Looking for a filter for alpha overlay

[Sergejack2]

I'm looking for a filter that would allow me to overlay a video at specific times with RGB pixels using alpha (transparency).
It can be done in ASS using VSFilter (with drawning pixels), but VSFilter isn't very friendly with using it much (a 320*240 pixels ASS defined image will require some hypercomputer even Bill Gate can't afford).

[communist]

A=Avisource("source.avi").ConvertToRGB32()
B=ImageReader("mypic.png",use_DevIL=true,pixel_type="rgb32",fps=25).trim(0,24)
Layer(A,B)

This will use the alpha channel from the png file as mask.
Tweak trim and fps values according to your source.