image: add extra test for monochrome images

This test needs tinygo-org/drivers#683 to pass,
but I think it's a useful test regardless.

image/image_test.go

@@ -99,3 +99,50 @@ func TestMono(t *testing.T) {
 		})
 	}
 }
+
+// Extra test to make sure decoding the image as pixel.Monochrome works as
+// expected. (It previously didn't due to
+// https://github.com/tinygo-org/drivers/pull/683).
+func TestMonoMonochrome(t *testing.T) {
+	data, err := os.ReadFile("testdata/tinygo-logo-monochrome.raw")
+	if err != nil {
+		t.Fatal("could not read input file:", err)
+	}
+	img, e := image.NewMono[pixel.Monochrome](true, false, 304, 255, string(data))
+	if e != nil {
+		t.Fatal("could not decode input file:", e)
+	}
+
+	// Decode the image.
+	width, height := img.Size()
+	buf := pixel.NewImage[pixel.Monochrome](width, height)
+	img.Draw(buf, 0, 0, 1)
+
+	// Load the reference image.
+	f, err := os.Open("testdata/tinygo-logo-monochrome.png")
+	if err != nil {
+		t.Fatal("could not open reference image:", err)
+	}
+	defer f.Close()
+	golden, err := png.Decode(f)
+	if err != nil {
+		t.Fatal("could not decode reference image:", err)
+	}
+
+	// Check that the decoded image matches the golden image.
+	fg := color.RGBA{192, 192, 192, 255}
+	bg := color.RGBA{32, 32, 32, 255}
+	for y := 0; y < height; y++ {
+		for x := 0; x < width; x++ {
+			c1 := bg
+			if buf.Get(x, y) {
+				c1 = fg
+			}
+			r2, g2, b2, _ := golden.At(x, y).RGBA()
+			c2 := color.RGBA{R: uint8(r2 >> 8), G: uint8(g2 >> 8), B: uint8(b2 >> 8), A: 255}
+			if c1 != c2 {
+				t.Fatalf("mismatch at %dx%d: expected %v got %v", x, y, c2, c1)
+			}
+		}
+	}
+}