[FFmpeg-devel] [PATCH] vf_overlay: add support to RGBA packed input and output

[Mark Himsley]

On 29/10/2011 16:26, Michael Niedermayer wrote:
> On Sat, Oct 29, 2011 at 04:47:41PM +0200, Stefano Sabatini wrote:

>> Regarding the second patch, I kept Mark's code but after some time
>> spent tinkering on it I couldn't figure out the meaning of the
>> equation:
>
>>      d[da] = ( (d[da]<<  8) + (256 - d[da]) * s[sa] )>>  8;
>
> correcter:
> d += ((255 - d) * s + 128) / 255;
>
> and /255 can be done by multiplication and shift

Sure, Michael is right.

I tried to optimise out the /255, and my code was within 1 of the 
correct result and never altered the main alpha if the overlay alpha was 0.

In the intervening months between writing that code and Stefano moulding 
it into something workable, I have learnt that GCC is better at 
optimising maths than I am :)

-- 
Mark