Skewed png images for certain resolutions when making thumbnail

[Kevin Sallee]

Summary of the bug:
How to reproduce:

% ffmpeg -i skew.png -vf "scale=force_original_aspect_ratio=decrease:w=2048:h=2048, thumbnail" -frames:v 1 skew_thumb.png
ffmpeg version 5.1.2
built on OSX

For certain PNG image resolutions, the command results in a skewed output. I am attaching examples of a PNG that does not cause the skew, and one that causes it.
Previous versions of ffmpeg were not exhibiting this behavior

The full output of the command failing:

[Kevin Sallee]

A PNG that causes a skew

[Kevin Sallee]

A PNG that does not cause it.

[Carl Eugen Hoyos]

I cannot reproduce, please test current FFmpeg git head, the only version supported on this bug tracker and provide the command line you tested together with the complete, uncut console output.

[Kevin Sallee]

The full output of the command:

ffmpeg -y -i skew.png -vf "scale=force_original_aspect_ratio=decrease:w=2048:h=2048:flags=fast_bilinear, thumbnail" -frames:v 1 skew_thumb.png
ffmpeg version N-107731-g5cdf4c0bed-tessus Copyright (c) 2000-2022 the FFmpeg developers

built with Apple clang version 11.0.0 (clang-1100.0.33.17)
configuration: --cc=/usr/bin/clang --prefix=/opt/ffmpeg --extra-version=tessus --enable-avisynth --enable-fontconfig --enable-gpl --enable-libaom --enable-libass --enable-libbluray --enable-libdav1d --enable-libfreetype --enable-libgsm --enable-libmodplug --enable-libmp3lame --enable-libmysofa --enable-libopencore-amrnb --enable-libopencore-amrwb --enable-libopenh264 --enable-libopenjpeg --enable-libopus --enable-librubberband --enable-libshine --enable-libsnappy --enable-libsoxr --enable-libspeex --enable-libtheora --enable-libtwolame --enable-libvidstab --enable-libvmaf --enable-libvo-amrwbenc --enable-libvorbis --enable-libvpx --enable-libwebp --enable-libx264 --enable-libx265 --enable-libxavs --enable-libxvid --enable-libzimg --enable-libzmq --enable-libzvbi --enable-version3 --pkg-config-flags=--static --disable-ffplay
libavutil 57. 32.101 / 57. 32.101
libavcodec 59. 42.100 / 59. 42.100
libavformat 59. 30.100 / 59. 30.100
libavdevice 59. 8.101 / 59. 8.101
libavfilter 8. 46.101 / 8. 46.101
libswscale 6. 8.102 / 6. 8.102
libswresample 4. 8.100 / 4. 8.100
libpostproc 56. 7.100 / 56. 7.100

Input #0, png_pipe, from 'skew.png':

Duration: N/A, bitrate: N/A
Stream #0:0: Video: png, rgb24(pc), 3121x1776 [SAR 2835:2835 DAR 3121:1776], 25 fps, 25 tbr, 25 tbn

Stream mapping:

Stream #0:0 -> #0:0 (png (native) -> png (native))

Press [q] to stop, ? for help
[Parsed_thumbnail_1 @ 0x7f9cdb8105c0] frame id #0 (pts_time=0.000000) selected from a set of 1 images
Output #0, image2, to 'skew_thumb.png':

Metadata:

encoder : Lavf59.30.100

Stream #0:0: Video: png, rgb24(pc, gbr/unknown/unknown, progressive), 2048x1165 [SAR 3635965:3637248 DAR 3121:1776], q=2-31, 200 kb/s, 25 fps, 25 tbn

Metadata:

encoder : Lavc59.42.100 png

[image2 @ 0x7f9cdb804f00] The specified filename 'skew_thumb.png' does not contain an image sequence pattern or a pattern is invalid.
[image2 @ 0x7f9cdb804f00] Use a pattern such as %03d for an image sequence or use the -update option (with -frames:v 1 if needed) to write a single image.
frame= 1 fps=0.0 q=-0.0 Lsize=N/A time=00:00:00.00 bitrate=N/A speed= 0x
video:1314kB audio:0kB subtitle:0kB other streams:0kB global headers:0kB muxing overhead: unknown

[Elon Musk]

Nothing to do with thumbnail filter. It is scale filter issue.