PATENT DOCUMENT

Publication Number: US-7688328-B2
Application Number: US-26588205-A
Country: US
Kind Code: B2

Title: Luminance point correction without luminance degradation

Abstract:
White point is corrected without degrading luminance on a display device. A white point manager modifies the balance between red, green and blue according to a target white point up to a threshold gray value. As the gray scale approaches white from the threshold gray value, the white point manager blends the balance between red, green and blue from the target white point substantially towards the native white point for the display device, so as not to degrade luminance as the gray scale approaches white.

Claims:
1. A method for correcting white point without degrading luminance on a display device, the display device having a native luminance point, the method executed by a processor and comprising:
 for a first set of gray values occupying a first side of a threshold gray value, modifying, by the processor, a balance among a plurality of constituent color components according to a target white point up to the threshold gray value; and 
 for a second set of gray values occupying a second side of the threshold gray value, blending, by the processor, the balance among the plurality of constituent color components from the target white point substantially towards the native luminance point in a plurality of consecutive gray levels between the target white point to the native white point for the display device. 
 
   
   
     2. The method of  claim 1  wherein blending the balance among the plurality of constituent color components further comprises:
 interpolating the plurality of constituent color components from the threshold gray value to the native luminance point for the display device. 
 
   
   
     3. The method of  claim 1  wherein blending the balance among the plurality of constituent color components further comprises:
 interpolating the plurality of constituent color components from the threshold gray value to a target value between the threshold gray value and the native luminance point for the display device. 
 
   
   
     4. The method of  claim 1  wherein the second set of gray values occupies the range from the threshold gray value to the native luminance point for the display device. 
   
   
     5. The method of  claim 1  wherein the second set of gray values occupies the range from the threshold gray value to a target value between the threshold gray value and the native luminance point for the display device. 
   
   
     6. The method of  claim 1  further comprising:
 receiving a control signal indicating the target white point. 
 
   
   
     7. The method of  claim 6  wherein receiving a control signal indicating the target white point further comprises:
 receiving the control signal from one of a group of sources consisting of:
 a user operated input device; 
 an optical display calibration device; and 
 an internal automated display calibration mechanism. 
 
 
   
   
     8. The method of  claim 7  further comprising:
 saving the target white point as a user preference. 
 
   
   
     9. The method of  claim 1  further comprising:
 modifying measured gamma for the display device to a desired gamma, thereby correcting gamma for the display device. 
 
   
   
     10. The method of  claim 1 , wherein the native luminance point for the display device is a native white point. 
   
   
     11. The method of  claim 1 , wherein the native luminance point for the display device is a native black point. 
   
   
     12. A computer system for correcting white point without degrading luminance on a display device, the display device having a native luminance point, the system comprising:
 logic configured to modify a balance among a plurality of constituent color components according to a target white point up to the threshold gray value, for a first set of gray values occupying a first side of a threshold gray value; and 
 logic configured to blend the balance among the plurality of constituent color components from the target white point substantially towards the native luminance point in a plurality of consecutive gray levels between the target white point to the native white point for the display device, for a second set of gray values occupying a second side of the threshold gray value. 
 
   
   
     13. The system of  claim 12  further comprising:
 logic configured to interpolate the plurality of constituent color components from the threshold gray value to the native luminance point for the display device. 
 
   
   
     14. The system of  claim 12  further comprising:
 logic configured to interpolate the plurality of constituent color components from the threshold gray value to a target value between the threshold gray value and the native luminance point for the display device. 
 
   
   
     15. The system of  claim 12  further comprising:
 logic configured to receive a control signal indicating the target white point. 
 
   
   
     16. The system of  claim 15  further comprising:
 logic configured to save the target white point as a user preference. 
 
   
   
     17. The system of  claim 12  further comprising:
 logic configured to modify measured gamma for the display device to a desired gamma, thereby correcting gamma for the display device. 
 
   
   
     18. The system of  claim 12 , wherein the native luminance point for the display device is a native white point. 
   
   
     19. The system of  claim 12 , wherein the native luminance point for the display device is a native black point. 
   
   
     20. The system of  claim 12 , wherein the logic configured to modify a balance and the logic configured to blend the balance are hardware logic. 
   
