Abstract:
The invention relates to a method and apparatus for processing video pictures, in particular for large area flicker effect and false contour effect reduction. This method concerns a new coding called Parallel Peak Coding. The general idea of the Parallel Peak Code is to have almost always the same energy in two packets of light and to encode the code words for these two packets differently so that changes in sub-field code word will not appear in the two packet code words simultaneously.

Description:
FIELD OF THE INVENTION  
       [0001]     The invention relates to a method and apparatus for processing video pictures, in particular for large area flicker effect reduction and false effect contours.  
       BACKGROUND OF THE INVENTION  
       [0002]     More specifically the invention is closely related to a kind of video processing for improving the picture quality of pictures which are displayed on matrix displays like plasma display panels (PDP), display devices with digital micro mirror arrays (DMD) and all kind of displays based on the principle of duty cycle modulation (pulse width modulation) of light emission.  
         [0003]     Although plasma display panels are known for many years, plasma displays are encountering a growing interest from TV manufacturers. Indeed, this technology now makes it possible to achieve flat colour panels of large size and with limited depths without any viewing angle constraints. The size of the displays may be much larger than the classical CRT picture tubes would have ever been allowed.  
         [0004]     Referring to the latest generation of European TV sets, a lot of work has been made to improve its picture quality. Consequently, there is a strong demand, that a TV set built in a new technology like the plasma display technology has to provide a picture so good or better than the old standard TV technology.  
         [0005]     A plasma display panel utilises a matrix array of discharge cells, which could only be switched ON or OFF. Also unlike a CRT or LCD in which grey levels are expressed by analogue control of the light emission, in a PDP the grey level is controlled by modulating the number of light pulses per frame. The eye will integrate this time-modulation over a period corresponding to the eye time response.  
         [0006]     For static pictures, this time-modulation, repeats itself, with a base frequency equal to the frame frequency of the displayed video norm. As known from the CRT-technology, a light emission with base frequency of 50 Hz, introduces large area flicker, which can be eliminated by field repetition in 100 Hz CRT TV receivers.  
         [0007]     Contrary to the CRTs, where the duty cycle of light emission is very short, the duty cycle of light emission in PDPs is ˜50% for middle grey. This reduces the amplitude of the 50 Hz frequency component in the spectrum, and thus large area flicker artefact, but due to the larger size of PDPs, with a larger viewing angle, even a reduced large area flicker becomes objectionable in terms of picture quality. The present trend of increasing size and brightness of PDPs, will also contribute to aggravate this problem in the future.  
         [0008]     A solution is known from the patent application EP 0 982 708 is to use a dual-peak code using two groups of identical subfields. But the above solution still suffer from much more false contour than a single peak code for the same number of sub-fields.  
       SUMMARY OF THE INVENTION  
       [0009]     It is an object of the present invention to disclose a method and an apparatus, which reduce the large area flicker artefact in PDPs in particular for 50 Hz video norms, without introducing false contour effects.  
         [0010]     This object is achieved by a method for processing video pictures consisting of pixels, the pixels being digitally coded, the digital code word determining the length of the time period during which the corresponding pixel of a display is activated, wherein to each bit of a digital code word a certain activation duration is assigned, hereinafter called sub-field, the sum of the duration of the sub-fields according to a given code word determining the length of the time period during which the corresponding pixel is activated, the sub-fields of a pixel being organised in two consecutive groups such that to a value of a pixel a code word is assigned which distributes the active sub-field periods equally over the two sub-field groups. According to the invention, substantially all the sub-fields of the two groups have different activation durations and, for all pixel values apart from exceptions in the low pixel value range up to a first predetermined limit and/or in the high pixel value range from a second limit on, the pixel value is split into first and second substantially equal values, said first and second values being encoded into first and second code words, said first code word being the part of the code word assigned to one of the two sub-field groups and said second code word being the part of the code word assigned to the other sub-field group.  
         [0011]     A dithering step can be possibly applied to said first and second values before being encoded into first and second code words.  
         [0012]     Furthermore, for a video pictures sequence with an increased frequency, for example 100 Hz, the pixel values of the even pictures of the sequence will be encoded with the sub-fields of one of the sub-field groups and the pixel values of the odd pictures of the sequence with the sub-fields of the other sub-field group.  
