               
                   
                   
               
             
            
               
                   
                 If (PEK&gt;TH Max  and AVE&gt; TH AVE ) 
               
               
                   
                  SG=m×k; 
               
               
                   
                 else 
               
               
                   
                  SG=k; 
               
               
                   
                   
               
            
           
         
       
     
       FIG. 6  shows an adaptive clamping gain adjustment mechanism of the invention. When an image is characterized by bright colors (i.e., AVE&gt;TH AVE  and PEK&gt;TH Max ), based on the adaptive clamping gain adjustment mechanism, the saturation compensation unit  416  sets the soft clamping gain (SG) to (m×k) in order to further enhancing the image contrast at the high levels. Otherwise, the saturation compensation unit  416  still sets the soft clamping gain (SG) to the k value. Finally, according to an input pixel value (I), the saturation compensation unit  416  selects one from the g value and the soft-clamping gain (SG) for output as the resultant contrast gain value (CG′). Accordingly, the multiplier  216  multiplies the input pixel value (I) by the CG′ value to obtain the output pixel value (P). Referring to  FIG. 6 , in one embodiment, if the input pixel value (I) is greater than or equal to the h value, the saturation compensation unit  416  selects the soft-clamping gain (SG) for output as the CG′ value. Otherwise, the saturation compensation unit  416  selects the g value for output as the CG′ value. p  FIG. 7  shows a flow chart of a contrast control method according to an embodiment of the invention. Referring to  FIG. 7 , the contrast control method of the invention is applied to an image display system, such as a liquid crystal display device. However, the invention is not limited to the above described examples. The contrast control method of the invention is described as follows. 
     Step S 702 : Calculate an average value (AVE) and detect a maximum value (PEK) according to a plurality of pixels of a current image. 
     Step S 704 : Determine a gain index and a brightness index according to the AVE value. As mentioned above, according to the AVE value, the brightness judging unit  208  generates a corresponding bright index BI to reduce different backlight brightness. Meanwhile, the gain judging unit  204  generates a corresponding gain index (GI) to enhance image brightness. Thus, the viewers perceive the same image quality and the power consumption is saved as well. In this embodiment, if the GI value is determined, the g value of the slope is also determined. 
     Step S 706 : Determine whether the output pixel value (P) will saturate at the high levels (including high luminance levels or/and high chrominance levels). For example, according to the g value of the slope and the PEK value, the saturation compensation unit  416  can determine whether the output pixel value (P) will saturate at the high levels (in other words, determine whether saturation will occur at the high levels). If “YES,” the flow goes to the step S 708 ; otherwise, the flow goes to the step S 714 . 
     Step S 708 : Determine whether the AVE value is greater than TH AVE  and the PEK value is greater than TH MAX . If “YES,” the flow goes to the step S 712 ; otherwise, the flow goes to the step S 710 . 
     Step S 710 : Determine the k value according to the AVE value. Besides, set the soft clamping gain (SG) to the k value and the flow goes to the step S 714 . 
     Step S 712 : Determine the m and k values according to the AVE value and the PEK value. Besides, set the soft clamping gain (SG) to (m×k), i.e., SG=m×k 
     Step S 714 : Generate a control signal (CTL) to adjust a backlight brightness in accordance with the brightness index (BI). Meanwhile, one of the g value and the soft-clamping gain (SG) is selected as a resultant contrast gain value (CG′). Accordingly, the input pixel value (I) is multiplied by the CG′ value to obtain the output pixel value (P). 
     While certain exemplary embodiments have been described and shown in the accompanying drawings, it is to be understood that such embodiments are merely illustrative of and not restrictive on the broad invention, and that this invention should not be limited to the specific construction and arrangement shown and described, since various other modifications may occur to those ordinarily skilled in the art.