A method of manipulating spectral content of a block of pixels for
compression includes the step of classifying each pixel within a selected
block of pixels as relevant or irrelevant. A forward transform is applied
to the selected block to generate a coefficient block. The coefficient
values are modified to generate a modified coefficient block subject to a
set of pre-determined constraints including a constraint that the relevant
pixels have a same value in an inverse transformation of the modified
coefficient block as in the selected block. Proceeding in a reverse zig
zag coefficient order the method is repeated for other coefficients having
non-zero quantized values.
