Plasma display apparatuses using plasma display panels as self-emission image displays have the advantage that thinning and larger screens are possible. Such a plasma display apparatus displays images by utilizing light emissions at the time of discharges of discharge cells forming pixels. Since a plasma display panel emits light in binary form, a sub-field method is used for the plasma display panel by which a halftone is displayed by temporally superimposing a plurality of binary images that are each weighted.
In the above-mentioned sub-field method, a single field is temporally divided into a plurality of sub-fields that are each weighted. The weight of each sub-field corresponds to the amount of emission for each sub-field. For example, the number of emissions is used as the weight, and the sum of the weights of all the sub-fields corresponds to the brightness, or the grayscale level of a video signal.
This sub-field method has a fixed order of emissions for the plurality of sub-fields. Therefore, when a viewer of emitting sub-fields moves his or her eye on a plurality of pixels, the viewer will see a different sub-field for each pixel. This causes the viewer to see a grayscale level that is considerably different from the grayscale level that should have been represented. In particular, if adjacent pixels are consecutive, the viewer visually perceives a striped false contour which seems as if the grayscale level had been lost. Such a false contour is known to degrade display quality very much. This false contour appearing only with moving images is referred to as a “false contour noise” (Institute of Television Engineers of Japan Technical Report. “False Contour Noise Observed in Display of Pulse Width Modulated Moving Images”, Vol. 19, No. 2, IDY 95-21, pp. 61-66).
FIG. 11 is a schematic diagram for illustrating a false contour noise that is visually perceived by a human eye moving on different pixels.
In FIG. 11, the white circles denote emission sub-fields, the black circles denote non-emission sub-fields, and the plurality of sub-fields are denoted as SF1 to SF10 in order of smaller weight. The rows A, B, C, D shown in FIG. 11 denote the numbers of pixel rows in the horizontal direction, and the column 1 denotes the number of a pixel column in the vertical direction.
In FIG. 11, when the human eye is fixed in the row A column 1 position or the row B column 1 position, the eye perceives the sub-fields SF1-SF10 for the pixel arranged in the row A column 1 or the sub-fields SF1-SF10 for the pixel arranged in the row B column 1. In this case, the emission patterns for these pixels are “1101110111” and “0111011111”, respectively, while the grayscale values perceived by the eye are “955” and “1006”, respectively. In this way, the grayscale value of the pixel arranged in the row A column 1 is originally perceived to be lower than the grayscale value of the pixel in the row B column 1.
When the sight of line I1 of a human moves from the pixel in the row A column 1 to the pixel in the row C column 1 as denoted by the solid arrow in FIG. 11, the eye perceives the sub-fields SF1-SF3 for the pixel in the row A column 1, the sub-fields SF4-SF8 for the pixel in the row B column 1, and the sub-fields SF9, SF10 for the pixel in row C column 1 in order. In this case, the emission pattern is “1101011111”, while the grayscale value that is perceived by the eye is “1003”.
Similarly, when the line of sight I2 of a human moves from the pixel in the row B column 1 to the pixel in the row D column 1 as denoted by the dotted arrow in FIG. 11, the eye perceives the sub-fields SF1-SF3 for the pixel in the row B column 1, the sub-fields SF4-SF8 for the pixel in the row C column 1, and the sub-fields SF9, SF10 for the pixel in the row D column 1 in order. In this case, the emission pattern is “0111110111”, while the grayscale value that is perceived by the eye is “956”.
In this way, because of the motion of the sight of line I1, the grayscale value is perceived as “1003” which is higher than the grayscale value “955” that should have originally been represented. Also, because of the motion of the line of sight I2, the grayscale value is perceived as “956” which is lower than the grayscale value “1006” that should have originally been represented. The relation between each adjacent pixels in the row 1 is thus reversed by the motion of the line of sight. Such changes in the grayscale values of pixels are perceived as false contour noises.
In one suggested method for reducing false contour noises, grayscale levels for which emission sub-fields are continuously present are selected as grayscale levels unlikely to cause a false contour noise, and only the selected grayscale levels are used for display. In this case, a grayscale level other than the selected ones can be represented by selecting two of grayscale levels between this grayscale level that are unlikely to cause a false contour noise, and displaying the two grayscale levels alternately for each field (refer to e.g. JP 2000-276100 A).
Another method for reducing false contour noises involves decreasing the number of sub-fields to reduce the generation of a false contour noise. In this case, in order to represent grayscale levels that cannot be represented due to the decreased number of sub-fields, four pixels that are vertically and horizontally adjacent to one other are assumed as a single set, and four dither coefficients different from one another are assigned and added to respective pixel data corresponding to the pixels of this set. This allows representation of the foregoing grayscale levels that could not be represented through an area ratio grayscale. The method also achieves reduction of noise due to dither patterns by varying the dither coefficient that is added for each field (refer to e.g. JP 10-98663 A).
However, with the above-described methods for reducing false contour noises, degradation of image quality occurs due to decreased number of grayscale levels that can be represented.