[FFmpeg-devel] [PATCH 2/4] avfilter/avfiltergraph: fix has_alpha in pick_format

[Michael Niedermayer]

On Mon, Apr 23, 2018 at 04:50:54AM +0200, Marton Balint wrote:
> 
> 
> On Mon, 23 Apr 2018, Michael Niedermayer wrote:
> 
> >On Sun, Apr 22, 2018 at 01:44:19PM +0200, Marton Balint wrote:
> >>
> >>
> >>On Fri, 20 Apr 2018, Michael Niedermayer wrote:
> >>
> >>>On Thu, Apr 19, 2018 at 09:32:19PM +0200, Marton Balint wrote:
> >>>>A pixel format which has a palette also has alpha, without this patch the
> >>>>format negotiation might have preferred formats without alpha even if the
> >>>>source had alpha.
> >>>>
> >>>>Signed-off-by: Marton Balint <cus at passwd.hu>
> >>>>---
> >>>>libavfilter/avfiltergraph.c | 2 +-
> >>>>1 file changed, 1 insertion(+), 1 deletion(-)
> >>>>
> >>>>diff --git a/libavfilter/avfiltergraph.c b/libavfilter/avfiltergraph.c
> >>>>index 4cc6892404..e18f733e23 100644
> >>>>--- a/libavfilter/avfiltergraph.c
> >>>>+++ b/libavfilter/avfiltergraph.c
> >>>>@@ -679,7 +679,7 @@ static int pick_format(AVFilterLink *link, AVFilterLink *ref)
> >>>>
> >>>>    if (link->type == AVMEDIA_TYPE_VIDEO) {
> >>>>        if(ref && ref->type == AVMEDIA_TYPE_VIDEO){
> >>>>-            int has_alpha= av_pix_fmt_desc_get(ref->format)->nb_components % 2 == 0;
> >>>>+            int has_alpha= av_pix_fmt_desc_get(ref->format)->nb_components % 2 == 0 || (av_pix_fmt_desc_get(ref->format)->flags & AV_PIX_FMT_FLAG_PAL);
> >>>
> >>>This causes various output files to grow in size with a unused alpha plane
> >>>
> >>>a random example: (there are likels better examples)
> >>>./ffmpeg -y -i ~/tickets/1116/1023.bmp -vf negate fileX.bmp
> >>>
> >>>Iam not sure unconditionally treating all palettes as if they have
> >>>non fully opaque entries is a good idea.
> >>
> >>Obviously not, but it is already treated this way in most places. Having a
> >>bigger image with alpha is better than losing alpha. And the user can always
> >>force losing alpha with a filter, and sometimes he has to do that anyway
> >>because for example fre0r filters have no way of signalling if they use
> >>alpha or not...
> >
> >you can, the average user certainly doesnt have the knowledge to adjust
> >anything alpha by hand, the average user isnt even aware of that the issue
> >is alpha or pal8 related probably
> >
> >also about "better", i saw a few cases that got bigger, i dont remember
> >seeing a case that was fixed.
> >Have you seen real usecases this fixes ?
> 
> A source file with a palette and alpha and a filter which supports formats
> with both alpha and without:
> 
> https://dab1nmslvvntp.cloudfront.net/images/blogs/design/8bit-trans.png
> 
> ffmpeg -i 8bit-trans.png -vf negate out.bmp

i assume fate doesnt cover this yet, so a new fate test probably makes sense

thx

[...]
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Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

Complexity theory is the science of finding the exact solution to an
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