# Patent

## Title
Method and apparatus for cross color and/or cross luminance suppression

## Abstract

A method and apparatus for cross color and/or cross luminance suppression. The apparatus checks for three conditions in determining the stillness of an image. Similarity between two frames, which are one frame apart, is examined. Similarity between two adjacent frames is also examined. Similarity among three successive frames is also examined. By integrating the results of such three conditions, the stillness of the image is determined. Only when the image is deemed still/stationary, is the cross color and/or cross luminance suppressing operation being performed.

## Background
BACKGROUND OF INVENTION
1. Field of the Invention
The present invention relates to a method for improving display quality of an image. More specifically, the present invention discloses a method of performing cross color and/or cross luminance suppression to improve display quality.
2. Description of the Prior Art
In composite video television systems such as NTSC and PAL, a luminance signal and a chrominance signal share a portion of the available bandwidth. In NTSC, for example, chrominance information is encoded through a sub-carrier having frequency equaling 3.57955 MHz. Within the chrominance band extending from roughly 2.3 MHz to 4.2 MHz, the luminance spectrum overlaps that chrominance spectrum. In other words, signal interference occurs.
It is well-known that a television decoder is implemented to extract both luminance information and chrominance information from the received composite signal; however, a typical simple television decoder cannot discern which of the higher frequency components are luminance information and which are chrominance information. As a result, such a television decoder generates incorrect chrominance information owing to the interference introduced via the luminance spectrum. The term “cross color” is commonly referred to corruption of the chrominance spectrum caused by the misinterpretation of high-frequency luminance information as wanted chrominance information. Conversely, the term “cross luminance ” is commonly referred to corruption of the luminance spectrum caused by the misinterpretation of chrominance information as high-frequency luminance information.
Typical conventional methods reduce cross color by operating upon chrominance information encoded on the chrominance subcarrier prior to demodulation into baseband chrominance information. These methods typically incorporate cross color suppression into the decoding process, focusing on improving the separation of the chrominance and luminance information to reduce both cross color and cross luminance.
However, cross color suppression is very desirable in applications where only demodulated baseband chrominance information is available, especially where demodulation was performed without much regard for suppressing cross color. In such applications, for practical reasons, cross color suppression must be performed in the baseband domain.
As such, Faroudja provides a technique for suppressing cross color in U.S. Pat. No. 5,305,120, the contents of which are hereby incorporated by reference. Although Faroudja suggests a feasible approach for post-decoding cross color suppression, a more optimized motion detection algorithm can further be proposed in order to minimize possible error occurrence in the outcome of cross color suppression caused by over-simplified stationary image judgment.
SUMMARY OF INVENTION
It is therefore one of the objectives of the claimed invention to provide a method of suppressing cross color and/or cross luminance of an image by introducing a well-designed motion detection algorithm.
According to embodiments of the present invention, a method for processing an image in a video data is disclosed. The video data comprises a plurality of frames. The method comprises obtaining a first difference set between pixel information of one of the frames and pixel information of another one of the frames, wherein the two frames involving the first difference set are one frame apart; examining a first criterion with the first difference set; and performing cross color suppressing operation on pixel information of a current frame of the plurality of frames according to a set of stationary image judgment information comprising the result of the first criterion examination.
According to embodiments of the present invention, the method further comprises obtaining a second difference set between pixel information of one of the frames and pixel information of another one of the frames, wherein the two frames involving the second difference set are adjacent to each other; and examining a second criterion with the second difference set; wherein the set of stationary image judgment information further comprising the result of the second criterion examination.
According to embodiments of the present invention, the method further comprises obtaining a third difference set among pixel information of three successive frames of the plurality of frames; and examining a third criterion with the third difference set; wherein the set of stationary image judgment information further comprising the result of the third criterion examination.
These and those objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the embodiments that are illustrated in the various figures and drawings.



BRIEF DESCRIPTION OF DRAWINGS
FIG. 1 is a block diagram of a cross color and cross luminance suppression apparatus according to an embodiment of the present invention.
FIG. 2 is a timing diagram conceptually illustrating a plurality of sequentially incoming image frames of the apparatus in FIG. 1.
FIG. 3 is a diagram illustrating the pixel allocation of a portion of the image frames in FIG. 2.
FIG. 4 is a timing diagram conceptually illustrating a plurality of sequentially incoming image fields of the apparatus in FIG. 1, embodied by the portion shown in FIG. 3 of a plurality of image frames in FIG. 2.
FIG. 5 is a flowchart illustrating the method of cross color and/or cross luminance suppression according to an embodiment of the present invention.