   
     21. The system of  claim 20 , wherein the logic configured to modify a balance and the logic configured to blend the balance are components of the display device. 
   
   
     22. The system of  claim 12 , wherein the logic configured to modify a balance and the logic configured to blend the balance are software logic. 
   
   
     23. The system of  claim 12 , wherein the logic configured to modify a balance and the logic configured to blend the balance are firmware logic. 
   
   
     24. A computer system for correcting white point without degrading luminance on a display device, the display device having a native luminance point, the system comprising:
 means for modifying a balance among a plurality of constituent color components according to a target white point up to the threshold gray value, for a first set of gray values occupying a first side of a threshold gray value; and 
 means for blending the balance among the plurality of constituent color components from the target white point substantially towards the native luminance point in a plurality of consecutive gray levels between the target white point to the native white point for the display device, for a second set of gray values occupying a second side of the threshold gray value. 
 
   
   
     25. The system of  claim 24  further comprising:
 means for interpolating the plurality of constituent color components from the threshold gray value to the native luminance point for the display device. 
 
   
   
     26. The system of  claim 24  further comprising:
 means for interpolating the plurality of constituent color components from the threshold gray value to a target value between the threshold gray value and the native luminance point for the display device. 
 
   
   
     27. The system of  claim 24  further comprising:
 means for receiving a control signal indicating the target white point. 
 
   
   
     28. The system of  claim 27  further comprising:
 means for saving the target white point as a user preference. 
 
   
   
     29. The system of  claim 24  further comprising:
 means for modifying measured gamma for the display device to a desired gamma, thereby correcting gamma for the display device. 
 
   
   
     30. The system of  claim 24 , wherein the native luminance point for the display device is a native white point. 
   
   
     31. The system of  claim 24 , wherein the native luminance point for the display device is a native black point. 
   
   
     32. A computer program product for correcting white point without degrading luminance on a display device, the display device having a native luminance point, the computer program product comprising a computer-readable medium containing computer program code for:
 modifying a balance among a plurality of constituent color components according to a target white point up to the threshold gray value, for a first set of gray values occupying a first side of a threshold gray value; and 
 blending the balance among the plurality of constituent color components from the target white point substantially towards the native luminance in a plurality of consecutive gray levels between the target white point to the native white point for the display device, for a second set of gray values occupying a second side of the threshold gray value. 
 
   
   
     33. The computer program product of  claim 32  further comprising:
 program code for interpolating the plurality of constituent color components from the threshold gray value to the native luminance point for the display device. 
 
   
   
     34. The computer program product of  claim 32  further comprising:
 program code for interpolating the plurality of constituent color components from the threshold gray value to a target value between the threshold gray value and the native luminance point for the display device. 
 
   
   
     35. The computer program product of  claim 32  further comprising:
 program code for receiving a control signal indicating the target white point. 
 
   
   
     36. The computer program product of  claim 35  further comprising:
 program code for saving the target white point as a user preference. 
 
   
   
     37. The computer program product of  claim 32  further comprising:
 program code for modifying measured gamma for the display device to a desired gamma, thereby correcting gamma for the display device. 
 
   
   
     38. The computer program product of  claim 32 , wherein the native luminance point for the display device is a native white point. 
   
   
     39. The computer program product of  claim 32 , wherein the native luminance point for the display device is a native black point. 
   
   
     40. A method for correcting white point without degrading luminance on a display device, the display device having a native luminance point, the method executed by a processor and comprising:
 determining at least one threshold gray value within a range of input values of the display device; 
 for at least a first set of gray values occupying a first side of the threshold gray value, modifying, by the processor, a balance among a plurality of constituent color components according to a target white point up to the threshold gray value; and 
 for at least a second set of gray values occupying a second side of the threshold gray value, blending, by the processor, the balance among the plurality of constituent color components from the target white point substantially towards the native luminance point in a plurality of consecutive gray levels between the target white point to the native white point for the display device. 
 
   
   
     41. A method for correcting white point without degrading luminance on a display device, the display device having a native white point, the method executed by a processor and comprising:
 for a first set of gray values occupying a first side of a threshold gray value, adjusting, by the computer, a balance among a plurality of constituent color components according to a target white point up to the threshold gray value; and 
 for a second set of gray values occupying a second side of the threshold gray value, using a smoothing function to blend the balance among the plurality of constituent color components from the target white point to maximum output value in a plurality of consecutive gray levels between the target white point to the maximum output value to preserve maximum luminance for the display device. 
 
