Re: [FFmpeg-devel] [PATCH] avfilter/vf_bwdif: Remove undesireable spatial preference logic

[Philip Langdale <philipl@overt.org>]

On Sat, 25 Mar 2023 00:02:03 +0100
Thomas Mundt <tmundt75@gmail.com> wrote:

> Hi Philip,
> 
> Philip Langdale <philipl@overt.org> schrieb am Fr., 24. März 2023,
> 23:21:
> 
> > bwdif inherited this check from yadif, which was originally
> > supposed to prefer the spatial predictor if the temporal predictor
> > was too far off.
> >
> > However, the core bwdif algorithm already accounts for the spatial
> > predictor, so this additional check actually ends up preferring a
> > worse value, reducing the overall quality.
> >
> > This was found by cyanreg while writing bwdif_vulkan, and the visual
> > improvement is pretty dramatic in some samples. If we agree that
> > this change is desirable, we should update all implementations.
> >
> > Signed-off-by: Philip Langdale <philipl@overt.org>
> > ---
> >  libavfilter/vf_bwdif.c | 5 -----
> >  1 file changed, 5 deletions(-)
> >
> > diff --git a/libavfilter/vf_bwdif.c b/libavfilter/vf_bwdif.c
> > index 65c617ebb3..441bb11e7b 100644
> > --- a/libavfilter/vf_bwdif.c
> > +++ b/libavfilter/vf_bwdif.c
> > @@ -106,11 +106,6 @@ typedef struct ThreadData {
> >              interpol = (c + e) >> 1;
> >
> >  #define FILTER2() \
> > -            if (interpol > d + diff) \
> > -                interpol = d + diff; \
> > -            else if (interpol < d - diff) \
> > -                interpol = d - diff; \
> > - \
> >  
> 
> Removing this will make lower thirds and other graphic jump up and
> down each frame. It is the main improvement over w3fdif that I have
> ported from yadif.
> Can you provide samples including still graphics that are improved
> with this patch?

Hi Thomas,

Sorry for this thread going unanswered for so long. With the Vulkan
hwaccel stuff sorted out, I wanted to come back to this. I discussed
more with Lynne and we're no longer convinced this change is obviously
desirable. You are right about the jumping behaviour - and although
there are samples (eg: the burosch ones on samples.ffmpeg.org) which
look better with the change, I don't think that's something to over
index on.

Thanks,

--phil
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