Make perfectly looping cross-fading video. Using FFmpeg with transparency support (on .webm and .mov).

cf.mjs

#!/usr/bin/env zx

// Establish program arguments
const input = argv.i;
const output = argv.o;
const fadeDuration = parseInt(argv.f);
const useTransparency = argv.t;
// Quality options
const bitRate = argv.b;
const constantRateFactor = argv.crf;

// Establish temporary files
const tmpDir = path.join(os.tmpdir(), Math.random().toString(36).slice(2));
await fs.mkdirp(tmpDir);

// Try encoding and then cleanup temporary files even if the process fails
try {
	await crossFade(input, output, fadeDuration);

	console.log(`Encoded ${output}`)
} finally {
	await cleanup();
}

// CrossFade provided `input` file as `output` file with `fadeDuration`
async function crossFade(input, output, fadeDuration) {
	// Determine input duration
	const duration = await determineDuration(input);

	// Construct ffmpeg command
	const args = [
		`-i "${input}"`,
		`-filter_complex '
[0]split[body][pre];
[pre]trim=duration='${fadeDuration}',format=yuva420p,fade=d='${fadeDuration}':alpha=1,setpts=PTS+('${duration - fadeDuration * 2}'/TB)[jt];
[body]trim='${fadeDuration}',format=yuva420p,setpts=PTS-STARTPTS[main];
[main][jt]overlay,format=yuva420p'`,
	];

	// Fix for .webm with transparency (force codec to VP8)
	if (input.endsWith('.webm') && useTransparency) {
		args.unshift(`-vcodec libvpx`);
		args.push('-auto-alt-ref 0');
	}
	if (output.endsWith('.mov') && useTransparency) {
		args.push(`-vcodec png`);
	}

	// Set configured quality
	if (constantRateFactor) {
		args.push(`-crf ${constantRateFactor}`, `-b:v ${bitRate}`);
	}

	// Avoid escaping args...
	$.quote = v => v;

	// Run command
	await $`ffmpeg -y ${args.join(' ')} "${output}"`;
}

// Determine duration in seconds (as integer)
async function determineDuration(input) {
	let duration = await $`ffprobe -v error -show_entries format=duration -of default=noprint_wrappers=1:nokey=1 ${input}`;

	return parseInt(duration);
}

// Remove temporary dir with all its files
async function cleanup() {
	await fs.rm(tmpDir, {recursive: true});
}

Usage

$ chmod +x cf.mjs
$ ./cf.mjs -i <input> -o <output> -f <fade-duration-seconds> [-b <bitrate>] [--crf <crf-value>] [-t]

You need npm i -g zx (google/zx)

Options

• -i path to input video
• -o path to output video
• -f fade duration as integer, in seconds
• -t set this flag to enable transparency fix
• -b set bitrate
• --crf set CRF