   
   
     42. The method of  claim 41 , wherein the smoothing function is linear interpolation. 
   
   
     43. The method of  claim 41 , wherein blending the balance among the plurality of constituent color components further comprises:
 interpolating the plurality of constituent color components from the threshold gray value to a target value without blending the gray levels all the way to maximum output value. 
 
   
   
     44. A system for correcting white point without degrading luminance on a display device, the display device having a native white point, the system comprising:
 a processor; 
 a white point manager configured by the processor to adjust a balance among a plurality of constituent color components according to a target white point up to a threshold gray value, for a first set of gray values occupying a first side of the threshold gray value; and 
 the white point manager further configured by the processor to blend the balance among the plurality of constituent color components from the target white point to maximum output value in a plurality of consecutive gray levels between the target white point to the maximum output value to preserve maximum luminance for the display device, for a second set of gray values occupying a second side of the threshold gray value. 
 
   
   
     45. The system of  claim 44  wherein the smoothing function is linear interpolation. 
   
   
     46. The system of  claim 44 , wherein the white point manager is further configured to:
 interpolate the plurality of constituent color components from the threshold gray value to a target value without blending the gray levels all the way to maximum output value. 
 
   
   
     47. A computer program product for correcting white point without degrading luminance on a display device, the display device having a native white point, the computer program product comprising a computer-readable medium containing computer program code for:
 adjusting a balance among a plurality of constituent color components according to a target white point up to a threshold gray value, for a first set of gray values occupying a first side of the threshold gray value; and 
 using a smoothing function to blend the balance among the plurality of constituent color components from the target white point to maximum output value in a plurality of consecutive gray levels between the target white point to the maximum output value to preserve maximum luminance for the display device, for a second set of gray values occupying a second side of the threshold gray value. 
 
   
   
     48. The computer program product of  claim 47 , further comprising:
 program code for interpolating the plurality of constituent color components from the threshold gray value to a target value without blending the gray levels all the way to maximum output value.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   This application is a continuation of, and claims priority under 35 USC § 120 to, U.S. patent application Ser. No. 10/641,214, filed Aug. 13, 2003 now U.S. Pat. No. 6,972,772 titled “White Point Correction without Luminance Degradation,” the contents of which are herein incorporated by reference. 

