code: func paint(canvas *ebiten.Image, x, y int) {
  	op := &ebiten.DrawImageOptions{}
  	op.GeoM.Translate(float64(x), float64(y))
  	// Scale the color and rotate the hue so that colors vary on each frame.
  	op.ColorM.Scale(1.0, 0.50, 0.125, 1.0)
  	tps := ebiten.MaxTPS()
  	theta := 2.0 * math.Pi * float64(count%tps) / float64(tps)
  	op.ColorM.RotateHue(theta)
  	canvas.DrawImage(brushImage, op)
  }
docs: // paint draws the brush on the given canvas image at the position (x, y).