[FFmpeg-devel] MPEG-4 ASP motion vector clamping correct?

[Michael Niedermayer]

On Fri, Dec 16, 2011 at 11:05:12PM +0100, Reimar Döffinger wrote:
> On Fri, Dec 16, 2011 at 10:11:58PM +0100, Michael Niedermayer wrote:
> > On Fri, Dec 16, 2011 at 09:45:59PM +0100, Reimar Döffinger wrote:
> > > Hello,
> > > I have a question about mpegvideo_common.h
> > > There is a lot of code like:
> > > av_clip(src_x, -16, s->width);
> > > However on the other hand we have the FF_BUG_EDGE flag that changes
> > > where we start the border extension.
> > > Why shouldn't this be using s->h_edge_pos instead to follow that?
> > > Even more so since I think this is also used for H.263
> > > for which the spec seems quite clear that the decoder should only
> > > ever care about the rounded-up width, not the non-mod16 part.
> >
> > I think you are correct, it should use h/v_edge_pos according to the
> > h263+ spec. I dont know though if this will fix or break real world
> > samples.
> > it would only make a difference for h263+UMV with %16!=0 and 4mv and
> > only if the further outside area is sufficiently different and the
> > encoder actually used such motion vectors.
>
> More specifically the motion vector must point exactly into the mod-16
> extended area - a bit outside in either direction is ok.
> As is decoding with FF_BUG_EDGE.
> However I think GMC is affected as well, not just 4mv.
> Question is: what should we do? Change it? Make it a "bug" option?
> ...?
> Do you know what our encoder will have used as reference? If our
> encoder did "the right thing" making the decoder match seems
> advisable even if it hurts quality a bit for some existing files.

i think the encoder too is affected by the av_clip()

[...]

--
Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

It is dangerous to be right in matters on which the established authorities
are wrong. -- Voltaire
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