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avfilter/vf_libplacebo: force premultiplied blending for linear texture

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Blending onto independent alpha framebuffers is not possible under the
constraints of the supported blend operators. While we could handle
blending premul-onto-premul, this would break if the base layer is YUV,
since premultiplied alpha does not survive the (nonlinear) YUV conversion.

Fortunately, blending independent-onto-premul is just as easy, and works in
all cases. So just force this mode when using a linear intermediate blend
texture, which is always RGBA.
This commit is contained in:
Niklas Haas
2025-09-11 01:48:32 +02:00
committed by Niklas Haas
parent 6f08ab6c4c
commit 899e497122
1 changed files with 4 additions and 1 deletions
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5
libavfilter/vf_libplacebo.c
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@@ -1033,6 +1033,7 @@ props_done:
.color = orig_target.color,
.rotation = orig_target.rotation,
};
target.repr.alpha = PL_ALPHA_PREMULTIPLIED;
target.color.transfer = PL_COLOR_TRC_LINEAR;
use_linear_compositor = true;
}
@@ -1052,7 +1053,8 @@ props_done:
update_crops(ctx, in, &target, target_pts);
pl_render_image_mix(in->renderer, &in->mix, &target, &tmp_params);
/* Force straight output and set correct blend mode */
/* Force straight output and set correct blend operator. This is
* required to get correct blending onto YUV target buffers. */
target.repr.alpha = PL_ALPHA_INDEPENDENT;
tmp_params.blend_params = &pl_alpha_overlay;
#if PL_API_VER >= 346
@@ -1065,6 +1067,7 @@ props_done:
if (use_linear_compositor) {
/* Blit the linear intermediate image to the output frame */
target.crop = orig_target.crop = (struct pl_rect2df) {0};
target.repr.alpha = PL_ALPHA_PREMULTIPLIED;
pl_render_image(s->linear_rr, &target, &orig_target, &opts->params);
target = orig_target;
} else if (!ref) {