grayscale.coffee

grayscale = (canvas) ->
    context = canvas.getContext('2d')
    width = canvas.width
    height = canvas.height
    img = context.getImageData(0, 0, width, height)
    pixels = img.data
    for i in [0...height]
        for j in [0...width]
            index = (i * 4) * width + (j * 4)
            [r, g, b, a] = [index, index + 1, index + 2, index + 3]
            average = (pixels[r] + pixels[g] + pixels[b]) / 3
            pixels[r] = average
            pixels[g] = average
            pixels[b] = average
    context.putImageData(img, 0, 0)