The error “error while loading shared libraries” in this case just requires installing the missing dependencies via apt:
apt install libavformat-dev
In case of other errors, just install the other dependencies:
ffmpeg
libsdl2-dev
libsdl2-image-dev
libavcodec-dev
libavformat-dev
libavutil-dev
libswscale-dev
libomp-dev
zlib1g-dev
That should fix it. If it does not resolve the problem, you need to extract the AppImage and install all the missing dependencies:
./float-ditherer-x86_64.AppImage --appimage-extract
cd ./squashfs-root/usr/bin
ldd ./ditherer
This should list all of the dependencies. If there is a missing .so file, you can identify the corresponding package and install it with apt.