Ebiten example - perspective

// +build example

package main

import (
        _ "image/jpeg"
        "log"

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

const (
        screenWidth  = 320
        screenHeight = 240
)

var (
        gophersImage *ebiten.Image
)

type parts struct {
        image *ebiten.Image
}

func (p parts) Len() int {
        _, h := p.image.Size()
        return h
}

func (p parts) Dst(i int) (x0, y0, x1, y1 int) {
        w, h := p.image.Size()
        width := w + i*3/4
        x := ((h - i) * 3 / 4) / 2
        return x, i, x + width, i + 1
}

func (p parts) Src(i int) (x0, y0, x1, y1 int) {
        w, _ := p.image.Size()
        return 0, i, w, i + 1
}

func update(screen *ebiten.Image) error {
        if ebiten.IsRunningSlowly() {
                return nil
        }
        op := &ebiten.DrawImageOptions{
                ImageParts: &parts{gophersImage},
        }
        w, h := gophersImage.Size()
        maxWidth := float64(w) + float64(h)*0.75
        op.GeoM.Translate(-maxWidth/2, -float64(h)/2)
        op.GeoM.Translate(screenWidth/2, screenHeight/2)
        screen.DrawImage(gophersImage, op)
        return nil
}

func main() {
        var err error
        gophersImage, _, err = ebitenutil.NewImageFromFile("_resources/images/gophers.jpg", ebiten.FilterNearest)
        if err != nil {
                log.Fatal(err)
        }
        if err := ebiten.Run(update, screenWidth, screenHeight, 2, "Perspective (Ebiten Demo)"); err != nil {
                log.Fatal(err)
        }
}