vf_ass: Optimize alpha multiply

The effect of alpha blending was calculated as
color = orig_color * alpha / 255
where alpha and color range from 0 to 255.
Change this to
color = (orig_color * alpha + 255) / 256
where the "/ 256" can be expressed as a shift whereas the compiler
would probably generate a multiply+shift for the original "/ 255".
This formula gives a result that is too high by 1 for some inputs.
However it gives the exact result if alpha is 0 or 255 which is
probably the case where small errors would matter most.

0 files changed, 0 insertions, 0 deletions