         [0013]     The invention concerns also an apparatus for processing video pictures consisting of pixels, the pixels being digitally coded, the digital code word determining the length of the time period during which the corresponding pixel of a display is activated, wherein to each bit of a digital code word a certain activation duration is assigned, hereinafter called sub-field, the sum of the duration of the sub-fields according to a given code word determining the length of the time period during which the corresponding pixel is activated, the sub-fields of a pixel being organised in two consecutive groups such that to a value of a pixel a code word is assigned which distributes the active sub-field periods equally over the two sub-field groups. According to the invention, substantially all the sub-fields of the two groups have different activation durations and encoding means are provided for splitting, for all pixel values apart from exceptions in the low pixel value range up to a first predetermined limit and/or the in the high pixel value range from a second limit on, each of said pixel values into first and second substantially equal values and encoding said first and second values into first and second code words, said first code word being the part of the code word assigned to one of the two sub-field groups and said second code word being the part of the code word assigned to the other sub-field group.  
         [0014]     The apparatus can further comprise dithering means for processing said first and second values. For a video pictures sequence with an increased frequency, the encoding means encode the pixel values of the even pictures of the sequence with the sub-fields of one of the two sub-field groups and the pixel values of the odd pictures of the sequence with the sub-fields of the other sub-field group. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]     Exemplary embodiments of the invention are illustrated in the drawings and in more detail in the following description.  
         [0016]     In the figures:  
         [0017]      FIG. 1  shows a first circuit implementation for encoding the image according to the inventive method; and  
         [0018]      FIG. 2  shows a second circuit implementation for encoding the image according to the inventive method. 
     
    
     DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0019]     The general idea of the Parallel Peak Code is to have almost always the same energy in two packets of light and to encode the code words for these two packets differently so that changes in sub-field code word will not appear in the two packet code words simultaneously. This aims at reducing the false contour effect.  
         [0020]     The invention can be used with any number of sub-fields. It will be illustrated by a Parallel Peak Coding with 15 sub-fields. Considering a frame comprising 15 sub-fields with the following weights:  
         [0021]     1-2-3-5-7-9-11-14-17-20-24-28-33-38-43  
         [0022]     In a first step, these sub-fields are organized in two consecutive groups. A part of a sub-field code word is assigned to each group. These two groups of sub-fields are used for generating the two packets of light.  
         [0023]     For example, the odd sub-fields are grouped in a first group, called G 1 , and the even sub-fields are grouped in a second group called G 2 .  
         [0024]     G 1 : 1-3-7-11-17-24-33-43  
         [0025]     G 2 : 2-5-9-14-20-28-38  
         [0026]     Of course, the distribution of the sub-fields between the two groups can be carried out differently. The only condition is that the two groups should comprise sub-fields of different weights. Furthermore, the sub-fields of the group G 1  could be put before or after the sub-fields of the group G 2 .  
         [0027]     In a second step, a different coding is selected for each group of sub-fields. The level of false contour and the dithering noise obtained with the Parallel Peak Code are then depending directly on the level of false contour and dithering noise for each of the two packets of light. For example, the following encoding tables can be used:  
         [0028]     for the group G 1  (1-3-7-11-17-24-33-43)  
                                                       level 0:   00000000           level 1:   10000000           level 3:   01000000           level 4:   11000000           level 7:   00100000           level 8:   10100000           level 10:   01100000           level 11:   11100000           level 14:   01010000           level 15:   11010000           level 18:   00110000           level 19:   10110000           level 21:   01110000           level 22:   11110000           level 24:   00101000           level 25:   10101000           level 27:   01101000           level 28:   11101000           level 29:   10011000           level 31:   01011000           level 32:   11011000           level 35:   00111000           level 36:   10111000           level 38:   01111000           level 39:   11111000           level 42:   00110100           level 43:   10110100           level 45:   01110100           level 46:   11110100           level 48:   00101100           level 49:   10101100           level 51:   01101100           level 52:   11101100           level 53:   10011100           level 55:   01011100           level 56:   11011100           level 59:   00111100           level 60:   10111100           level 62:   01111100           level 63:   11111100           level 65:   11011010           level 68:   00111010           level 69:   10111010           level 71:   01111010           level 72:   11111010           level 75:   00110110           level 76:   10110110           level 78:   01110110           level 79:   11110110           level 81:   