Method and apparatus for coding image data

Abstract

Input image data is divided into a plurality of blocks. An orthogonal transform device subjects the input image data to orthogonal transform every block. A memory temporarily stores output data from the orthogonal transform device. A first quantizer quantizes output data from the memory. A coding device codes output data from the first quantizer into variable-length code words. A code amount accumulation device accumulates a code amount of output data from the coding device. A second quantizer separates the output data from the orthogonal transform device into K different groups, and assigns M different scale factors to the respective blocks and quantizes the output data from the orthogonal transform device in the blocks with the related scale factors, where K and M denote predetermined natural numbers. A block code amount calculation device calculates a code amount per block. An initial scale factor calculation device determines an initial scale factor designed to make one of a frame or field into a predetermined code amount. A group target code amount calculation device predicts target code amounts in the respective groups from the block code amounts represented by the output data from the block code amount calculation device. A scale factor correcting device corrects the initial scale factor in response to a prediction error between an actual code amount accumulated by the code amount accumulation device and the predicted target code amount each time the coding device outputs the data of one group.