Ebiten example - noise

package main

import (
    "fmt"
    "image"
    "log"

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

const (
    screenWidth  = 320
    screenHeight = 240
)

var (
    noiseImage *image.RGBA
)

type rand struct {
    x, y, z, w uint32
}

func (r *rand) next() uint32 {
    // math/rand is too slow to keep 60 FPS on web browsers.
    // Use Xorshift instead: http://en.wikipedia.org/wiki/Xorshift
    t := r.x ^ (r.x << 11)
    r.x, r.y, r.z = r.y, r.z, r.w
    r.w = (r.w ^ (r.w >> 19)) ^ (t ^ (t >> 8))
    return r.w
}

var randInstance = &rand{12345678, 4185243, 776511, 45411}

func update(screen *ebiten.Image) error {
    const l = screenWidth * screenHeight
    for i := 0; i < l; i++ {
        x := randInstance.next()
        noiseImage.Pix[4*i] = uint8(x >> 24)
        noiseImage.Pix[4*i+1] = uint8(x >> 16)
        noiseImage.Pix[4*i+2] = uint8(x >> 8)
        noiseImage.Pix[4*i+3] = 0xff
    }
    screen.ReplacePixels(noiseImage.Pix)
    ebitenutil.DebugPrint(screen, fmt.Sprintf("FPS: %f", ebiten.CurrentFPS()))
    return nil
}

func main() {
    noiseImage = image.NewRGBA(image.Rect(0, 0, screenWidth, screenHeight))
    if err := ebiten.Run(update, screenWidth, screenHeight, 2, "Noise (Ebiten Demo)"); err != nil {
        log.Fatal(err)
    }
}