Cutting Out the Noise - A Practical Guide to Video and Audio Compression - page 595

Frame 1 Frame 2 Differences

Figure 36-16 Temporal noise and its compression effects.

Solution?

1. Apply grain removal.
2. Apply blurring.
3. Motion-compensated averaging across frames might solve this, but you would
need expensive or complex restoration software/hardware in order to deploy that
capability.

36.4.18 Gibbs Effect

An artifact similar to dot crawl and cross color that is evident after compression is known
as the Gibbs effect (Figure 36-17). This is where letters acquire a shimmering cross-color
effect around their edges. It is a consequence of losing the high-resolution detail when the
entropy filtering is engaged. Screen credits must be processed separately from the rest of
the content. It is worth completely redesigning and retyping them to eliminate the prob-
lems arising from processing them like normal moving video. The Gibbs effect is also
known as mosquito noise.
The example shows the word ENGLAND originally drawn on a solid background.
The Gibbs effect has caused some harmonic effects of the DCT transform to introduce res-
onant frequencies that have added detail that was not there in the original. This might be
due to the entropy coding having compressed the data too much. The effect often happens
around the edges of text. To fix it, you have to sacrifice some compression efficiency or
draw the text in a way that compresses more easily.