[FFmpeg-devel] [PR] Revert "lavfi: make sure frame SAR matches the link value" (PR #21482)

[Marton Balint via ffmpeg-devel <ffmpeg-devel@ffmpeg.org>]

PR #21482 opened by Marton Balint (cus)
URL: https://code.ffmpeg.org/FFmpeg/FFmpeg/pulls/21482
Patch URL: https://code.ffmpeg.org/FFmpeg/FFmpeg/pulls/21482.patch

This reverts commit fcc33ada47e594d6b35b71c4a998af796f56492e.
    
It breaks playback of files with changing aspect ratio in ffplay, such as [1], because the filter chain is initialized with the initial aspect ratio, so any change is overwritten.

Michael already reported this issue at the time of patch submission [2], but it was applied anyway.

Ideally we could reinitialize the filter chain, but we never wanted to do that just for an aspect ratio change, because a filter chain reinitialization in the middle of playback / encoding can cause even worse issues. So I think it is better to just revert this.
    
[1] https://samples.ffmpeg.org/archive/container/mpegts/mpegts+mpeg2video+mp2++aspect.ts
[2] https://ffmpeg.org/pipermail/ffmpeg-devel/2023-July/312175.html


>From b306cc24bf8d023d579b0637f58ce8cb0039cfb8 Mon Sep 17 00:00:00 2001
From: Marton Balint <cus@passwd.hu>
Date: Fri, 16 Jan 2026 00:11:57 +0100
Subject: [PATCH 1/2] avfilter: do not overwrite frame aspect ratio with source
 frame props in histogram and vectorscrope

Signed-off-by: Marton Balint <cus@passwd.hu>
---
 libavfilter/vf_histogram.c   | 1 +
 libavfilter/vf_vectorscope.c | 1 +
 2 files changed, 2 insertions(+)

diff --git a/libavfilter/vf_histogram.c b/libavfilter/vf_histogram.c
index f77148cdfe..1a5626b317 100644
--- a/libavfilter/vf_histogram.c
+++ b/libavfilter/vf_histogram.c
@@ -594,6 +594,7 @@ static int filter_frame(AVFilterLink *inlink, AVFrame *in)
     }
 
     av_frame_copy_props(out, in);
+    out->sample_aspect_ratio = outlink->sample_aspect_ratio;
     av_frame_free(&in);
     s->x_pos++;
     if (s->x_pos >= s->width) {
diff --git a/libavfilter/vf_vectorscope.c b/libavfilter/vf_vectorscope.c
index eeee166172..483132332e 100644
--- a/libavfilter/vf_vectorscope.c
+++ b/libavfilter/vf_vectorscope.c
@@ -1469,6 +1469,7 @@ static int filter_frame(AVFilterLink *inlink, AVFrame *in)
         return AVERROR(ENOMEM);
     }
     av_frame_copy_props(out, in);
+    out->sample_aspect_ratio = outlink->sample_aspect_ratio;
 
     s->vectorscope(s, in, out, s->pd);
     s->graticulef(s, out, s->x, s->y, s->pd, s->cs);
-- 
2.52.0


>From 8b13e72ad8bb6c1d64606481049f7dbaf0c1cd85 Mon Sep 17 00:00:00 2001
From: Marton Balint <cus@passwd.hu>
Date: Wed, 14 Jan 2026 23:03:26 +0100
Subject: [PATCH 2/2] Revert "lavfi: make sure frame SAR matches the link
 value"

This reverts commit fcc33ada47e594d6b35b71c4a998af796f56492e.

It breaks playback of files with changing aspect ratio in ffplay, such as [1],
because the filter chain is initialized with the initial aspect ratio, so any
change is overwritten.

[1] https://samples.ffmpeg.org/archive/container/mpegts/mpegts+mpeg2video+mp2++aspect.ts
---
 libavfilter/avfilter.c | 2 --
 1 file changed, 2 deletions(-)

diff --git a/libavfilter/avfilter.c b/libavfilter/avfilter.c
index 169c2baa42..bb0b665f0c 100644
--- a/libavfilter/avfilter.c
+++ b/libavfilter/avfilter.c
@@ -1084,8 +1084,6 @@ int ff_filter_frame(AVFilterLink *link, AVFrame *frame)
             if (av_pix_fmt_desc_get(link->format)->flags & AV_PIX_FMT_FLAG_ALPHA)
                 av_assert1(frame->alpha_mode == link->alpha_mode);
         }
-
-        frame->sample_aspect_ratio = link->sample_aspect_ratio;
     } else {
         if (frame->format != link->format) {
             av_log(link->dst, AV_LOG_ERROR, "Format change is not supported\n");
-- 
2.52.0

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