Post processing transform coded images using edges
William E. Lynch, Amy R. Reibman, Baoxing Liu · 2002
Discrete cosine transform coding, a popular image compression strategy, results in two visible artifacts: blocking and ringing. These are both high frequency artifacts. Since images contain high frequency information the artifacts are removed using a space varying low pass filter as a post processor. Low frequency blocks and flat regions of blocks containing a strong edge are filtered. Low frequency blocks are identified in the transform coefficient domain; edge blocks are identified in the spatial domain. This does not require any alterations in the compressed bit stream. Improvement is demonstrated both subjectively and objectively.