Re: [FFmpeg-devel] [PATCH] avfilter/vf_alphamerge: use refcounting for planar formats

[James Almer]

On 8/7/2025 4:58 PM, Jorge Estrada wrote:

Use reference plane when handling planar formats. Should address
https://code.ffmpeg.org/FFmpeg/FFmpeg/issues/20153

Example:
ffmpeg -f lavfi -i "color=red:s=640x480:d=5,format=yuva420p" \
        -f lavfi -i 
"color=black:s=640x480:d=5,format=yuv420p,geq=lum='255*gt(150,hypot(X-W/2,Y-H/2))':a=0"
 \
        -f lavfi -i "color=blue:s=640x480:d=5,format=yuv420p" \
        -filter_complex 
"[0:v][1:v]alphamerge[merged_with_alpha];[2:v][merged_with_alpha]overlay" \
        -c:v libx264 \
        -y out.mp4
---
  libavfilter/vf_alphamerge.c | 26 +++++++++++++++++++-------
  1 file changed, 19 insertions(+), 7 deletions(-)

diff --git a/libavfilter/vf_alphamerge.c b/libavfilter/vf_alphamerge.c
index f5779484a9..9f53806537 100644
--- a/libavfilter/vf_alphamerge.c
+++ b/libavfilter/vf_alphamerge.c
@@ -28,12 +28,12 @@
  #include "libavutil/imgutils.h"
  #include "libavutil/opt.h"
  #include "libavutil/pixfmt.h"
+#include "libavutil/frame.h"
  #include "avfilter.h"
  #include "drawutils.h"
  #include "formats.h"
  #include "filters.h"
  #include "framesync.h"
-#include "video.h"

              }
          }
      } else {
-        const int main_linesize = main_buf->linesize[A];
-        const int alpha_linesize = alpha_buf->linesize[Y];
-        av_image_copy_plane(main_buf->data[A], main_linesize,
-                            alpha_buf->data[Y], alpha_linesize,
-                            FFMIN(main_linesize, alpha_linesize), 
alpha_buf->height);
+        AVBufferRef *alpha_plane_buf = av_frame_get_plane_buffer(alpha_buf, Y);
+
+        if (!alpha_plane_buf) {
+            av_log(ctx, AV_LOG_ERROR, "Could not get buffer for alpha 
plane.\n");
+            return AVERROR(EINVAL);
+        }
+

enum { Y, U, V, A }; @@ -78,11 +78,23 @@ static int do_alphamerge(FFFrameSync *fs)

+        av_buffer_unref(&main_buf->buf[A]);
+
+        main_buf->buf[A] = av_buffer_ref(alpha_plane_buf);

av_buffer_replace()

+        if (!main_buf->buf[A]) {
+            av_log(ctx, AV_LOG_ERROR, "Failed to reference alpha plane 
buffer.\n");
+            return AVERROR(ENOMEM);
+        }
+
+        main_buf->data[A] = alpha_buf->data[Y];
+        main_buf->linesize[A] = alpha_buf->linesize[Y];
      }

@@ -212,4 +224,4 @@ const FFFilter ff_vf_alphamerge = {
      FILTER_QUERY_FUNC2(query_formats),
      .uninit         = uninit,
      .activate       = activate,
-};
+};
\ No newline at end of file

return ff_filter_frame(ctx->outputs[0], main_buf);

Please fix this.

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