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

[Michael Niedermayer]

On Tue, Apr 24, 2018 at 02:10:53AM +0200, Michael Niedermayer wrote:
> 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

i still think it would be more ideal if this is fully fixed 
(by alpha / non alpha pal8) or other.

Because as is we have a set of bugs, and with this patchset we fix some while
introducing other (maybe less important though) bugs.
Then later behavior changes again when this is all fixed.
A smaller number of behavior changes should be better and less confusing to
users.

thx

[...]

-- 
Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

He who knows, does not speak. He who speaks, does not know. -- Lao Tsu
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