[FFmpeg-devel] [PATCH] avfilter/vf_libplacebo: two blending-related fixes (PR #20491)

[Niklas Haas via ffmpeg-devel <ffmpeg-devel@ffmpeg.org>]

PR #20491 opened by Niklas Haas (haasn)
URL: https://code.ffmpeg.org/FFmpeg/FFmpeg/pulls/20491
Patch URL: https://code.ffmpeg.org/FFmpeg/FFmpeg/pulls/20491.patch


>From 71d4a88136e731bddf2673e13ed393efe9c2fdd5 Mon Sep 17 00:00:00 2001
From: Niklas Haas <git@haasn.dev>
Date: Thu, 11 Sep 2025 01:39:46 +0200
Subject: [PATCH 1/2] avfilter/vf_libplacebo: use temporary params struct for
 per-pass params

Instead of directly mutating `opts->params`. Avoids any possible leak of
overriden params between invocations of this function, as well as the later
`pl_render_image` during the linear output pass.
---
 libavfilter/vf_libplacebo.c | 17 ++++++-----------
 1 file changed, 6 insertions(+), 11 deletions(-)

diff --git a/libavfilter/vf_libplacebo.c b/libavfilter/vf_libplacebo.c
index 5e0a678ff2..8f30eb193b 100644
--- a/libavfilter/vf_libplacebo.c
+++ b/libavfilter/vf_libplacebo.c
@@ -1038,12 +1038,7 @@ props_done:
     }
 
     /* Draw first frame opaque, others with blending */
-    opts->params.blend_params = NULL;
-#if PL_API_VER >= 346
-    opts->params.background = opts->params.border = PL_CLEAR_COLOR;
-#else
-    opts->params.skip_target_clearing = false;
-#endif
+    struct pl_render_params tmp_params = opts->params;
     for (int i = 0; i < s->nb_inputs; i++) {
         LibplaceboInput *in = &s->inputs[i];
         FilterLink *il = ff_filter_link(ctx->inputs[i]);
@@ -1053,17 +1048,17 @@ props_done:
             pl_renderer_flush_cache(in->renderer);
             continue;
         }
-        opts->params.skip_caching_single_frame = high_fps;
+        tmp_params.skip_caching_single_frame = high_fps;
         update_crops(ctx, in, &target, target_pts);
-        pl_render_image_mix(in->renderer, &in->mix, &target, &opts->params);
+        pl_render_image_mix(in->renderer, &in->mix, &target, &tmp_params);
 
         /* Force straight output and set correct blend mode */
         target.repr.alpha = PL_ALPHA_INDEPENDENT;
-        opts->params.blend_params = &pl_alpha_overlay;
+        tmp_params.blend_params = &pl_alpha_overlay;
 #if PL_API_VER >= 346
-        opts->params.background = opts->params.border = PL_CLEAR_SKIP;
+        tmp_params.background = tmp_params.border = PL_CLEAR_SKIP;
 #else
-        opts->params.skip_target_clearing = true;
+        tmp_params.skip_target_clearing = true;
 #endif
     }
 
-- 
2.49.1


>From ba24b8bf6f6003901af92c83a442d49263cab9d3 Mon Sep 17 00:00:00 2001
From: Niklas Haas <git@haasn.dev>
Date: Thu, 11 Sep 2025 01:48:32 +0200
Subject: [PATCH 2/2] avfilter/vf_libplacebo: force premultiplied blending for
 linear texture

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.
---
 libavfilter/vf_libplacebo.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/libavfilter/vf_libplacebo.c b/libavfilter/vf_libplacebo.c
index 8f30eb193b..80d3b273bb 100644
--- a/libavfilter/vf_libplacebo.c
+++ b/libavfilter/vf_libplacebo.c
@@ -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) {
-- 
2.49.1

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