Ebiten example - masking

package main

import (
    "image"
    "image/color"
    _ "image/jpeg"
    "log"
    "math"

    "github.com/hajimehoshi/ebiten"
    "github.com/hajimehoshi/ebiten/ebitenutil"
)

const (
    screenWidth  = 320
    screenHeight = 240
)

var (
    gophersImage   *ebiten.Image
    fiveyearsImage *ebiten.Image
    maskImage      *ebiten.Image
    spotLightImage *ebiten.Image
    spotLightX     = 0
    spotLightY     = 0
    spotLightVX    = 1
    spotLightVY    = 1
)

func update(screen *ebiten.Image) error {
    spotLightX += spotLightVX
    spotLightY += spotLightVY
    if spotLightX < 0 {
        spotLightX = -spotLightX
        spotLightVX = -spotLightVX
    }
    if spotLightY < 0 {
        spotLightY = -spotLightY
        spotLightVY = -spotLightVY
    }
    w, h := spotLightImage.Size()
    maxX, maxY := screenWidth-w, screenHeight-h
    if maxX < spotLightX {
        spotLightX = -spotLightX + 2*maxX
        spotLightVX = -spotLightVX
    }
    if maxY < spotLightY {
        spotLightY = -spotLightY + 2*maxY
        spotLightVY = -spotLightVY
    }

    if err := maskImage.Clear(); err != nil {
        return err
    }

    op := &ebiten.DrawImageOptions{}
    op.GeoM.Translate(float64(spotLightX), float64(spotLightY))
    if err := maskImage.DrawImage(spotLightImage, op); err != nil {
        return err
    }

    op = &ebiten.DrawImageOptions{}
    op.CompositeMode = ebiten.CompositeModeSourceOut
    if err := maskImage.DrawImage(fiveyearsImage, op); err != nil {
        return err
    }

    if err := screen.Fill(color.RGBA{0x00, 0x00, 0x80, 0xff}); err != nil {
        return err
    }
    if err := screen.DrawImage(gophersImage, &ebiten.DrawImageOptions{}); err != nil {
        return err
    }
    if err := screen.DrawImage(maskImage, &ebiten.DrawImageOptions{}); err != nil {
        return err
    }

    return nil
}

func max(a, b int) int {
    if a < b {
        return b
    }
    return a
}

func min(a, b int) int {
    if a < b {
        return a
    }
    return b
}

func main() {
    var err error
    gophersImage, _, err = ebitenutil.NewImageFromFile("_resources/images/gophers.jpg", ebiten.FilterNearest)
    if err != nil {
        log.Fatal(err)
    }
    fiveyearsImage, _, err = ebitenutil.NewImageFromFile("_resources/images/fiveyears.jpg", ebiten.FilterNearest)
    if err != nil {
        log.Fatal(err)
    }
    maskImage, err = ebiten.NewImage(screenWidth, screenHeight, ebiten.FilterNearest)
    if err != nil {
        log.Fatal(err)
    }

    as := image.Point{128, 128}
    a := image.NewAlpha(image.Rectangle{image.ZP, as})
    for j := 0; j < as.Y; j++ {
        for i := 0; i < as.X; i++ {
            r := as.X / 2
            d := math.Sqrt(float64((i-r)*(i-r) + (j-r)*(j-r)))
            b := uint8(max(0, min(0xff, 3*0xff-int(d*3*0xff)/r)))
            a.SetAlpha(i, j, color.Alpha{b})
        }
    }
    spotLightImage, err = ebiten.NewImageFromImage(a, ebiten.FilterNearest)
    if err != nil {
        log.Fatal(err)
    }
    if err := ebiten.Run(update, screenWidth, screenHeight, 2, "Masking (Ebiten Demo)"); err != nil {
        log.Fatal(err)
    }
}