00101110           level 82:   10101110           level 84:   01101110           level 85:   11101110           level 86:   10011110           level 88:   01011110           level 89:   11011110           level 92:   00111110           level 93:   10111110           level 95:   01111110           level 96:   11111110           level 98:   01011101           level 99:   11011101           level 102:   00111101           level 103:   10111101           level 105:   01111101           level 106:   11111101           level 107:   01011011           level 108:   11011011           level 111:   00111011           level 112:   10111011           level 114:   01111011           level 115:   11111011           level 118:   00110111           level 119:   10110111           level 121:   01110111           level 122:   11110111           level 124:   00101111           level 125:   10101111           level 127:   01101111           level 128:   11101111           level 129:   10011111           level 131:   01011111           level 132:   11011111           level 135:   00111111           level 136:   10111111           level 138:   01111111           level 139:   11111111                      
 
         [0029]     for the group G 2  (2-5-9-14-20-28-38)  
                                                       level 0:   0000000           level 2:   1000000           level 5:   0100000           level 7:   1100000           level 9:   0010000           level 11:   1010000           level 14:   0110000           level 16:   1110000           level 19:   0101000           level 21:   1101000           level 23:   0011000           level 25:   1011000           level 28:   0111000           level 30:   1111000           level 34:   0110100           level 36:   1110100           level 39:   0101100           level 41:   1101100           level 43:   0011100           level 45:   1011100           level 48:   0111100           level 50:   1111100           level 53:   1011010           level 56:   0111010           level 58:   1111010           level 59:   1010110           level 62:   0110110           level 64:   1110110           level 67:   0101110           level 69:   1101110           level 71:   0011110           level 73:   1011110           level 76:   0111110           level 78:   1111110           level 81:   0011101           level 83:   1011101           level 86:   0111101           level 88:   1111101           level 89:   0011011           level 91:   1011011           level 94:   0111011           level 96:   1111011           level 97:   1010111           level 100:   0110111           level 102:   1110111           level 105:   0101111           level 107:   1101111           level 109:   0011111           level 111:   1011111           level 114:   0111111           level 116:   1111111                      
 
         [0030]     All the video levels can not be achieved. So, the missing video levels are expressed from the available levels by a classical dithering step. Two independent dithering blocks will be needed for this purpose.  
         [0031]     Then, according to the invention, the same light energy should be emitted during the two packets of light. It is not always possible, for example for the video levels greater than 232 in the present example. Furthermore, for the lowest video levels, the sub-fields of only one sub-field group are preferably used in order to reduce the dithering noise. The low video levels will have only one packet of light but it is not so important since these levels do not generate any flicker.  
         [0032]     For example, if i designates an input video level, a the value of the part of code word assigned to the first packet of light, and b the value of the part of code word assigned to the second packet of light, the values a and b can be determined as follows:  
         [0033]     For 0≦i≦1, a=i and b=0.  
         [0034]     For 1≦i≦2, a=1 and b=i−1.  
         [0035]     For 2≦i≦232, a=i/2 and b=i/2. (232=2×116)  
         [0036]     For 232≦i≦255, a=i−116 and b=116.  
         [0037]     In  FIG. 1 , a block diagram of a possible circuit implementation for encoding the video levels into sub-field code word as described above is illustrated. Input R,G,B video data, IN[ 9 : 0 ], coming for example from a video degamma unit, are forwarded to splitting means  10  used for outputting, for each input video data, the values a and b. These means comprise for example two Look-Up Tables (LUTs), one for delivering the value a and one for delivering the value b. The value a (respectively b) is then advantageously transmitted to a dithering block  11  (resp.  21 ) for generating, if need be, values encodable by the subfield group G 1  (resp. G 2 ). The dithered value is then forwarded to a subfield coding block  12  (resp.  22 ) for outputting the corresponding subfield code word. This sub-field code word will be used by the display panel for driving the lighting period of the cells of the panel.  
         [0038]     It is also possible to use the Parallel Peak Code with a frame frequency twice as high. For example, instead of having a 50 Hz video input, it is also possible to have a 100 Hz video input and to use, depending on the parity of the frame (odd or even), the first or the second group of sub-fields (G 1  or G 2 ) and the corresponding encoding table. Of course, it is not limited to 100 Hz; it can also be used for other frequencies like 72, 75, 80, 85, 90 or even 120 Hz.  FIG. 2  is illustrated this possibility. Count is a 1-bit counter, which is incremented at each frame. Depending on its value (0 or 1), the video is encoded with sub-field group G 1  and the encoding table assigned to this first group (case 0) or with sub-field group G 2  and the encoding table assigned to this second group (case 1).