Image compressing apparatus, image compressing method, image decompressing apparatus, and storage medium

An image compressing method for compressing image data that has pixel data for each pixel, comprising:

• generating a plurality of second pixel data groups by dividing a first pixel data group including a plurality of pixels existing in block that divides an image;
• performing orthogonal transformations in a plurality of layers to the second pixel data group using only a predetermined number of coefficients from a lowest frequency side from among coefficients obtained by a plurality of orthogonal transformations performed in lower layer;
• and
• generating compressed data by encoding a predetermined number of coefficients from lowest frequency side from among coefficients obtained in orthogonal transformation of a highest layer and coefficients not used in orthogonal transformations from among coefficients obtained in a layer other than the highest layer.

A computer-accessible storage medium on which is recorded a program for enabling a computer that can be used as an image compressing apparatus compressing image data having pixel data for every pixel to execute a process, the process comprising:

• generating a plurality of second pixel data groups by dividing a first pixel data group including a plurality of pixels existing in block that divides an image;
• performing an orthogonal transformation for each of the second pixel data groups generated by the generating, and outputting a plurality of coefficients that represent frequency components;
• performing an orthogonal transformation using a predetermined number of coefficients from a lowest frequency side from among the plurality of coefficients obtained by the orthogonal transformation for each piece of the second pixel data group;
• and
• generating compressed data from coefficients output by the orthogonal transformation and coefficients not used by the orthogonal transformation with the predetermined number of the coefficients.