Add golden tests for Thumbnail

resize_test.go

@@ -630,6 +630,24 @@ func TestThumbnail(t *testing.T) {
 	}
 }
 
+func TestThumbnailGolden(t *testing.T) {
+	for name, filter := range map[string]ResampleFilter{
+		"out_thumbnail_nearest.png": NearestNeighbor,
+		"out_thumbnail_linear.png":  Linear,
+		"out_thumbnail_catrom.png":  CatmullRom,
+		"out_thumbnail_lanczos.png": Lanczos,
+	} {
+		got := Thumbnail(testdataBranchesPNG, 150, 100, filter)
+		want, err := Open("testdata/" + name)
+		if err != nil {
+			t.Fatalf("failed to open image: %v", err)
+		}
+		if !compareNRGBA(got, toNRGBA(want), 0) {
+			t.Fatalf("resulting image differs from golden: %s", name)
+		}
+	}
+}
+
 func BenchmarkResize(b *testing.B) {
 	for _, dir := range []string{"Down", "Up"} {
 		for _, filter := range []string{"NearestNeighbor", "Linear", "CatmullRom", "Lanczos"} {