testing jpeg-ls compression rates

png2jls.js

const fs = require('fs');
const child_process = require('child_process');
const charls = new Promise((resolve) => {
	const charlsJS = require('./charlsjs.js'); //https://github.com/chafey/charls-js/raw/master/dist/charlsjs.js
	//in same directory: https://github.com/chafey/charls-js/blob/master/dist/charlsjs.wasm?raw=true
	charlsJS.onRuntimeInitialized = () => { resolve(charlsJS); };
});

(async () => {
	const file = 'image5.png';
	const encoder = new (await charls).JpegLSEncoder();
	encoder.setNearLossless(0);
	
	console.log((await fs.promises.stat(file)).size);
	
	const info = await new Promise(async (resolve, reject) => {
		let error = '';
		try {
			const identify = child_process.spawn(
				'identify', [
					'-format', '{"width":%w, "height":%h, "bitsPerSample":%z, "componentCount":3}',
					'-'
				]
			);
			let json = '';
			identify
				.stdout
				.on('data', (data) => { json += data; })
				.on('end', () => { resolve(JSON.parse(json)); })
			;
			identify.stderr.on('data', (data) => { error += data; });
			fs.createReadStream(file).pipe(identify.stdin);
		}
		catch (e) {
			if (error)
				console.err(error);
			reject(e);
		}
	});
	console.log(info);
	
	await new Promise(async (resolve, reject) => {
		let error = '';
		try {
			const convert = child_process.spawn('convert', ['png:-', 'bgr:-']);
			const bytes = encoder.getDecodedBuffer(info);
			console.log(bytes.length);
			let index = 0;
			convert
				.stdout
				.on('data', (data) => {
					data.copy(bytes, index);
					index += data.length;
				})
				.on('end', () => {
					console.log(index, bytes.length);
					resolve();
				})
			;
			convert.stderr.on('data', (data) => { error += data; });
			fs.createReadStream(file).pipe(convert.stdin);
		}
		catch (e) {
			if (error)
				console.err(error);
			reject(e);
		}
	});
	
	encoder.encode();
	console.log(encoder.getEncodedBuffer().length);
	await fs.promises.writeFile(
		'image5.jls',
		encoder.getEncodedBuffer(),
		{encoding:null}
	);
	
	encoder.delete();
})();

jpeg-2000 (via imagemagick, graphicsmagick does lossy j2 compression) wins by a factor of 1.6 (over IrfanView's jls plugin implementation, see below)

maybe charls-js is broken, it just outputs files twice as big as PNG (~1/3 the size of the RGB raw, but still 1.7x larger than IrfanView's jls output!)