ELBG Posterizer creates flickering effect

[erjiang]

At some point the ELBG filter changed, and now creates a flickering effect that it did not use to create. The below example renders a video where the black color sometimes appears and sometimes does not.

$ ffmpeg -version
ffmpeg version N-63101-gc92edd969a-static https://johnvansickle.com/ffmpeg/  Copyright (c) 2000-2022 the FFmpeg developers
built with gcc 8 (Debian 8.3.0-6)
configuration: --enable-gpl --enable-version3 --enable-static --disable-debug --disable-ffplay --disable-indev=sndio --disable-outdev=sndio --cc=gcc --enable-fontconfig --enable-frei0r --enable-gnutls --enable-gmp --enable-libgme --enable-gray --enable-libaom --enable-libfribidi --enable-libass --enable-libvmaf --enable-libfreetype --enable-libmp3lame --enable-libopencore-amrnb --enable-libopencore-amrwb --enable-libopenjpeg --enable-librubberband --enable-libsoxr --enable-libspeex --enable-libsrt --enable-libvorbis --enable-libopus --enable-libtheora --enable-libvidstab --enable-libvo-amrwbenc --enable-libvpx --enable-libwebp --enable-libx264 --enable-libx265 --enable-libxml2 --enable-libdav1d --enable-libxvid --enable-libzvbi --enable-libzimg
libavutil      57. 36.101 / 57. 36.101
libavcodec     59. 42.104 / 59. 42.104
libavformat    59. 30.101 / 59. 30.101
libavdevice    59.  8.101 / 59.  8.101
libavfilter     8. 48.100 /  8. 48.100
libswscale      6.  8.108 /  6.  8.108
libswresample   4.  9.100 /  4.  9.100
libpostproc    56.  7.100 / 56.  7.100
$ ffmpeg -i Jane_M._Byrne_Interchange_Traffic_1080p.webm -vf elbg=codebook_length=3:nb_steps=2 output-new.mp4 2> stderr.log 1> stdout.log

Source video file can be downloaded from: ​https://invent.kde.org/erjiang/kdenlive-backcompat/-/blob/master/projects/footage/Jane_M._Byrne_Interchange_Traffic_1080p.webm

ffmpeg 4.4.2 with libavfilter=7.110.100 is known to work (doesn’t have the flickering).

[erjiang]

ffmpeg output from stderr

[mkver]

Regression since c1a49a1264926039fabdeb1c51909fc2c34e2414.