Further compression of data allowing economical storage of data for
extended periods of time in high-speed access memory is performed in
reduced time by performing further compression during transcoding in the
transform domain and without restoring image data to its original image
data form. The reduction in processing time is achieved by exploiting the
large number of zero-valued quantization transform coefficients and not
changing quantized transform coefficients at zig-zag scan positions where
non-zero coefficients are rare during range reduction of the entropy
decoded quantized transformed data. The range can be restored by
computation or estimation of an altered quantization table which is
stored with the further compressed a quantization values. Further
advantages accrue from use of JPEG packed format for the data during
transcoding.