Ebiten example - infinitescroll

// +build example

package main

import (
        "fmt"
        _ "image/png"
        "log"
        "math"

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

const (
        screenWidth  = 320
        screenHeight = 240
)

var (
        theViewport     = &viewport{}
        bgImage         *ebiten.Image
        repeatedBgImage *ebiten.Image
        groundImage     *ebiten.Image
)

type viewport struct {
        x16 int
        y16 int
}

func round(x float64) float64 {
        return math.Floor(x + 0.5)
}

func (p *viewport) Move() {
        w, h := bgImage.Size()
        mx := w * 16
        my := h * 16

        p.x16 += w / 32
        p.y16 += h / 32

        for mx <= p.x16 {
                p.x16 -= mx
        }
        for my <= p.y16 {
                p.y16 -= my
        }
        for p.x16 < 0 {
                p.x16 += mx
        }
        for p.y16 < 0 {
                p.y16 += my
        }
}

func (p *viewport) Position() (int, int) {
        return p.x16, p.y16
}

func updateGroundImage(ground *ebiten.Image) {
        ground.Clear()
        x16, y16 := theViewport.Position()
        op := &ebiten.DrawImageOptions{}
        op.GeoM.Translate(float64(-x16)/16, float64(-y16)/16)
        ground.DrawImage(repeatedBgImage, op)
}

func drawGroundImage(screen *ebiten.Image, ground *ebiten.Image) {
        op := &ebiten.DrawImageOptions{}
        screen.DrawImage(ground, op)
}

func update(screen *ebiten.Image) error {
        theViewport.Move()
        if ebiten.IsRunningSlowly() {
                return nil
        }
        updateGroundImage(groundImage)
        drawGroundImage(screen, groundImage)

        msg := fmt.Sprintf("FPS: %0.2f", ebiten.CurrentFPS())
        ebitenutil.DebugPrint(screen, msg)
        return nil
}

func main() {
        var err error
        bgImage, _, err = ebitenutil.NewImageFromFile("_resources/images/tile.png", ebiten.FilterNearest)
        if err != nil {
                log.Fatal(err)
        }
        w, h := bgImage.Size()
        const repeat = 5
        repeatedBgImage, _ = ebiten.NewImage(w*repeat, h*repeat, ebiten.FilterNearest)
        for j := 0; j < repeat; j++ {
                for i := 0; i < repeat; i++ {
                        op := &ebiten.DrawImageOptions{}
                        op.GeoM.Translate(float64(w*i), float64(h*j))
                        repeatedBgImage.DrawImage(bgImage, op)
                }
        }
        groundImage, _ = ebiten.NewImage(screenWidth, screenHeight, ebiten.FilterNearest)

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