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

Marton Balint cus at passwd.hu
Sun Apr 22 14:44:19 EEST 2018

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...

> Doesnt this patchset replace a problem by another problem ?
> It may be better to not rush this and find a complete solution
> a 2nd pixfmt and or scaning the 256 entries for their alpha values
> is maybe the direction we could go

I don't think scaning can work for anything other than still images, as 
the palette can change per-frame AFAIK. Introducing a new pixel format 
seems the better approach, however keeping in mind compatibility and 
existing code, I'd rather make the newly introduced one the one without 
alpha, and move things gradually to it. Still seems like a fair amount of 


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