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vf_overlay: add support to alpha pre-multiplication in the RGBA path

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Based on the work of Mark Himsley <mark@mdsh.com>.

See thread:
Subject: [FFmpeg-devel] libavfilter: extending overlay filter
Date: Sun, 13 Mar 2011 14:18:42 +0000
This commit is contained in:
Stefano Sabatini 2011-10-29 00:29:25 +02:00
parent 3013bfa81e
commit 2f7c8aefa8
1 changed files with 29 additions and 0 deletions
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29
libavfilter/vf_overlay.c
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@ -360,18 +360,35 @@ static void blend_slice(AVFilterContext *ctx,
const int dr = over->main_rgba_map[R];
const int dg = over->main_rgba_map[G];
const int db = over->main_rgba_map[B];
const int da = over->main_rgba_map[A];
const int dstep = over->main_pix_step[0];
const int sr = over->overlay_rgba_map[R];
const int sg = over->overlay_rgba_map[G];
const int sb = over->overlay_rgba_map[B];
const int sa = over->overlay_rgba_map[A];
const int sstep = over->overlay_pix_step[0];
const int main_has_alpha = over->main_has_alpha;
if (slice_y > y)
sp += (slice_y - y) * src->linesize[0];
for (i = 0; i < height; i++) {
uint8_t *d = dp, *s = sp;
for (j = 0; j < width; j++) {
alpha = s[sa];
// if the main channel has an alpha channel, alpha has to be calculated
// to create an un-premultiplied (straight) alpha value
if (main_has_alpha && alpha != 0 && alpha != 255) {
// apply the general equation:
// alpha = alpha_overlay / ( (alpha_main + alpha_overlay) - (alpha_main * alpha_overlay) )
alpha =
// the next line is a faster version of: 255 * 255 * alpha
( (alpha << 16) - (alpha << 9) + alpha )
/
// the next line is a faster version of: 255 * (alpha + d[da])
( ((alpha + d[da]) << 8 ) - (alpha + d[da])
- d[da] * alpha );
}
switch (alpha) {
case 0:
break;
@ -387,6 +404,18 @@ static void blend_slice(AVFilterContext *ctx,
d[dg] = FAST_DIV255(d[dg] * (255 - alpha) + s[sg] * alpha);
d[db] = FAST_DIV255(d[db] * (255 - alpha) + s[sb] * alpha);
}
if (main_has_alpha) {
switch (alpha) {
case 0:
break;
case 255:
d[da] = s[sa];
break;
default:
// apply alpha compositing: main_alpha += (1-main_alpha) * overlay_alpha
d[da] += FAST_DIV255((255 - d[da]) * s[sa]);
}
}
d += dstep;
s += sstep;
}