[FFmpeg-devel] [PATCH] avfilter/vf_chromakey: The chromakey filter preserves non-opaque alpha transparency.

[Gavin Smith]

On 12/06/2020 16:58, Gavin Smith wrote:
> On 12/06/2020 12:59, Timo Rothenpieler wrote:
>> On 12.06.2020 00:31, Gavin Smith wrote:
>>>
>>> On 11/06/2020 22:03, Timo Rothenpieler wrote:
>>>> On 11.06.2020 22:27, Gavin Smith wrote:
>>>>> This is to address trac ticket #8724. The filter did not preserve 
>>>>> alpha transparency. Items that were transparent in the input would 
>>>>> appear black on the output or pixels that were semi-tranparent 
>>>>> would appear opaque.
>>>>>
>>>>
>>>> What is the performance impact for inputs without an alpha channel?
>>>
>>> Firstly, I'm new to this world of filters.  Now my patch only 
>>> applies to chromakey and not chromahold. On that note, is it not the 
>>> case that the chromakey mandates that all its inputs surfaces 
>>> contain alpha (as per query_formats function)? Would any 
>>> AVPixelFormat that does not match query_formats get converted to a 
>>> format containing alpha transparency?
>>
>> But it still adds new code, and given it's in the very hot path of 
>> the filter, it can easily add a performance penalty.
> Sure. I understand. I'll see what numbers I can get.  Having a quick 
> cursory look over the code, I wonder if my code actually might be 
> slightly more performant because it checks the case if a0 != 0. 
> However, this jmp/cmp may negatively impact performance.

vf_chromakey wll now have 2 implementations of chromakey depending on 
whether "use_alpha" is selected. What are your feelings on the best way 
to approach this?

1. Add an additional function pointer to ChromakeyContext: In 
do_chromakey_slice(..), call the said func ptr in the loop each iteration?

2. Add a conditional to do_chromakey_slice(...): In the loop iteration, 
check "use_alpha" variable and execute and jmp to pertinent code?

3. Any other suggestions?

Thanks.

>>
>>>>
>>>> Generally looks fine to me, but might need hidden behind an option, 
>>>> as to not break existing setups that rely on this filter 
>>>> discarding/ignoring the input alpha channel.
>>>
>>> Yes. No problem. "preserve_transparency" and default to the 
>>> equivalent of 'false'?
>>
>> That seems a bit clunky. Maybe something like "use_alpha"?
> Sure.
>>
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