Ebiten example - paint

package main

import (
    "fmt"
    "image"
    "image/color"
    "log"
    "math"

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

const (
    screenWidth  = 320
    screenHeight = 240
)

var (
    count       int
    brushImage  *ebiten.Image
    canvasImage *ebiten.Image
)

func update(screen *ebiten.Image) error {
    if ebiten.IsMouseButtonPressed(ebiten.MouseButtonLeft) {
        count++
    }

    mx, my := ebiten.CursorPosition()

    if ebiten.IsMouseButtonPressed(ebiten.MouseButtonLeft) {
        op := &ebiten.DrawImageOptions{}
        op.GeoM.Translate(float64(mx), float64(my))
        op.ColorM.Scale(1.0, 0.50, 0.125, 1.0)
        theta := 2.0 * math.Pi * float64(count%60) / ebiten.FPS
        op.ColorM.RotateHue(theta)
        if err := canvasImage.DrawImage(brushImage, op); err != nil {
            return err
        }
    }

    if err := screen.DrawImage(canvasImage, nil); err != nil {
        return err
    }

    if err := ebitenutil.DebugPrint(screen, fmt.Sprintf("(%d, %d)", mx, my)); err != nil {
        return err
    }
    return nil
}

func main() {
    var err error
    const a0, a1, a2 = 0x40, 0xc0, 0xff
    pixels := []uint8{
        a0, a1, a1, a0,
        a1, a2, a2, a1,
        a1, a2, a2, a1,
        a0, a1, a1, a0,
    }
    brushImage, err = ebiten.NewImageFromImage(&image.Alpha{
        Pix:    pixels,
        Stride: 4,
        Rect:   image.Rect(0, 0, 4, 4),
    }, ebiten.FilterNearest)
    if err != nil {
        log.Fatal(err)
    }

    canvasImage, err = ebiten.NewImage(screenWidth, screenHeight, ebiten.FilterNearest)
    if err != nil {
        log.Fatal(err)
    }
    canvasImage.Fill(color.White)

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