perspective

package main

import (
    "github.com/hajimehoshi/ebiten"
    "github.com/hajimehoshi/ebiten/ebitenutil"
    _ "image/jpeg"
    "log"
)

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 {
    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("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)
    }
}