Method and device for coding video levels in a plasma display panel
View Patent ↗The invention relates to a method and a device for coding video levels of pixels of a color component of a picture to be displayed on a display device into code words. The invention is more particularly applicable to Plasma Display Panels (PDP) to improve the picture quality (grayscale enhancement, dynamic false contour reduction). The main idea of the invention is to automatically change the floor and ceiling levels used classically for dithering each pixel value depending on the neighboring pixels of the same color component. The size of the window determining the neighboring pixels is dependent on the definition of the wanted flat field size.
1. A method for coding the video level of a current pixel of a colour component of a picture to be displayed on a display device into a code word belonging to at least a first code set or a second code set or both, the second code set being a subset of the first code set, the code words of the second code set being selected among the code words of the first code set according to the rule that the temporal centre of gravity for the light generation of the corresponding sub-field code words grows continuously with the video level of said code words by selecting certain levels, with an exception in which the temporal centre of gravity does not grow continuously in the low video level range up to a first predetermined number of low level code words or in the high video level range from a second predetermined number of high level code words or in both,
comprising:
determining, by an adaptive coder, for a predetermined number of neighbouring pixels belonging to a window including said current pixel, at least a first floor level and a first ceiling level that can be coded by a code word of the first code set and a second floor level and a second ceiling level that can be coded by a code word of the second code set, the first and second floor levels being respectively the highest video level among the video levels corresponding to the code words of the first and second code sets equal to or lower than the video level of the current pixel and the first and second ceiling levels being respectively the lowest video level among the video levels corresponding to the code words of the first and second code sets equal to or higher than the video level of the current pixel,
determining, for said current pixel, the minimum level between the first floor level of said current pixel and the ceiling levels of said predetermined number of neighbouring pixels of the window including the current pixel and determining for said current pixel the maximum level between the first ceiling level of said current pixel and the floor levels of said predetermined number of neighbouring pixels, said minimum level being called floor level of the current pixel and said maximum level being called ceiling level of the current pixel,
substituting, for said current pixel, the floor level by the second floor level if the said floor level is lower than the second floor level and the ceiling level by the second ceiling level if said ceiling level is higher than the second ceiling level, and
selecting, for said current pixel, the code word among the code word of the floor level and the code word of the ceiling level according to dithering for coding the video level of the current pixel.
2. The method according to claim 1 , further comprising
encoding, for said current pixel, the selected code word into a subfield code word, wherein to each bit of the subfield code word a certain duration is assigned, hereinafter called sub-field, during which a pixel can be activated for light generation.
3. The method according to claim 1 , wherein the selecting the code word of the current pixel among the code word of the floor level and the code word of the ceiling level according to dithering comprises the following:
computing a coefficient α such that α=[l(x)−f(x)]/[c(x)−f(x)] where
l(x) is the video level of the current pixel x,
C(x p ) is the ceiling level of the current pixel,
F(x p ) is the floor level of the current pixel, and
α is a weight factor resembling the participation of C(x p ) in l(x),
generating a random number N rnd such that 0≦N rnd ≦1,
selecting the ceiling level if the coefficient α is greater than or equal to N rnd and selecting the floor level if the coefficient α is lower than the random number N rnd .
4. A device for coding the video level of a current pixel of a colour component of a picture to be displayed on a display device into a code word belonging to at least a first code set or a second code set or both, the second code set being a subset of the first code set, the code words of the second code set being selected among the code words of the first code set according to the rule that the temporal centre of gravity for the light generation of the corresponding sub-field code words grows continuously by selecting certain levels, with the video level of said code words with an exception in which the temporal centre of gravity does not grow continuously in the low video level range up to a first predetermined number of low level code words or in the high video level range from a second predetermined number of high level code words or in both,
comprising:
means for determining, for a predetermined number of neighbouring pixels belonging to a window including said current pixel, at least a first floor level and a first ceiling level coded by a code word of the first code set and a second floor level and a second ceiling level coded by a code word of the second code set, the first and second floor levels being respectively the highest video level among the video levels corresponding to the code words of the first and second code sets equal to or lower than the video level of the current pixel and the first and second ceiling levels being respectively the lowest video level among the video levels corresponding to the code words of the first and second code sets equal to or higher than the video level of the current pixel,
means for determining, for said current pixel, the minimum level between the first floor level of said current pixel and the ceiling levels of said predetermined number of neighbouring pixels of the window including the current pixel and determining, for said current pixel, the maximum level between the first ceiling level of said current pixel and the floor levels of said predetermined number of neighbouring pixels, said minimum level being called floor level of the current pixel and said maximum level being called ceiling level of the current pixel,
means for substituting, for said current pixel, the floor level by the second floor level if the current floor level is lower than the second floor level and the ceiling level by the second ceiling level if said ceiling level is higher than the second ceiling level, and
dithering means for selecting, for said current pixel, the code word among the code word of the floor level and the code word of the ceiling level according to dithering for coding the video level of the current pixel.
5. The device according to claim 4 , further comprising subfield encoding means for encoding, for said current pixel, the selected code word into a subfield code word, wherein to each bit of the subfield code word a certain duration is assigned, hereinafter called sub-field, during which a pixel can be activated for light generation.
6. The device according to claim 4 , wherein the dithering means comprises:
means for computing a coefficient α such that α=[l(x)−f(x)]/[c(x)−f(x)] where
l(x) is the video level of the current pixel x,
c(x) is the ceiling level of the current pixel,
f(x) is the floor level of the current pixel, and
α is a weight factor resembling the participation of C(x p ) in l(x),
means for generating a random number N rnd such that 0≦N rnd ≦1,
means for selecting the ceiling level if the coefficient α is greater than or equal to N rnd and selecting the floor level if the coefficient α is lower than the random number N rnd .