[FFmpeg-devel] Re: [PATCH] apng: fix rgb24 blending bug for non-alpha frames

[Michael Niedermayer via ffmpeg-devel <ffmpeg-devel@ffmpeg.org>]

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On Fri, Jan 02, 2026 at 01:29:32PM +0200, Hassan Hany via ffmpeg-devel wrote:
> From: "U-DESKTOP-70DF2U5\\GIGABYT" <hassanhanyrashad@gmail.com>
> 
> Delta frame optimization in APNG encoder caused gray bars for RGB24 frames.
> This patch disables delta optimization for non-alpha frames, ensuring correct
> rendering. Delta optimization is preserved for RGBA frames.
> 
> Fixes: trac Ticket #9602
> 
> Signed-off-by: U-DESKTOP-70DF2U5\GIGABYT <hassanhanyrashad@gmail.com>
> ---
> This bug caused visible artifacts such as gray bars in non-alpha 
> pixel formats such as RGB24 or GRAY. This patch disabled delta frame
> optimization only for non alpha channels. the output size increased
> from 512kb to 1.1mb on the test sample but correctness was restored
> and the relevant APNG FATE tests were ran and no regression occurred
>  libavcodec/pngenc.c | 15 ++++++++++++++-
>  1 file changed, 14 insertions(+), 1 deletion(-)
> 
> diff --git a/libavcodec/pngenc.c b/libavcodec/pngenc.c
> index e627bf83fc..3e46363b00 100644
> --- a/libavcodec/pngenc.c
> +++ b/libavcodec/pngenc.c
> @@ -39,7 +39,7 @@
>  #include "libavutil/opt.h"
>  #include "libavutil/rational.h"
>  #include "libavutil/stereo3d.h"
> -
> +#include "libavutil/pixdesc.h"
>  #include <zlib.h>
>  
>  #define IOBUF_SIZE 4096
> @@ -894,6 +894,19 @@ static int apng_encode_frame(AVCodecContext *avctx, const AVFrame *pict,
>          best_fctl_chunk->blend_op = APNG_BLEND_OP_SOURCE;
>          return encode_frame(avctx, pict);
>      }
> +    
> +    const AVPixFmtDescriptor *desc = av_pix_fmt_desc_get(pict->format);
> +        
> +    if(!(desc->flags & AV_PIX_FMT_FLAG_ALPHA)){
> +         best_fctl_chunk->width = pict->width;
> +         best_fctl_chunk->height = pict->height;
> +         best_fctl_chunk->x_offset = 0;
> +         best_fctl_chunk->y_offset = 0;
> +         best_fctl_chunk->blend_op = APNG_BLEND_OP_SOURCE;
> +         best_fctl_chunk->dispose_op = APNG_DISPOSE_OP_NONE;
> +          return encode_frame(avctx, pict);
> +         } 
> +    

This looks like a hack not like a bugfix
besides the indention is wrong and there are tabs

thx

[...]
-- 
Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

If the United States is serious about tackling the national security threats 
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values corporate profits and government espionage over security.-Bruce Schneier

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