Alpha compositing in JavaScript

composite.js

function _composite(under, over) {
	var alphaO = over[3],
		alphaU = under[3],
		invAlphaO = 1 - alphaO,
		i,
		len;

	for(i = 0, len = under.length - 1; i < len; ++i) {
		under[i] = Math.round((over[i] * alphaO) 
			+ ((under[i] * alphaU) * invAlphaO));
	}
}