code: func (im *Image) ForExp(each func(x, y int) (r, g, b, a uint8)) {
  	var x, y, i int
  	var r, g, b, a uint8
  	for x = im.Rect.Min.X; x < im.Rect.Max.X; x++ {
  		for y = im.Rect.Min.Y; y < im.Rect.Max.Y; y++ {
  			i = im.PixOffset(x, y)
  			r, g, b, a = each(x, y)
  
  			im.Pix[i+0] = b
  			im.Pix[i+1] = g
  			im.Pix[i+2] = r
  			im.Pix[i+3] = a
  		}
  	}
  }
docs: // ForExp is like For, but bypasses image.Color types.
  // (So it should be faster.)