Abstract: When an N-times density binary image having N+1 tones is compressed, only the tone of each pixel is converted into a compressed code. The compressed code has a number of bits necessary and sufficient to distinguish between the tones. When a triple-density image having 4 tones is compressed, for example, the tone of each pixel is converted into a 2-bit compressed code. When the original N-times density is restored from the compressed code, there is formed a dot pattern in which the number of dots corresponding to the tone, as represented by the compressed code of each pixel, are situated closer to the darker one of the two adjoining pixels.

Abstract: In the case of a 4-fold density image, for example, 2-pixel areas in the 4-fold density image which are adjacent to each other are allocated with one 4-bit compression code, respectively. In this case, the possible patterns which can be produced by the 8-dot pattern of the 2-pixel area are limited to 16 kinds of patterns in which there is one or less black and white change. With respect to 2-bit portions of 4-bit compression codes which correspond to pixels, correspondence relationships between 2-bit portions and 4-dot patterns are set so that, when a 2-bit portion is inverted or converted into "11" in all bits or "00" in all bits, also the 4-dot pattern of the corresponding pixel is similarly inverted or converted into a pattern of all white dots or all black dot. Next, by combining the correspondence relationships of the 2-bit portions, 12 kinds of correspondence relationships between 4-bit compression codes and 8-dot patterns of two pixels are produced.