DETAILED DESCRIPTION
Please refer to FIG. 1, which is a block diagram illustrating a cross color and cross luminance suppression apparatus 100 according to an embodiment of the present invention. In this embodiment, the apparatus 100 comprises a motion detector 102 and a processor 104. The motion detector 102 successively receives a plurality of pixel information, which may include, in one embodiment, luminance information (Y) and chrominance information (U and V, or Cr and Cb), representative of a series of pixel data in image frames. The motion detector 102 then generates a motion control signal according to the received pixel information. The motion control signal indicates whether a current pixel is deemed stationary (i.e., still), or deemed to be with motion. The processor 104 also receives the pixel information, here, the luminance information and the chrominance information, and performs cross color suppressing operation on the chrominance information, as well as cross luminance suppressing operation on the luminance information. By doing so, the pixel information outputted by the processor 104 is free of cross color and cross luminance influence.
Before further explain the operation of the present invention apparatus 100, certain preliminary knowledge pertaining image frame composition should be understood. Here, the well-known NTSC systems are hereby taken as an example for explanatory purpose. Please refer to FIG. 2, which is a timing diagram conceptually illustrating a plurality of sequentially incoming image frames of the apparatus 100. As is well-known in the art, in NTSC systems pixel data are sequentially transmitted and processed in a separate even field and odd field fashion, as illustrated in FIG. 4, while pixel data in the even field and in the odd field are interlaced to constitute a full image frame, as illustrated in FIG. 3. In FIG. 2, FIG. 3, and FIG. 4, four image frames 3, 2, 1, 0 are sequentially arranged at time T-3, T-2, T-1, and T in a timely fashion. It is to be noted that in FIG. 3 and FIG. 4 only a portion of an image frame (9 pixels in each field, or 18 pixels in each frame) is illustrated for simplicity, wherein the notations A, B, C, D, E, F, G, H, and I are representative of pixel data at respective location of a field, the substripts “e” and “o” correspond to the even field and the odd field, respectively, and the superstripts correspond to frame numbers.
As is well known to one of ordinary skill in the art, in NTSC systems, which are adopted as an example in the following description of the embodiment of the invention, the chrominance subcarrier phase rotates by 180 degrees between successive frames. This rotation causes luminance information to be misinterpreted as chrominance information, which oscillates between two complementary colors such as red and green; that is, the luminance appears to be spectral energy which oscillates between two colors represented by chrominance information 180 degrees out of phase with each other. Similar 180-degree phase rotation between successive frames can also be observed when examining the cross luminance phenomenon, i.e., the corruption of the luminance spectrum by the chrominance information.
Therefore, by averaging the chrominance information in two successive frames the out-of-phase cross color information cancels thereby allowing chrominance information to be obtained which is free of cross color. Likewise, the cross luminance information can also be cancelled by similar averaging operation. However, this technique works most effective only when the image is stationary, or still. As a result, a well-designed motion detection algorithm (or a stationary judgment algorithm) may serve to enhance the cross color suppression and/or cross luminance suppression effect, as well as the resultant display quality of the outcome of the processing, since improper cross color and/or cross luminance suppressing operation based on a poor motion detection algorithm degrades the display quality drastically.
As such, in an embodiment of the present invention, a motion detection algorithm adopted by the motion detector 102 is to be provided as in the following descriptions. Take the sequentially incoming image fields shown in FIG. 4 as an example, for each of the incoming pixel data (for example, the pixel Eo as a current pixel), the motion detector 102 checks for a number of conditions. As a first condition, the motion detector 102 checks for the similarity between the current frame 0 at time T and the previous frame 2 at time T-2, which is two frames prior to the current frame 0, for they are of the same phase in cross color/cross luminance phenomena. This condition may be implemented by observing the values of the following functions:

dY1=|YE0−Y′′E0|<Thl—Y1  (1)

dU1=|UE0−U′′E0|<Thl—U1  (2)

dV1=|VE0−V′′E0|<Thl—V1  (3)

wherein Y, U, and V represent the one luminance information and two chrominance information of the corresponding pixel data, respectively, and Thl_Y, Thl_U, and Thl_V are threshold values, whose amounts should be determined according to actual applications. In this embodiment, only when the values of the above three functions (1), (2), and (3) are all true, is the first condition asserted to be true.
Please note that, although in this embodiment differences in pixel data between two frames are adopted to indicate the degree of similarity between two frames, other known way of indicating similarity may also be utilized. Please also note that, although in this embodiment only the pixel information of the current pixel (i.e., Eo) is adopted for similarity determination, more pixels may be incorporated into such determination. For example, the function of (1) may also be substituted by the following function:




