The horizontal and vertical offset for the MN DCT block boundaries are
determined for a decompressed image originally produced by a DCT-based compression
system, regardless of the amount of compression and/or cropping that had been applied
to the image. The method includes the steps of: a) computing one or more selected
DCT coefficients from nonoverlapping, contiguous MN blocks of the decompressed
image, beginning at a selected offset; b) computing a coefficient histogram from
a set of values for each selected DCT coefficient obtained from the blocks of the
decompressed image; c) computing a dispersion metric from each coefficient histogram;
d) repeating steps (a) through (c) for a plurality of offsets within an MN
block; e) comparing the dispersion metrics corresponding to the plurality of offsets
to determine the minimum dispersion coefficient histogram; and f) selecting the
offset that corresponds to the minimum dispersion coefficient histogram as the
offset for the DCT block boundaries.
