// +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 (
        bgImage        *ebiten.Image
        fgImage        *ebiten.Image
        maskedFgImage  *ebiten.Image
        spotLightImage *ebiten.Image
        spotLightX     = 0
        spotLightY     = 0
        spotLightVX    = 1
        spotLightVY    = 1
)

func init() {
        var err error
        bgImage, _, err = ebitenutil.NewImageFromFile("_resources/images/gophers.jpg", ebiten.FilterDefault)
        if err != nil {
                log.Fatal(err)
        }

        fgImage, _, err = ebitenutil.NewImageFromFile("_resources/images/fiveyears.jpg", ebiten.FilterDefault)
        if err != nil {
                log.Fatal(err)
        }

        maskedFgImage, _ = ebiten.NewImage(screenWidth, screenHeight, ebiten.FilterDefault)

        // Initialize the spot light image.
        const r = 64
        alphas := image.Point{r * 2, r * 2}
        a := image.NewAlpha(image.Rectangle{image.ZP, alphas})
        for j := 0; j < alphas.Y; j++ {
                for i := 0; i < alphas.X; i++ {
                        // d is the distance between (i, j) and the (circle) center.
                        d := math.Sqrt(float64((i-r)*(i-r) + (j-r)*(j-r)))
                        // Alphas around the center are 0 and values outside of the circle are 0xff.
                        b := uint8(max(0, min(0xff, int(3*d*0xff/r)-2*0xff)))
                        a.SetAlpha(i, j, color.Alpha{b})
                }
        }
        spotLightImage, _ = ebiten.NewImageFromImage(a, ebiten.FilterDefault)
}

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 ebiten.IsRunningSlowly() {
                return nil
        }

        // Reset the maskedFgImage.
        maskedFgImage.Fill(color.White)
        op := &ebiten.DrawImageOptions{}
        op.CompositeMode = ebiten.CompositeModeCopy
        op.GeoM.Translate(float64(spotLightX), float64(spotLightY))
        maskedFgImage.DrawImage(spotLightImage, op)

        // Use 'source-in' composite mode so that the source image (fgImage) is used but the alpha
        // is determined by the destination image (maskedFgImage).
        //
        // The result image is the source image with the destination alpha. In maskedFgImage, alpha
        // values in the hole is 0 and alpha values in other places are 0xff. As a result, the
        // maskedFgImage draws the source image with a hole that shape is spotLightImage. Note that
        // RGB values in the destination image are ignored.
        //
        // See also https://www.w3.org/TR/compositing-1/#porterduffcompositingoperators_srcin.
        op = &ebiten.DrawImageOptions{}
        op.CompositeMode = ebiten.CompositeModeSourceIn
        maskedFgImage.DrawImage(fgImage, op)

        screen.Fill(color.RGBA{0x00, 0x00, 0x80, 0xff})
        screen.DrawImage(bgImage, &ebiten.DrawImageOptions{})
        screen.DrawImage(maskedFgImage, &ebiten.DrawImageOptions{})

        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() {
        if err := ebiten.Run(update, screenWidth, screenHeight, 2, "Masking (Ebiten Demo)"); err != nil {
                log.Fatal(err)
        }
}