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

[Timo Rothenpieler]

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.

>>
>> 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"?