[FFmpeg-devel] [PATCH 08/24] fftools/ffmpeg_filter: remove an unnecessary sub2video_push_ref() call

[Nicolas George]

Anton Khirnov (12023-11-04):
> It only seems to produce duplicate frames.

Yes it does, that is the point of it. Without the duplicated frame, a
filter with synchronized inputs (like overlay) will accumulate video
frames while waiting for the next sub2video frame.

It is actually super easy to test: any file with both video and bitmap
subtitles will cause a huge memory consumption if there is a long enough
time without subtitles.

Since I am feeling super helpful, here is how I just tested:

./ffmpeg_g -lavfi testsrc2=s=720x480:d=150 -preset ultrafast -y /tmp/dummy.mkv
mkvmerge -o /tmp/dummy_with_sub.mkv /tmp/dummy.mkv $ffmpeg_fate/sub/vobsub.idx
limit addressspace 2G
./ffmpeg_g -xerror -i /tmp/dummy_with_sub.mkv -preset ultrafast -lavfi '[0:v][0:s]overlay' -y /tmp/dummy_with_hardsub.mkv
mplayer /tmp/dummy_with_hardsub.mkv -ss 2:05

The limit addressspace 2G is there so that the huge memory consumption
will hit something. Another way is to log the number of queued frames:

--- a/libavfilter/framequeue.c
+++ b/libavfilter/framequeue.c
@@ -90,2 +90,3 @@ int ff_framequeue_add(FFFrameQueue *fq, AVFrame *frame)
     fq->queued++;
+    av_log(0, 16, "queued = %zd\n", fq->queued);
     fq->total_frames_head++;

ffmpeg version N-112710-g86e0dea620 Copyright (c) 2000-2023 the FFmpeg developers
  built with gcc 13 (Debian 13.2.0-5)
  configuration: --enable-shared --disable-static --enable-gpl --enable-libx264 --enable-libopus --enable-libass --enable-libfreetype --enable-opengl --assert-level=2
  libavutil      58. 31.100 / 58. 31.100
  libavcodec     60. 32.102 / 60. 32.102
  libavformat    60. 17.100 / 60. 17.100
  libavdevice    60.  4.100 / 60.  4.100
  libavfilter     9. 13.100 /  9. 13.100
  libswscale      7.  6.100 /  7.  6.100
  libswresample   4. 13.100 /  4. 13.100
  libpostproc    57.  4.100 / 57.  4.100
Input #0, matroska,webm, from '/tmp/dummy_with_sub.mkv':
[…]
queued = 2210
queued = 2211
queued = 2212
queued = 2213
[h264 @ 0x562874ac4f40] get_buffer() failed
[h264 @ 0x562874ac4f40] thread_get_buffer() failed
[h264 @ 0x562874ac4f40] decode_slice_header error
[h264 @ 0x562874ac4f40] no frame!
[…]

-- 
  Nicolas George
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