  
    
      
        
          dY
          1
        
        =
        
          
            ∑
            
              X
              =
              
                Ao
                ∼
                Io
              
            
          
          ⁢
          
            
          
          ⁢
          
            |
            
              
                Y
                X
              
              -
              
                Y
                X
                ′′
              
            
            |
            
              <
              
                Thl_Y
                1
              
            
          
        
      
    
    
      
        
          (
          1
          )
        
        ′
      
    
  




That is, besides of the current pixel Eo, the surrounding eight pixels in the same field are also incorporated into the similarity determination. Of course, the number and position of pixels incorporated may be altered, and similar substitutions may also be asserted to functions (2) and/or (3).
Moreover, in determining whether the image is stationary or not for the current pixel, here, Eo, it may also be instructive to check for the similarity between a corresponding pixel (Ee) in the complementary field, here, the even field, of the same frame, and a pixel (Ee′′′′) two frames prior thereto. This is because pixels in the complementary field of the same frame contributes half of the frame, and therefore should be indicative in determining if an image being stationary or not. Likewise, the above-mentioned adoption of Y, U, and/or V pixel information in determining similarity, and the adoption of multiple pixels around the current pixel, may also be applied to such checking for similarity between the complememtary field and its predecessor two frames ahead.
When deriving a result for the first condition, it may also be meaningful to optionally check for the similarity between the preceding frame 1 at time T-1 of the current frame 0, and the frame 3 at time T-3, which is two frames prior to the frame 1. As an example, the following three functions can be utilized:

dY2=|Y′E0−Y′′′E0|<Thl—Y2   (4)

dU2=|U′E0−U′′′E0|<Thl—U2   (5)

dV2=|V′E0−V′′′E0|<Thl—V2   (6)

This is because in determining whether an image at time T is stationary or not, it might also be indicative to check if the image one frame ahead (i.e., at time T-1) is also stationary, for stillness of an image is construed in a consecutive context. In an embodiment where these functions are incorporated in determining the outcome of the first condition, only when the values of the functions (1), (2), (3), (4), (5), and (6) are all true, is the first condition asserted to be true.
Of course as can be appreciated by those of ordinary skill in the art, the above-mentioned adoption of multiple pixels around the current pixel, and the adoption of pixel information of the complementary fields may also be applied to such checking for stillness between the frame 1 and its predecessor frame 3, which is two frames ahead.
In addition to the first condition, a second condition, wherein the similarity between a frame that is one frame ahead of the current frame (i.e., the frame 1 at time T-1), and an adjacent frame thereof (for example, the frame 2 at time T-2), is further considered in determining the stillness of the image for the current pixel. The checking for similarity between two adjacent frames, though may not as significant due to the 180 degree out-of-phase characteristic in cross color of the NTSC systems, can still be of meaning in determining stillness, considering the consecutive nature of the stillness in image. In this embodiment, the second condition may be implemented by observing the values of the following functions:

dY3=|Y′E0−Y′′E0|<Thl—Y3   (7)

dU3=|U′E0−U′′E0|<Thl—U3   (8)

dV3=|V′E0−V′′E0|<Thl—V3   (9)

Similarly in this embodiment, only when the values of the above three functions (7), (8), and (9) are all true, is the second condition asserted to be true.
On top of the three functions (7), (8), and (9), further observations can be incorporated in determining the outcome of the second condition, such as the similarity between the frame which is two frames ahead of the current frame (i.e., the frame 2 at time T-2) and an adjacent frame thereof (for example, the frame 3 at time T-3). The following functions may serve as one such example:

dY4=|Y′′E0−Y′′′E0|<Thl—Y4   (10)

dU4=|U′′E0−U′′′E0|<Thl—U4   (11)

dV4=|V′′E0−V′′′E0|<Thl—V4   (12)

