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

[Lynne <dev@lynne.ee>]

Jun 11, 2023, 04:53 by 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.
>

I don't recall agreeing that this is desirable.
bwdif_vulkan still looks better than bwdif or bwdif_cuda (and it's got proper edge handling).
I just forgot to investigate this, that's all.
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