When decoding apng the image has uncleaned parts.

[Ted Zyzsdy]

Summary of the bug:

I need to convert the format of a set of apng files to gif. But I found in the conversion result that part of the image was not cleaned properly, but overlapped on top of the canvas. To determine whether the problem was in decoding apng or encoding gif, I converted the apng as a sequence of jpeg images and still found the same problem.

Then, I checked at the apng file, and in the fcTL block at frame 7, the dispose_op is marked as 0x01 APNG_DISPOSE_OP_BACKGROUND, indicating that the background needs to be cleared before the frame is rendered, but ffmpeg doesn't actually do that.

How to reproduce:

% ffmpeg.exe -i apng_image_uncleaned_part.png -f image2 de-%03d.jpg
ffmpeg version 2022-01-17-git-dcc9454ab9-full_build-www.gyan.dev Copyright (c) 2000-2022 the FFmpeg developers
  built with gcc 11.2.0 (Rev5, Built by MSYS2 project)
  configuration: --enable-gpl --enable-version3 --enable-static --disable-w32threads --disable-autodetect --enable-fontconfig --enable-iconv --enable-gnutls --enable-libxml2 --enable-gmp --enable-bzlib --enable-lzma --enable-libsnappy --enable-zlib --enable-librist --enable-libsrt --enable-libssh --enable-libzmq --enable-avisynth --enable-libbluray --enable-libcaca --enable-sdl2 --enable-libdav1d --enable-libdavs2 --enable-libuavs3d --enable-libzvbi --enable-librav1e --enable-libsvtav1 --enable-libwebp --enable-libx264 --enable-libx265 --enable-libxavs2 --enable-libxvid --enable-libaom --enable-libopenjpeg --enable-libvpx --enable-mediafoundation --enable-libass --enable-frei0r --enable-libfreetype --enable-libfribidi --enable-libvidstab --enable-libvmaf --enable-libzimg --enable-amf --enable-cuda-llvm --enable-cuvid --enable-ffnvcodec --enable-nvdec --enable-nvenc --enable-d3d11va --enable-dxva2 --enable-libmfx --enable-libshaderc --enable-vulkan --enable-libplacebo --enable-opencl --enable-libcdio --enable-libgme --enable-libmodplug --enable-libopenmpt --enable-libopencore-amrwb --enable-libmp3lame --enable-libshine --enable-libtheora --enable-libtwolame --enable-libvo-amrwbenc --enable-libilbc --enable-libgsm --enable-libopencore-amrnb --enable-libopus --enable-libspeex --enable-libvorbis --enable-ladspa --enable-libbs2b --enable-libflite --enable-libmysofa --enable-librubberband --enable-libsoxr --enable-chromaprint
  libavutil      57. 18.100 / 57. 18.100
  libavcodec     59. 20.100 / 59. 20.100
  libavformat    59. 17.101 / 59. 17.101
  libavdevice    59.  5.100 / 59.  5.100
  libavfilter     8. 25.100 /  8. 25.100
  libswscale      6.  5.100 /  6.  5.100
  libswresample   4.  4.100 /  4.  4.100
  libpostproc    56.  4.100 / 56.  4.100
Input #0, apng, from 'apng_image_uncleaned_part.png':
  Duration: N/A, bitrate: N/A
  Stream #0:0: Video: apng, rgba(pc), 370x300, 20 fps, 20 tbr, 100k tbn
Stream mapping:
  Stream #0:0 -> #0:0 (apng (native) -> mjpeg (native))
Press [q] to stop, [?] for help
[swscaler @ 00000264be1f9280] [swscaler @ 00000264be206240] deprecated pixel format used, make sure you did set range correctly
[swscaler @ 00000264be1f9280] [swscaler @ 00000264be219600] deprecated pixel format used, make sure you did set range correctly
[swscaler @ 00000264be1f9280] [swscaler @ 00000264be446340] deprecated pixel format used, make sure you did set range correctly
[swscaler @ 00000264be1f9280] [swscaler @ 00000264be81dfc0] deprecated pixel format used, make sure you did set range correctly
[swscaler @ 00000264be1f9280] [swscaler @ 00000264be83afc0] deprecated pixel format used, make sure you did set range correctly
[swscaler @ 00000264be1f9280] [swscaler @ 00000264be8594c0] deprecated pixel format used, make sure you did set range correctly
[swscaler @ 00000264be1f9280] [swscaler @ 00000264be8800c0] deprecated pixel format used, make sure you did set range correctly
[swscaler @ 00000264be1f9280] [swscaler @ 00000264be8adcc0] deprecated pixel format used, make sure you did set range correctly
[swscaler @ 00000264be1f9280] [swscaler @ 00000264be8cdcc0] deprecated pixel format used, make sure you did set range correctly
Output #0, image2, to 'de-%03d.jpg':
  Metadata:
    encoder         : Lavf59.17.101
  Stream #0:0: Video: mjpeg, yuvj444p(pc, progressive), 370x300, q=2-31, 200 kb/s, 20 fps, 20 tbn
    Metadata:
      encoder         : Lavc59.20.100 mjpeg
    Side data:
      cpb: bitrate max/min/avg: 0/0/200000 buffer size: 0 vbv_delay: N/A
frame=   20 fps=0.0 q=24.8 Lsize=N/A time=00:00:01.00 bitrate=N/A speed=19.9x
video:236kB audio:0kB subtitle:0kB other streams:0kB global headers:0kB muxing overhead: unknown

