[Theora-dev] Very visible artefacts on still frames

[Alen Ladavac]

Hi all,

We have bunch of CG rendered movies in 640x368, NTSC, encoded with Theora 
using illiminable's DirectShow filters with default params 
(targetbitrate=400000, log2(keyframefreq)=64, quality=30).

While quality is excellent during scenes with motion, all still frames have 
some very annoying problems. A still frame usually appears for dramatic 
pause (a character's face usually), or when there is a black screen with 
text-only ("A few hours later..."). When the still frame initially appears, 
it looks good, but as it stands still, its quality quickly degrades. Along 
object borders in the image, dots appear in what looks like dithering 
patterns grouped in small square blocks.

Does anyone know what this is and/or how to get rid of that problem? It is 
not a small glitch, this is quite obvious and looks very bad.

I can provide example movie if needed.

Thanks in advance for any info you can give me,
Alen