   BACKGROUND 
   1. Field of Invention 
   The present invention relates generally to display device color correction, and specifically to correcting white point without degrading luminance. 
   2. Background of Invention 
   It is often desirable to adjust the native white point of a display device to a target white point, in order to achieve an optimal display of images on the specific display device. Applications typically assume that the native white point will display correctly, and write to the display device accordingly. In practice, the display of native white is not optimal on many display devices. Therefore, it is necessary to modify the displayed white point for the device at a system level, so as to output optimal images. Prior art techniques for white point adjustment modify the balance between red, green and blue for the entire range of gray scale, from black to white. Such techniques succeed in correcting the white point from the native to the target, but have the pronounced, undesirable side effect of noticeably reducing the luminance of the display, particularly as the gray scale approaches white. By adjusting the balance to achieve the target white point, the prior art techniques lower red, green and/or blue values as the gray scale approaches white, thereby undesirably reducing the brightness of the output. 
   Although it is desirable to correct white point to target white, the resulting decrease in luminance in the prior art techniques is very noticeable to the users, and is thus highly undesirable. What is needed are methods, systems and computer program products for correcting white point without degrading luminance on a display device. 
   SUMMARY OF INVENTION 
   The majority of natural images (e.g., photographs, video) mostly use only the lower 95 percent of the gray scale range. By modifying the balance between red, green and blue according to the target white point for only the majority of the lower range (e.g., 95 percent) of input levels as opposed to the entire range, most images can be displayed optimally. However, the upper range of input levels requires additional special processing in order to achieve optimal display. Accordingly, the balance between red, green and blue for the remaining upper portion of the range (e.g., five percent) can be blended from the ratio for the target white point to the native white point. Thus, as input values approach the maximum range of gray scale, the output is adjusted towards native white, thereby preserving the maximum luminance. For example, suppose a display has the gray scale adjusted according to a target white point (e.g., D50) for the gray levels from 0 to a desired threshold in the upper part of the range, e.g., 242 in a system with a range of 0 to 255 (eight bits of color). The balance can then be gradually changed from D50 to native between 242 and 255, resulting in maximum output values for all three color channels at white. In other words, as input values approach 255, the output values will also approach 255 as opposed to being adjusted according to the target white point. Thus, the maximum luminance of the display will not be compromised by the white point adjustment. The transition of the red, green and blue balance can be implemented using linear interpolation or any other type of interpolation that minimizes the visual effect of the change of the white point. 
   Because the vast majority of natural images use on average only the lower 95 percent of the gray scale range, the images will generally be displayed according to the target white point balance, and yet luminance will not be compromised as the gray scale approaches white. The appearance of displayed images is improved according to the adjustment of the white point to target, but the user does not perceive a change in maximum brightness. 
   The features and advantages described in this summary and the following detailed description are not all-inclusive, and particularly, many additional features and advantages will be apparent to one of ordinary skill in the art in view of the drawings, specification, and claims hereof. Moreover, it should be noted that the language used in the specification has been principally selected for readability and instructional purposes, and may not have been selected to delineate or circumscribe the inventive subject matter, resort to the claims being necessary to determine such inventive subject matter. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a block diagram illustrating a high level overview of a system for practicing some embodiments of the present invention. 
       FIG. 2  is a block diagram illustrating white point correction according to the prior art. 
       FIG. 3  is a block diagram illustrating white point correction according to some embodiments of the present invention. 
       FIG. 4  is a flowchart diagram illustrating steps for correcting white point according to various embodiments of the present invention. 
   

