switch cb {
		case cbG8:
			if gray != nil {
				offset := (y - b.Min.Y) * gray.Stride
				copy(cr[0][1:], gray.Pix[offset:offset+b.Dx()])
			} else {
				for x := b.Min.X; x < b.Max.X; x++ {
					c := color.GrayModel.Convert(m.At(x, y)).(color.Gray)
					cr[0][i] = c.Y
					i++
				}
			}
		case cbTC8:
			// We have previously verified that the alpha value is fully opaque.
			cr0 := cr[0]

		{"CMYKToRGB", Func, 5},
		{"Color", Type, 0},
		{"Gray", Type, 0},
		{"Gray.Y", Field, 0},
		{"Gray16", Type, 0},
		{"Gray16.Y", Field, 0},
		{"Gray16Model", Var, 0},
		{"GrayModel", Var, 0},
		{"Model", Type, 0},
		{"ModelFunc", Func, 0},
		{"NRGBA", Type, 0},
		{"NRGBA.A", Field, 0},
		{"NRGBA.B", Field, 0},
		{"NRGBA.G", Field, 0},
		{"NRGBA.R", Field, 0},