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Trying to debug blending images over video led to this observation:

ffmpeg -i https://lh3.googleusercontent.com/vt1VzlKCpjJjxRGBd3oAVgOHau22m2257fDBaW3KfHsNbhOcw1--OY7oISUM93Zp0SjwCSdlHP5PXXbFHdaflxTJAJ_B32MSrUH4AerS_0u6gv3X -i https://lh3.googleusercontent.com/MMzUU7GklKAqtdcGN4k2UVF22Wc2l35A2NDEeB4GhgM7eU06nKSQgitHAoWttj605G1yjoOEODt-dMKNPoWUdCmtFxe-u-qH5H228E9DxpoZsZk -filter_complex "[0:v] format=rgba [bg]; [1:v] format=rgba [fg]; [bg][fg] blend=all_mode='overlay':all_opacity=1, format=rgba"  out.png

results in (with odd fringes): enter image description here

desired image is:

enter image description here

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The blend filter ignores alpha from second input. You will need to recombine alpha component back into other components somehow. This one works for me:

ffmpeg -i a.png -f lavfi -i color=gray:s=512x512,format=gbrp -i b.png -lavfi "[1][2]overlay=format=gbrp[a],[0:v][a]blend=all_mode=overlay" out.png

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