// An image.YCbCr is always fully opaque, and so if the
					// mask is nil (i.e. fully opaque) then the op is
					// effectively always Src. Similarly for image.Gray and
					// image.CMYK.
					if imageutil.DrawYCbCr(dst0, r, src0, sp) {
						return
					}
				case *image.Gray:
					drawGray(dst0, r, src0, sp)
					return
				case *image.CMYK:
					drawCMYK(dst0, r, src0, sp)
					return
				}

		// out the 'Adobe inversion' described in the applyBlack doc comment
		// above, so in practice, only the fourth channel (black) is inverted.
		bounds := d.img3.Bounds()
		img := image.NewRGBA(bounds)
		imageutil.DrawYCbCr(img, bounds, d.img3, bounds.Min)
		for iBase, y := 0, bounds.Min.Y; y < bounds.Max.Y; iBase, y = iBase+img.Stride, y+1 {
			for i, x := iBase+3, bounds.Min.X; x < bounds.Max.X; i, x = i+4, x+1 {