Improved transform coding [imaging coding]

A novel method for coding of images using the discrete cosine transform is presented. It is based on a modification of the idea of reducing subthreshold coefficients to zero during transform coding. A simple analysis shows that if the subthreshold coefficients are represented by reduced magnitudes other than zero, then the net error that results in a reconstructed pixel is less than if the coefficients have zero value. Based on this reasoning, the subthreshold coefficients are grouped according to their magnitudes, and only the average value of the coefficients in each group is selected in place of all the members of that group during coding. During image reconstruction, the average value of each group is used to generate all the members of that group. By selecting only one coefficient to represent many, the compression ratio is greatly improved