However, when I try to convert with an older version of ffmpeg 4.3.1 downloaded the year before, this problem does not exist.

% ffmpeg.exe -i apng_image_uncleaned_part.png -f image2 de-%03d.jpg
ffmpeg version 4.3.1-2020-11-08-full_build-www.gyan.dev Copyright (c) 2000-2020 the FFmpeg developers
  built with gcc 10.2.0 (Rev3, Built by MSYS2 project)
  configuration: --enable-gpl --enable-version3 --enable-static --disable-w32threads --disable-autodetect --enable-fontconfig --enable-iconv --enable-gnutls --enable-libxml2 --enable-gmp --enable-lzma --enable-libsnappy --enable-zlib --enable-libsrt --enable-libssh --enable-libzmq --enable-avisynth --enable-libbluray --enable-libcaca --enable-sdl2 --enable-libdav1d --enable-libzvbi --enable-librav1e --enable-libwebp --enable-libx264 --enable-libx265 --enable-libxvid --enable-libaom --enable-libopenjpeg --enable-libvpx --enable-libass --enable-frei0r --enable-libfreetype --enable-libfribidi --enable-libvidstab --enable-libvmaf --enable-libzimg --enable-amf --enable-cuda-llvm --enable-cuvid --enable-ffnvcodec --enable-nvdec --enable-nvenc --enable-d3d11va --enable-dxva2 --enable-libmfx --enable-libcdio --enable-libgme --enable-libmodplug --enable-libopenmpt --enable-libopencore-amrwb --enable-libmp3lame --enable-libshine --enable-libtheora --enable-libtwolame --enable-libvo-amrwbenc --enable-libilbc --enable-libgsm --enable-libopencore-amrnb --enable-libopus --enable-libspeex --enable-libvorbis --enable-ladspa --enable-libbs2b --enable-libflite --enable-libmysofa --enable-librubberband --enable-libsoxr --enable-chromaprint
  libavutil      56. 51.100 / 56. 51.100
  libavcodec     58. 91.100 / 58. 91.100
  libavformat    58. 45.100 / 58. 45.100
  libavdevice    58. 10.100 / 58. 10.100
  libavfilter     7. 85.100 /  7. 85.100
  libswscale      5.  7.100 /  5.  7.100
  libswresample   3.  7.100 /  3.  7.100
  libpostproc    55.  7.100 / 55.  7.100
Input #0, apng, from 'apng_image_uncleaned_part.png':
  Duration: N/A, bitrate: N/A
    Stream #0:0: Video: apng, rgba(pc), 370x300, 20 fps, 20 tbr, 100k tbn, 100k tbc
Stream mapping:
  Stream #0:0 -> #0:0 (apng (native) -> mjpeg (native))
Press [q] to stop, [?] for help
[swscaler @ 000002071a438f80] deprecated pixel format used, make sure you did set range correctly
Output #0, image2, to 'de-%03d.jpg':
  Metadata:
    encoder         : Lavf58.45.100
    Stream #0:0: Video: mjpeg, yuvj444p(pc), 370x300, q=2-31, 200 kb/s, 20 fps, 20 tbn, 20 tbc
    Metadata:
      encoder         : Lavc58.91.100 mjpeg
    Side data:
      cpb: bitrate max/min/avg: 0/0/200000 buffer size: 0 vbv_delay: N/A
frame=   20 fps=0.0 q=24.8 Lsize=N/A time=00:00:01.00 bitrate=N/A speed=17.6x
video:233kB audio:0kB subtitle:0kB other streams:0kB global headers:0kB muxing overhead: unknown

This version outputs the expected result.

The original apng file uploaded as attachment apng_image_uncleaned_part.png
Expected result (frame 7) uploaded as attachment apng_uncleaned_part_frame_7_expected_result.jpg
The problem result (frame 7) uploaded as attachment apng_uncleaned_part_frame_7_defected_result.jpg

[mkver]

Fixed in a84087eaf8b806380dc8d50ea367816b12b99580.