   The figures depict embodiments of the present invention for purposes of illustration only. One skilled in the art will readily recognize from the following discussion that alternative embodiments of the structures and methods illustrated herein may be employed without departing from the principles of the invention described herein. 
   DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIG. 1  illustrates a high level overview of a system  100  for practicing some embodiments of the present invention. A white point manager  101  adjusts the red channel  103 , green channel  105  and/or blue channel  107  of a display device. The implementation mechanics for adjusting color channels are known to those of ordinary skill in the relevant art. According to the present invention, the white point manager  101  modifies the balance between red  103 , green  105  and blue  107  according to a target white point  102  up to a threshold gray value  111 . As the gray scale approaches white from the threshold gray value  111 , the white point manager  101  blends the balance between red  103 , green  105  and blue  107  from the target white point  102  substantially towards the native white point for the display device, so as not to degrade luminance as the gray scale approaches white. This procedure is discussed in greater detail below. 
   As illustrated in  FIG. 1 , in some embodiments the white point manager receives a control signal  109  indicating the target white point  102  to utilize in the channel modification and blending. As will be apparent to one of ordinary skill in the relevant art in light of this specification, such a control signal  109  can originate from a variety sources. In some embodiments, a chromaticity diagram is visually displayed to a user. The user indicates the target white point  102  by visually inspecting the chromaticity diagram, and operating an input device (e.g., a pointing device or keyboard) to make adjustments. The input device then generates the control signal  109  indicating the target white point  102 . In other embodiments, a visual white point indicator other than a chromaticity diagram is displayed to the user, such as a color wheel. In some embodiments, the target white point  102  is saved as a user preference, for example in a configuration file. The saved target white point can then be utilized in the future without having to be reselected or calibrated. 
   In other embodiments, the visual white point indicator can be measured by automatic optical calibrating hardware (not shown). Such hardware can optically measure display output (in this case the displayed chromaticity diagram) by measuring the photons emitted from the display. Such hardware can generate control signals  109  to modify the display properties (in this case, the white point adjustment). 
   It is to be understood that in other embodiments, the control signal  109  can be generated other ways as desired, for example by utilizing hardware and/or software to measure and modify signals internal to the computing system. 
   As illustrated in  FIG. 1 , the white point manager  101  defines a threshold gray value  111  in the upper part of the gray range. The nature and use of this threshold gray value is discussed in detail below. 
   It is to be understood that although the white point manager  101  is illustrated as a single entity, as the term is used herein a white point manager  101  refers to a collection of functionalities which can be implemented as software, hardware, firmware or any combination of the three. Where the white point manager  101  is implemented as software, it can be implemented as a standalone program, but can also be implemented in other ways, for example as part of a larger program, as a plurality of separate programs, or as one or more statically or dynamically linked libraries. In another embodiment of the invention, the white point manager  101  forms a computer program product and is stored on a computer-readable medium such as a storage device of the system  100 . 
   Before continuing the discussion of the white point manager  101  correcting the white point, it is first necessary to explain white point correction according to the prior art, which is illustrated in  FIG. 2 . As illustrated in  FIG. 2 , prior art techniques for white point adjustment modify the balance between red  103 , green  105  and blue  107  for the entire range of gray scale, from black to white. The specific adjustments made will be a function of the target white point  102 . In the example illustrated in  FIG. 2 , the modification to the balance is made by adjusting red  103  up and green  105  and blue  107  down. In other words, in order to generate the target white point, the amount of red  103  actually displayed in the output  201  is adjusted up from the amount indicated in the input  203 , whereas the amounts of green  105  and blue  107  are adjusted down from the input  203  values. Of course, the specific white point correction illustrated is only an example, and those of ordinary skill in the relevant art will understand that other adjustments can be made depending upon the specific white point correction desired for the specific display device. 
   The prior art technique succeeds in correcting the white point from the native to the target  102 , but as  FIG. 2  illustrates, the technique reduces the luminance of the display. By adjusting the balance to achieve target white point  102 , the red  103 , green  105  and/or blue  107  values are reduced. Because displaying white according to native white point for a display typically comprises outputting maximum values for red  103 , green  105  and blue  107 , any adjustment thereto must be down for at least one color channel, thereby necessitating a degradation of luminance. Thus, adjusting white point according to the prior art undesirably degrades the brightness of the output  201 , which is noticeable to the user, particularly as the gray scale approaches white. 
     FIG. 3  illustrates a technique for correcting white point without degrading luminance as the gray scale approaches white, according to some embodiments of the present invention. As mentioned above, the white point manager  101  defines a threshold gray value  111  in the upper part of the gray range. For example, in some embodiments the threshold gray value  111  is defined such that only approximately the upper five percent of the gray range is above it. To illustrate the example more specifically, in a system with a gray range of 0 to 255 (eight bits of color), the threshold gray value could be 242. The numbers 95 percent and 242 are examples only. As will be readily apparent to those of ordinary skill in the relevant art in light of this specification, in other embodiments other threshold gray values  111  can be utilized, as desired. The specific threshold gray value  111  to utilize is a design choice. 
