examples/alphablending: Refactoring: Reduce draw calls

examples/alphablending/main.go

@@ -29,11 +29,30 @@ const (
 )
 
 var (
-	count           int
+	count       int
-	tmpRenderTarget *ebiten.Image
+	ebitenImage *ebiten.Image
-	ebitenImage     *ebiten.Image
 )
 
+type imageParts struct {
+	diff float64
+}
+
+func (p *imageParts) Src(i int) (int, int, int, int) {
+	w, h := ebitenImage.Size()
+	return 0, 0, w, h
+}
+
+func (p *imageParts) Dst(i int) (int, int, int, int) {
+	x := int(float64(i%10)*p.diff + 15)
+	y := int(float64(i/10)*p.diff + 20)
+	w, h := ebitenImage.Size()
+	return x, y, x + w, y + h
+}
+
+func (p *imageParts) Len() int {
+	return 10 * 10
+}
+
 func update(screen *ebiten.Image) error {
 	count++
 	count %= ebiten.FPS * 10
@@ -45,27 +64,15 @@ func update(screen *ebiten.Image) error {
 		diff = float64(480-count) * 0.2
 	}
 
-	if err := tmpRenderTarget.Clear(); err != nil {
+	screen.Fill(color.NRGBA{0x00, 0x00, 0x80, 0xff})
+	op := &ebiten.DrawImageOptions{}
+	op.ColorM.Scale(1.0, 1.0, 1.0, 0.5)
+	op.ImageParts = &imageParts{
+		diff: diff,
+	}
+	if err := screen.DrawImage(ebitenImage, op); err != nil {
 		return err
 	}
-	for i := 0; i < 10; i++ {
-		op := &ebiten.DrawImageOptions{}
-		op.GeoM.Translate(15+float64(i)*diff, 20)
-		op.ColorM.Scale(1.0, 1.0, 1.0, 0.5)
-		if err := tmpRenderTarget.DrawImage(ebitenImage, op); err != nil {
-			return err
-		}
-	}
-
-	screen.Fill(color.NRGBA{0x00, 0x00, 0x80, 0xff})
-	for i := 0; i < 10; i++ {
-		op := &ebiten.DrawImageOptions{}
-		op.GeoM.Translate(0, float64(i)*diff)
-		if err := screen.DrawImage(tmpRenderTarget, op); err != nil {
-			return err
-		}
-	}
-
 	return nil
 }
 
@@ -75,10 +82,6 @@ func main() {
 	if err != nil {
 		log.Fatal(err)
 	}
-	tmpRenderTarget, err = ebiten.NewImage(screenWidth, screenHeight, ebiten.FilterNearest)
-	if err != nil {
-		log.Fatal(err)
-	}
 	if err := ebiten.Run(update, screenWidth, screenHeight, 2, "Alpha Blending (Ebiten Demo)"); err != nil {
 		log.Fatal(err)
 	}