Image encoding apparatus, image decoding apparatus, control method therefor, computer program, and computer-readable storage medium

Abstract

An image having few colors can be losslessly encoded at a higher compression ratio. For this purpose, pixel data are input in raster order and temporarily stored in a buffer. A neighborhood matching determination unit generates first information representing whether a pixel having the same color as that of the pixel of interest exists in neighboring pixels and second information specifying whether a pixel having the same color as that of the pixel of interest exists, and if the pixel having the same color exists, specifying the neighboring pixel. A pixel matching detection unit counts the number of colors contained in the neighboring pixels and generates information representing whether the number of colors is two or less, or three or less. On the basis of the information, a code generation unit outputs one or both of encoded data from a matched-pixel position encoding unit and a prediction error encoding unit.