And in such an embodiment, only when the values of the functions (7), (8), (9), (10), (11), and (12) are all true, is the second condition asserted to be true.
Please note that, when the required pixel information is available, the above-mentioned functions (7), (8), (9) may also be adapted to check the similarity between the frame 1 and the other adjacent frame thereof, which is the current frame 0, and the above-mentioned function (10), (11), (12) may also be adapted to check the similarity between the frame 2 and the other adjacent frame thereof, which is the frame 1. Also as can be appreciated by those of ordinary skill in the art, the above-mentioned adoption of multiple pixels around the current pixel, and the adoption of pixel information of the complementary fields may also be applied to such checking for similarity between two adjacent frames.
Besides of the first and the second conditions, a third condition, which is termed as the “high-frequency stillness” condition, is further examined in determining the stillness of the image for the current pixel. The third condition checks for the consecutive stationary situation of frames 0, 1, and 2 respectively at time T, T-1, and T-2 by utilizing, as an example, the following operations. First, the following operators are so defined:

dNext—Y=Y′E0−YE0

dPre—Y=Y′E0−Y′′E0

dNext—U=U′E0−UE0

dPre—U=U′E0−U′′E0

dNext—V=V′E0−VE0

dPre—V=V′E0−V′′E0

Then, the following condition pertaining the operator dNext_Y is checked to find out the value of an additional operator Next_Y:















if(dNext_Y > Thl_Y5)



Next_Y = 1(True)



elseif(dNext_Y < −Thl_Y5)



Next_Y = −1(True)



else



Next_Y = 0(False)










Similar conditions respectively pertaining the operators dPre_Y, dNext_U, dPre_U, dNext_V, and dPre_V are also checked to find out corresponding operators Pre_Y, Next_U, Pre_U, Next_V, and Pre_V. And lastly, the following condition pertaining the operators Next_Y and Pre_Y is further checked to find out the value of yet another operator NextPre_Y:















if(Next_Y > 0&&Pre_Y > 0)



NextPre_Y = 1(True)



elseif(Next_Y < 0&&Pre_Y < 0)



NextPre_Y = 1(True)



else



NextPre_Y = 0(False)










Similar conditions respectively pertaining the operators Next_U and Pre_U, Next_V and Pre_V, are also checked to find out corresponding operators NextPre_U, and NextPre_V. Here if the value of the operator NextPre_Y is true, high-frequency alternation in Y domain is deemed existing, and high-frequency alternation phenomenon is not desirable for an image regarded as stationary. Therefore, in this embodiment, only when the values of the operators NextPre_Y, NextPre_U, and NextPre_V are all false, is the third condition asserted to be true. Please note that, for a person of ordinary skill in the art, it is understood that such a high-frequency stillness condition may also be expanded to incorporate laterally adjacent pixels (Do and/or Fo) and/or vertically adjacent pixels (Bo and/or Ho) of the current pixel Eo.
After all these operations, the motion detector 102 determines whether the image is stationary or not for the current pixel. In this embodiment, the image is deemed stationary for the current pixel only when the first, the second,  and the third conditions are all asserted true.
Please note that the hardware requirement, particularly the memory requirement, of the motion detector 102 to perform the above-mentioned condition check varies according to the complexity of conditions actually adopted, from 8 field buffers to 4 field buffers, and is obvious to those of ordinary skill in the art. Also note that here, the estimated field buffer requirement needs not include currently incoming image field, taking the advantage of a pixel-by-pixel operation.
After the motion detector 102 decides on whether the image is stationary or not for the current pixel, the motion control signal is then passed to the processor 104 to inform the processor 104 of the determination of the motion detector 102. If the image is deemed stationary for the current pixel, the cross color suppression and/or the cross luminance suppression operation is launched by, in this embodiment, averaging the pixel information across two consecutive image frames (for example, (YEo+YE0)/2, (UEo+UEo)/2, and (VEo+VEo)/2), or other suppression methods known to a skilled artisan. If the image is deemed not stationary (i.e., with motion), in this embodiment the current pixel is output as received.
Please refer to FIG. 5, which is a flowchart illustrating the aforementioned condition checks and cross color and/or cross luminance suppression operation according to an embodiment of the present invention. A person of ordinary skill in the art should be able to understand that the order of performing the checking steps 502, 504, and 506 in FIG. 5 serves only as an example, is not meant to be limiting and prone to be changed. Also note that although when any of the first, the second, and the third condition is not met, pixel data are outputted as received in step 510 in this embodiment, further processing on the pixel data outputted may also be done in other embodiments.
Although the detailed description of the embodiments of the invention has been focused on the application in NTSC systems, the present invention may also be adapted to other display systems, such as the PAL systems. One point worth noting is that for PAL systems, the chrominance subcarrier phase rotates by 90 degrees between successive frames, as is the case for the luminance subcarrier. Therefore, the misinterpretation of luminance information as chrominance information rotates in phase by 90 degrees for each incoming frame. Bear this in mind, a skilled artisan should be able to substitute the claimed invention into a PAL system, and gain from similar improved display quality.
Those skilled in the art will readily observe that numerous modifications and alterations of the device may be made while retaining the teachings of the invention. Accordingly, the above disclosure should be construed as limited only by the metes and bounds of the appended claims.

