colorutil.js

function applyAlpha(source, target) {
    var sourceData = source.getContext('2d').getImageData(0,0,source.width,source.height),
        targetCtx = target.getContext('2d'),
        targetData = targetCtx.getImageData(0,0,target.width,target.height);
    for(var p = 3; p < targetData.data.length; p += 4) {
        targetData.data[p] = sourceData.data[p];
    }
    targetCtx.putImageData(targetData,0,0);
}