[FFmpeg-user] PNG-->GIF produces weird color aliasing?

[Paul B Mahol]

On Sun, May 19, 2024 at 7:02 PM Steven Kan <steven at kan.org> wrote:

> I have a series of 476 PNGs, exported from Excel in a loop through a
> series of values to control the plotted range:
>
> https://www.kan.org/download/1.png
> https://www.kan.org/download/2.png
> https://www.kan.org/download/3.png
> https://www.kan.org/download/4.png
> https://www.kan.org/download/5.png
> etc.
>
> If I convert them to MP4:
>
> ffmpeg -i %d.png -vf "pad=ceil(iw/2)*2:ceil(ih/2)*2,scale=1920:-1" -c:v
> libx264 -y WeightLogLapse.mp4
>
> the resulting video looks as I would expect:
>
> https://www.kan.org/download/WeightLogLapse.mp4
>
> But if I convert to GIF:
>
> ffmpeg -i %d.png -vf "pad=ceil(iw/2)*2:ceil(ih/2)*2,scale=1920:-1" -y
> WeightLogLapse.gif
>
> I get this weird color aliasing:
>
> https://www.kan.org/download/WeightLogLapse.gif
>
> Am I doing this incorrectly? Thanks!
>

Such wrong yellow colors are caused by buggy X -> RGB8 pixel format
conversion in libswscale code.

To get smaller and higher quality gifs use palettegen+paletteuse filters
approach.


>
> ffmpeg -i %d.png -vf "pad=ceil(iw/2)*2:ceil(ih/2)*2,scale=1920:-1" -y
> WeightLogLapse.gif
> ffmpeg version 7.0 Copyright (c) 2000-2024 the FFmpeg developers
>   built with Apple clang version 13.1.6 (clang-1316.0.21.2.5)
>   configuration: --prefix=/Volumes/tempdisk/sw
> --extra-cflags=-fno-stack-check --arch=arm64 --cc=/usr/bin/clang
> --enable-gpl --enable-libvmaf --enable-libopenjpeg --enable-libopus
> --enable-libmp3lame --enable-libx264 --enable-libx265 --enable-libvpx
> --enable-libwebp --enable-libass --enable-libfreetype --enable-fontconfig
> --enable-libtheora --enable-libvorbis --enable-libsnappy --enable-libaom
> --enable-libvidstab --enable-libzimg --enable-libsvtav1
> --enable-libharfbuzz --enable-libkvazaar --pkg-config-flags=--static
> --enable-ffplay --enable-postproc --enable-neon --enable-runtime-cpudetect
> --disable-indev=qtkit --disable-indev=x11grab_xcb
>   libavutil      59.  8.100 / 59.  8.100
>   libavcodec     61.  3.100 / 61.  3.100
>   libavformat    61.  1.100 / 61.  1.100
>   libavdevice    61.  1.100 / 61.  1.100
>   libavfilter    10.  1.100 / 10.  1.100
>   libswscale      8.  1.100 /  8.  1.100
>   libswresample   5.  1.100 /  5.  1.100
>   libpostproc    58.  1.100 / 58.  1.100
> Input #0, image2, from '%d.png':
>   Duration: 00:00:18.64, start: 0.000000, bitrate: N/A
>   Stream #0:0: Video: png, rgba(pc, gbr/bt709/iec61966-2-1), 2808x1550
> [SAR 5669:5669 DAR 1404:775], 25 fps, 25 tbr, 25 tbn
> Stream mapping:
>   Stream #0:0 -> #0:0 (png (native) -> gif (native))
> Press [q] to stop, [?] for help
> Output #0, gif, to 'WeightLogLapse.gif':
>   Metadata:
>     encoder         : Lavf61.1.100
>   Stream #0:0: Video: gif, bgr8(pc, gbr/bt709/iec61966-2-1, progressive),
> 1920x1060 [SAR 6201:6200 DAR 1404:775], q=2-31, 200 kb/s, 25 fps, 100 tbn
>       Metadata:
>         encoder         : Lavc61.3.100 gif
> [out#0/gif @ 0x600001210000] video:13110KiB audio:0KiB subtitle:0KiB other
> streams:0KiB global headers:0KiB muxing overhead: 0.000149%
> frame=  466 fps= 31 q=-0.0 Lsize=   13110KiB time=00:00:18.64
> bitrate=5761.8kbits/s speed=1.23x
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