Method and apparatus for image data compression

A method and apparatus for selecting a reference scan line for two-dimensional image coding preferably compatible with known facsimile transmission equipment are disclosed. Selection of a reference scan line in accordance with the method and apparatus of the invention allows data compression for facsimile transmission so that the cost for use of the communication link is reduced. More than one previous scan line is considered as a candidate reference scan line. A reference scan line is selected from among a plurality of previous scan lines, for example, the immediately preceding ten scan lines. The preselected or adaptively selected scan line among the multiple prior scan lines, more particularly, the previous scan line which is most similar to the current scan line to be coded, is selected as the reference scan line. The reference scan line is then fed with the current scan line to be coded to a two-dimensional data compression coding process so as to yield optimum data compression. This reduces the amount of image data transmitted and lowers the cost of operation of the facsimile transmission apparatus.