examples/rotate: Refactoring

examples/rotate/main.go

@@ -31,7 +31,7 @@ const (
 )
 
 var (
-	count        int
+	count        = 0
 	gophersImage *ebiten.Image
 )
 
@@ -42,8 +42,17 @@ func update(screen *ebiten.Image) error {
 	}
 	w, h := gophersImage.Size()
 	op := &ebiten.DrawImageOptions{}
+
+	// Move the image's center to the screen's upper-left corner.
+	// This is a prepartion for rotating. When geometry matrices are applied,
+	// the origin point is the upper-left corner.
 	op.GeoM.Translate(-float64(w)/2, -float64(h)/2)
+
+	// Rotate the image. As a result, the anchor point of this rotate is
+	// the center of the image.
 	op.GeoM.Rotate(float64(count%360) * 2 * math.Pi / 360)
+
+	// Move the image to the screen's center.
 	op.GeoM.Translate(screenWidth/2, screenHeight/2)
 	screen.DrawImage(gophersImage, op)
 	return nil