[XviD-devel] Scenechange detection

Radek Czyz syskin at ihug.com.au
Wed Dec 31 23:01:17 CET 2003


I wrote:
 >I'd say that this was the reason... pure black didn't add to SAD but
 >added at least 300 to picture's complexity, and it usually works good
 >this way.

Come to think of it, this works good for p/b part of the decision - pure 
black should bias towards a b-frame. For I-frame decision however, pure 
black should be reasonably neutral - it's almost as easy to code in an 
i-frame as in a p-frame. Perhaps we should count the values separatly.

> Ah, okay, I didn't remember those. Still, it had a strange effect in this
> clip. Btw, do those extra keyframes get removed in second pass if a
> keyframe within the limit is detected? I think they should, even if the 
> overhead is small. 

They are not, and unfortunately it's difficult to tell if a frame became 
an i-frame because it was definitely a scene change or because the limit 
decreased the threshold so much (we can expect that from an 298th frame 
in a row, but we never know for sure)

> XviD set 42 keyframes. P-frame at nondetected scenechanges were about 
> same size as I-frames, so no gain or loss from this. I-frames at non-scene
> changes were of course larger. 

Yeah I think the current code does that - doesn't care if a scene change 
which is very close to a keyframe is missed, but rather tries not to 
make any non-scane-change a keyframe. It appears to work well ;)


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