[FFmpeg-soc] vf_overlay with transparancy

[zeger at customsoft.nl]

Vitor Sessak wrote: 
> A patch to add alpha handling to vf_overlay.c has been posted to
> ffmpeg-devel (search for the thread "Support alphablending in the 
> overlay filter"). The add_alpha filter has also been posted to 
> ffmpeg-devel, see vf_alpha.patch in thread "[RFC] Alpha support"

After adding the vf_overlay blend patch as suggested I wrote an
updated vf_alpha filter since I needed Y based transparency and did 
not like the YUV -> RGB -> YUV conversions.

I ran into a slight problem when combining the vf_alpha filter with
vf_scale; div by zero. I am not sure how to fix this other than 
patching vf_alpha. 

I've been using the following filter chain to test the overlay with 
alpha and scaling combination:

 input ----------------------> deltapts0 --> overlay --> output
                                               ^
 movie --> alpha --> scale --> deltapts1 ------|

Zeger Knops 
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