## Claims

1. A method for processing an image in a video data, the video data comprising a plurality of frames, the method comprising:
obtaining a first difference set between pixel information of one of the frames and pixel information of another one of the frames, wherein the two frames involving the first difference set are one frame apart;
examining a first criterion with the first difference set;
performing cross color suppressing operation on pixel information of a current frame of the plurality of frames according to a set of stationary image judgment information comprising the result of the first criterion examination;
obtaining a third difference set among pixel information of three successive frames of the plurality of frames; and
examining a third criterion with the third difference set;
wherein the set of stationary image judgment information further comprising the result of the third criterion examination.

2. The method of claim 1, wherein the two frames involving the first difference set comprises the current frame, and a frame, which is two frames prior to the current frame.

3. The method of claim 1, wherein the two frames involving the first difference set comprises a frame, which is one frame prior to the current frame, and a frame, which is three frames prior to the current frame.

4. The method of claim 1, wherein the first difference set comprises a difference between pixel information of a target pixel in one of the two frames involving the first difference set and pixel information of a corresponding pixel in the other one of the two frames involving the first difference set.

5. The method of claim 4, wherein the first difference set further comprises a plurality of differences, each between pixel information of one of a plurality of pixels in the vicinity of the target pixel and pixel information of a corresponding pixel in the other one of the two frames involving the first difference set, respectively.

6. The method of claim 1, wherein the first criterion comprises a comparison between a difference in the first difference set and a first threshold value.

7. The method of claim 1, further comprising:
obtaining a second difference set between pixel information of one of the frames and pixel information of another one of the frames, wherein the two frames involving the second difference set are adjacent to each other; and
examining a second criterion with the second difference set;
wherein the set of stationary image judgment information further comprising the result of the second criterion examination.

8. The method of claim 7, wherein the two frames involving the second difference set comprises the current frame, and a frame, which is one frame prior to the current frame.

9. The method of claim 7, wherein the two frames involving the second difference set comprises a frame, which is one frame prior to the current frame, and a frame, which is two frames prior to the current frame.

10. The method of claim 7, wherein the two frames involving the second difference set comprises a frame, which is two frame prior to the current frame, and a frame, which is three frames prior to the current frame.

11. The method of claim 7, wherein the second difference set comprises a difference between pixel information of a target pixel in one of the two frames involving the second difference set and pixel information of a corresponding pixel in the other one of the two frames involving the second difference set.

12. The method of claim 11, wherein the second difference set further comprises a plurality of differences, each between pixel information of one of a plurality of pixels in the vicinity of the target pixel and pixel information of a corresponding pixel in the other one of the two frames involving the second difference set, respectively.

13. The method of claim 7, wherein the second criterion comprises a comparison between a difference in the second difference set and a second threshold value.

14. The method of claim 1, wherein the three successive frames involving the third difference set comprises the current frame, a frame, which is one frame prior to the current frame, and a frame, which is two frames prior to the current frame.

15. The method of claim 1, wherein the cross color suppression operation comprises averaging pixel information of the current frame and pixel information of a previous frame adjacent to the current frame.

16. The method of claim 1, further comprising:
performing cross luminance suppressing operation on pixel information of the current frame according to the set of stationary image judgment information.

17. The method of claim 16, wherein the cross luminance suppression operation comprises averaging pixel information of the current frame and pixel information of a previous frame adjacent to the current frame.

18. The method of claim 1, wherein the pixel information comprises luminance information.

19. The method of claim 1, wherein the pixel information comprises chrominance information.