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| author | Uoti Urpala <uau@symbol.nonexistent.invalid> | 2008-07-23 01:47:30 +0300 |
|---|---|---|
| committer | Uoti Urpala <uau@symbol.nonexistent.invalid> | 2008-07-23 02:05:55 +0300 |
| commit | 4dab4347f5d7bb3642a2c5918fc29855f8f62482 (patch) | |
| tree | 4f1f572dcd5cfee177658d28de3e1c3ce015dd65 /libmpcodecs | |
| parent | 2d4656e070697cebe8cb66568b3e8dd8919b5eed (diff) | |
| download | mpv-4dab4347f5d7bb3642a2c5918fc29855f8f62482.tar.bz2 mpv-4dab4347f5d7bb3642a2c5918fc29855f8f62482.tar.xz | |
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.
Diffstat (limited to 'libmpcodecs')
| -rw-r--r-- | libmpcodecs/vf_ass.c | 8 |
1 files changed, 4 insertions, 4 deletions
diff --git a/libmpcodecs/vf_ass.c b/libmpcodecs/vf_ass.c index 2f7a188246..bbe3bea371 100644 --- a/libmpcodecs/vf_ass.c +++ b/libmpcodecs/vf_ass.c @@ -296,10 +296,10 @@ static void my_draw_bitmap(struct vf_instance* vf, unsigned char* bitmap, int bi dstv = vf->priv->planes[2] + dst_x + dst_y * vf->priv->outw; for (i = 0; i < bitmap_h; ++i) { for (j = 0; j < bitmap_w; ++j) { - unsigned k = ((unsigned)src[j]) * opacity / 255; - dsty[j] = (k*y + (255-k)*dsty[j]) / 255; - dstu[j] = (k*u + (255-k)*dstu[j]) / 255; - dstv[j] = (k*v + (255-k)*dstv[j]) / 255; + unsigned k = (src[j] * opacity + 255) >> 8; + dsty[j] = (k*y + (255-k)*dsty[j] + 255) >> 8; + dstu[j] = (k*u + (255-k)*dstu[j] + 255) >> 8; + dstv[j] = (k*v + (255-k)*dstv[j] + 255) >> 8; } src += stride; dsty += dmpi->stride[0]; |