ffmpeg: pad filter with transparent background for GIF input/output? - ffmpeg

Question

I'm trying to pad the GIF source to a square thumbnail and the extended area should be transparent.
(image source: https://store.steampowered.com/app/2113680/The_Dot/)
To achieve the goal, I used the following command:
ffmpeg -i ./test/source.gif -filter_complex "[0]scale=255:144[s1];[s1]pad=256:256:0:-56:blue[s2];[s2]split[s3][s4];[s3]palettegen=reserve_transparent=on:transparency_color=blue[p];[s4][p]paletteuse[s5]" -map [s5] -loop 0 ./test/output.gif -y
[0]scale=255:144[s1] First I scale down the image.
[s1]pad=256:256:0:-56:blue[s2] Then pad the image with an uncommon background color: blue
[s2]split[s3][s4] And I split to two streams. One for palettegen and another for paletteuse.
[s3]palettegen=reserve_transparent=on:transparency_color=blue[p] Generate the color palette that indicates blue as the transparency_color
[s4][p]paletteuse[s5] Use the new color palette for output. The padded image with blue background color should be treated as a transparent background.
But the output animated GIF still has a visible blue background color no matter if I put the paletteuse at first or in the end.
Related posts:
Making GIFs with ffmpeg: what does "transparency_color" do?
How do I resize an animated GIF and keep transparency?
Transparency is gone while resizing (scaling) gif using ffmpeg

Answer 1

overlay on transparent background:
ffmpeg -i 74bRi.gif -filter_complex "
color=s=256x256:d=100,format=argb,colorchannelmixer=aa=0.0[b];
[0]scale=255:144[f];
[b][f]overlay=(W-w)/2:(H-h)/2:shortest=1,split[s1][s2];
[s1]palettegen[p];
[s2][p]paletteuse
" -loop 0 output.gif
pad using transparent color ffffff00
ffmpeg -i 74bRi.gif -filter_complex "
[0]scale=255:144,pad=256:256:0:-1:ffffff00,split[s1][s2];
[s1]palettegen[p];
[s2][p]paletteuse
" -loop 0 out.gif