Gif output not preserving source duration for last frame

[bjorn]

Summary of the bug:
How to reproduce:

% ffmpeg -i source_lightning.gif out.gif
ffmpeg version
ffmpeg version 4.1.3 Copyright (c) 2000-2019 the FFmpeg developers
built with Apple LLVM version 10.0.1 (clang-1001.0.46.4)
configuration: --prefix=/usr/local/Cellar/ffmpeg/4.1.3_1 --enable-shared --enable-pthreads --enable-version3 --enable-hardcoded-tables --enable-avresample --cc=clang --host-cflags='-I/Library/Java/JavaVirtualMachines/adoptopenjdk-11.0.2.jdk/Contents/Home/include -I/Library/Java/JavaVirtualMachines/adoptopenjdk-11.0.2.jdk/Contents/Home/include/darwin' --host-ldflags= --enable-ffplay --enable-gnutls --enable-gpl --enable-libaom --enable-libbluray --enable-libmp3lame --enable-libopus --enable-librubberband --enable-libsnappy --enable-libtesseract --enable-libtheora --enable-libvorbis --enable-libvpx --enable-libx264 --enable-libx265 --enable-libxvid --enable-lzma --enable-libfontconfig --enable-libfreetype --enable-frei0r --enable-libass --enable-libopencore-amrnb --enable-libopencore-amrwb --enable-libopenjpeg --enable-librtmp --enable-libspeex --enable-videotoolbox --disable-libjack --disable-indev=jack --enable-libaom --enable-libsoxr
libavutil      56. 22.100 / 56. 22.100
libavcodec     58. 35.100 / 58. 35.100
libavformat    58. 20.100 / 58. 20.100
libavdevice    58.  5.100 / 58.  5.100
libavfilter     7. 40.101 /  7. 40.101
libavresample   4.  0.  0 /  4.  0.  0
libswscale      5.  3.100 /  5.  3.100
libswresample   3.  3.100 /  3.  3.100
libpostproc    55.  3.100 / 55.  3.100

out.gif will have the wrong timing for the last frame. you can see this visually or in a tool such as imagemagick:

$ identify -format "%T\n" source_lightning.gif 
1
1
1
1
1
5
1
1
450
$ identify -format "%T\n" out.gif 
10
10
10
10
10
5
10
10
10

[bjorn]

This looks similar to ​https://trac.ffmpeg.org/ticket/6294

[Jacob Graff]

One fix for this in fftools/ffmpeg.c is to copy the duration from the frame to the packet during do_video_out calls, eg:

while (1) {
    ret = avcodec_receive_packet(enc, &pkt);
    if (pkt.duration <= 0)
        pkt.duration = av_rescale_q(in_picture->pkt_duration, ost->mux_timebase, enc->time_base);

And then use the packet duration, rather than the pts, during the gif encode step. However, this fix is probably not general enough, since other which attempts to write out gif packets could still face this issue. Ideally, packet duration should always be copied (and not zeroed out) during filtering, however it appears that at some point between reading packets in and writing them out, the duration gets dropped.

[Elon Musk]

Fixed in ff72256235aeaa2a4e197b54f69bad36d61a57d0