Ebiten example - masking

// +build example

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)
        }
}