A script to transform Animated Webp files into Webm files. Tested on Windows only. Depends on ffmpeg and libwebp. Make sure to add them to your PATH.

aww.js

/**
 * This script transforms animated webp files which are not widely supported yet
 * to simple animated webm. Please state the source of your share this.
 *
 * # How this works
 * This script depends on ffmpeg and libwebp provided by Google. Make sure
 * they're in your PATH.
 *
 * The way this works is based off this answer https://askubuntu.com/a/1141049.
 * It starts by extracting the input webp frames to a ./frames directory and,
 * using webpmux, determines whether it's actually animated or not. If it is
 * animated then it proceeds to use ffmpeg to create a webm based on the
 * extracted frames. Cleanup is done after the script finishes successfully.
 *
 * It also takes a switch that determines whether you want to permanently delete
 * the original animated webp file.
 *
 * libwebp: https://developers.google.com/speed/webp/docs/precompiled
 * ffmpeg: https://ffmpeg.org/
 */

const child = require('child_process')
const fs = require('fs')
const path = require('path')
const util = require('util')

const exec = util.promisify(child.exec)

const terminateWithError = (error = '[fatal] error') => {
	console.log(error)
	process.exit(1)
}

const params = process.argv.slice(2)
if (!params[0]) terminateWithError('[fatal] need file name or absolute path')

const deleteOriginal = params[1] === 'true'
const filename = path.isAbsolute(params[0])
	? path.basename(params[0])
	: params[0]

if (deleteOriginal) console.log('[info] will delete original when successful')

const nameWithoutExt = filename.replace('.webp', '')
const frames = path.resolve(process.cwd(), 'frames')

if (fs.existsSync(frames)) fs.rmdirSync(frames, { recursive: true })
fs.mkdirSync(frames)

process.chdir('frames')
console.log('[info]', process.cwd())

console.log('[info]', `anim_dump ../${filename}`)
exec(`anim_dump ../${filename}`)
	.then(() => {
		process.chdir('..')
		console.log('[info]', process.cwd())

		const command = `webpmux -info ./${filename}`

		console.log('[info]', command)
		return exec(command)
	})
	.then(({ stdout, stderr }) => {
		if (stderr) return Promise.reject(stderr)

		const isAnimation = stdout.match(/Features present: animation/) !== null
		if (!isAnimation) return Promise.reject('This is not an animated webp file')

		const firstLine = stdout.match(/1:.+[\r]?\n/g)
		if (!firstLine) return

		const frameLength = firstLine[0].split(/\s+/g)[6]
		const framerate = Math.round(1000 / frameLength) // frames/second
		const dump = path.resolve(frames, 'dump_%04d.png')
		const command = `ffmpeg -framerate ${framerate} -i "${dump}" "${nameWithoutExt}.webm" -y`

		console.log('[info]', command)
		return exec(command)
	})
	.then(({ stdout, stderr }) => {
		if (/error/gm.test(stderr)) return Promise.reject(stderr)

		// cleanup
		fs.rmdirSync(frames, { recursive: true })
		if (deleteOriginal) fs.rmSync(path.resolve(process.cwd(), filename))

		console.log('[info] Success!\n')
	})
	.catch(err => {
		terminateWithError(`[fatal] ${err}`)
		fs.rmdirSync(frames, { recursive: true })
	})

@enzomtpYT I never did that but you can probably just switch up the extensions in the script and have it work just as well