[Ffmpeg-devel] few remarks for h264 decoder
Sat Dec 31 11:38:05 CET 2005
On Sat, 31 Dec 2005, G?bor Kov?cs wrote:
> I think I found another one, but it's mmx releated which I don't actually use
> and I don't even have sample clip for this, so there is a chance I'am wrong.
> It's about deblocking. alpha_table has zero values for low qscale. In this
> case ABS( p0 - q0 ) < alpha should always fail. The problem with mmx code
> that it uses alpha-1. In H264_DEBLOCK_MASK() packuswb will saturate -1 to 0
> and the |p0-q0| > alpha-1
> condition may allow deblocking when |p0-q0| = 0. Probably rare problem.
If I read this correctly:
alpha=0 and beta=0 occur in the same place in the tables (qp<=15), so the
failure case is that it will erroneously enable filtering when p1=p0=q0=q1
... where it does nothing, because the block is flat anyway.
So it will only trigger when alpha_c0_offset != beta_offset.
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