   As illustrated in  FIG. 3 , the white point manager  101  modifies the balance between red  103 , green  105  and blue  107  according to a target white point  102  up to the threshold gray value  111 . However, as the gray scale approaches white from the threshold gray value  111 , the white point manager  101  blends the balance between red  103 , green  105  and blue  107  from the target white point  102  substantially towards the native white point for the display device, so as not to degrade luminance as the gray scale approaches white. In some embodiments, this blending from the threshold gray value  111  to white comprises interpolating red  103 , green  105  and blue  107  from the threshold gray value  111  to maximum output  201 . In other words, the white point manager  101  uses a smoothing function to blend the balance between red  103 , green  105  and blue  107  from the target white point  102  to maximum output  201  for all three color channels. Various smoothing functions are known in the art, for example linear interpolation. 
   To illustrate a specific example, a Samsung display of an Apple PowerBook G4 has a native white point of D65 and a target white point  102  of D50. Assume for the sake of the example a gray scale range of 0 to 255, and a threshold gray value  111  of 242. In this example, the white point manager  101  could adjust the balance between red  103 , green  105  and blue  107  from (243, 242, 210) corresponding to D50 at level 242 to (255, 255, 255) corresponding to D65 at level 255, in thirteen consecutive gray levels from 243 to 255. 
   As noted above, the majority of images mostly use only the lower 95 percent of the gray scale range. Therefore, by modifying the balance between red  103 , green  105  and blue  107  according to the target white point  102  for the majority of the lower range of input  203  levels only, the present invention enables most images to be displayed optimally. Because the color balance for the remaining upper portion of the range is blended to maximum output  201 , maximum luminance is preserved as the gray scale approaches white. 
   In other embodiments, the white point manager  101  interpolates red  103 , green  105  and blue  107  from the threshold gray value  111  to a target value between the threshold gray value  111  and maximum output  201 . Thus, the white point manager  101  still blends the balance between red  103 , green  105  and blue  107  from the target white point  102  substantially towards the native white point, but in these embodiments the white point manager  101  does not blend the levels all the way to maximum output  201 . The specific target value to utilize is a design choice. Because these embodiments do not blend the color balance all the way to maximum output  201 , they allow some reduction in luminance in the upper gray scale range. However, these embodiments keep the white point in the upper gray range closer to target  102  than the previously described embodiments, with significantly less luminance degradation than the prior art white point correction techniques. 
     FIG. 4  illustrates steps for correcting white point according to various embodiments of the present invention. As explained above, in various embodiment the white point manager modifies  401  a balance between red  103 , green  105  and blue  107  according to a target white point  102  up to a threshold gray value  111 . The white point manager  101  blends  403  the balance between red  103 , green  105  and blue  107  from the target white point  102  substantially towards the native white point for the display device, as the gray scale approaches white from the threshold gray value  111 . 
   As explained above, in some embodiments the white point manager  101  blends  403  the balance between red  103 , green  105  and blue  107  from the target white point  102  substantially towards the native white point for the display device by interpolating  405  red  103 , green  105  and blue  107  from the threshold gray value  111  to maximum output  201 . In other embodiments, the white point manager  101  blends  403  the balance between red  103 , green  105  and blue  107  from the target white point  102  substantially towards the native white point for the display device by interpolating  407  red  103 , green  105  and blue  107  from the threshold gray value  111  to a target value less than maximum output  201 . 
   It is to be understood that white point correction according to the present invention is distinct from gamma correction for a display. As those of ordinary skill in the relevant art know, gamma correction involves altering measured gamma (contrast associated with a device&#39;s gamma curve) to equal desired gamma. It will be apparent to those of ordinary skill in the relevant art in light of this specification that white point correction according to the present invention and gamma correction can be made independently of each other. As  FIG. 4  illustrates, in some embodiments of the present invention, in addition to correcting the white point as described above, measured gamma for the display device is modified  409  to a desired gamma, thereby correcting gamma for the display device. The implementation mechanics of gamma correction are known to those of ordinary skill in the relevant art. 
   As will be understood by those familiar with the art, the invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. Likewise, the particular naming and division of the modules, features, attributes, managers, methodologies and other aspects are not mandatory or significant, and the mechanisms that implement the invention or its features may have different names, divisions and/or formats. Furthermore, as will be apparent to one of ordinary skill in the relevant art, the modules, features, attributes, managers, methodologies and other aspects of the invention can be implemented as software, hardware, firmware or any combination of the three. Of course, wherever a component of the present invention is implemented as software, the component can be implemented as a standalone program, as part of a larger program, as a plurality of separate programs, as a statically or dynamically linked library, as a kernel loadable module, as a device driver, and/or in every and any other way known now or in the future to those of skill in the art of computer programming. Additionally, the present invention is in no way limited to implementation in any specific programming language, or for any specific operating system or environment. Accordingly, the disclosure of the present invention is intended to be illustrative, but not limiting, of the scope of the invention, which is set forth in the following claims.

Metadata:
Filing Date: 20051102
Publication Date: 20100330
Grant Date: 20100330
Priority Date: 20030813
Inventors: CHEN KOK
MARCU GABRIEL G.
Assignee: APPLE INC
CPC Classifications: [{"code": "G09G2320/0666", "inventive": false, "first": false, "tree": "[]"}, {"code": "G09G2320/0673", "inventive": false, "first": false, "tree": "[]"}, {"code": "G09G2320/0666", "inventive": false, "first": false, "tree": "[]"}, {"code": "G09G5/02", "inventive": true, "first": true, "tree": "[]"}, {"code": "G09G5/02", "inventive": true, "first": true, "tree": "[]"}, {"code": "G09G2320/0673", "inventive": false, "first": false, "tree": "[]"}]
Family ID: 35430467