Important note: Telegram supports WebM stickers since 2022. Use /newvideo to create a pack of such stickers.
It made this tool effectively obsolete. The tool still might have some limited scope (e.g., if you want to add stickers to an existing animated pack), or if the source GIFs are well-suitable for it (if you don't know what you're doing, they're most probably not), but otherwise it's now useless.
Theoretically, it might receive some updates (and I'm ready to accept PRs), so I'm not archiving it yet. But most likely, I won't ever return to it. So long, and thanks for the fish.
python3 read.py input.gif output.tgs
Requires gifsicle.
Python dependencies can be installed with:
pip3 install -r requirements.txt
This is an early prototype. It's already able to successfully convert simple animations, though even tiny ones rarely fit the Telegram limitations (64 kiB for gzipped and 1 MB for ungzipped data). Fortunately, there is still a big room for compression improvements and for optional vectorization.
Also, you can already use this for other Lottie-enabled applications, regardless of Telegram limits (if you really have some reason to use Lottie instead of GIFs).
The solution is completely independent from After Effects. The Lottie encoder is built from scratch by investigating the JSON schema in lottie-web and a bit of reverse engineering. The GIFs are read with a slightly modified gif2numpy library.