Ebiten example - alphablending

package main

import (
    "image/color"
    _ "image/png"
    "log"

    "github.com/hajimehoshi/ebiten"
    "github.com/hajimehoshi/ebiten/examples/common"
)

const (
    screenWidth  = 320
    screenHeight = 240
)

var (
    count       int
    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
    diff := float64(count) * 0.2
    switch {
    case 480 < count:
        diff = 0
    case 240 < count:
        diff = float64(480-count) * 0.2
    }

    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
    }
    return nil
}

func main() {
    var err error
    ebitenImage, _, err = common.AssetImage("ebiten.png", 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)
    }
}