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

> +        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];
>       }
>   
>       return ff_filter_frame(ctx->outputs[0], main_buf);
> @@ -212,4 +224,4 @@ const FFFilter ff_vf_alphamerge = {
>       FILTER_QUERY_FUNC2(query_formats),
>       .uninit         = uninit,
>       .activate       = activate,
> -};
> +};
> \ No newline at end of file
Please fix this.

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