code: func OverlayCenter(background, img image.Image, opacity float64) *image.NRGBA {
  	bgBounds := background.Bounds()
  	bgW := bgBounds.Dx()
  	bgH := bgBounds.Dy()
  	bgMinX := bgBounds.Min.X
  	bgMinY := bgBounds.Min.Y
  
  	centerX := bgMinX + bgW/2
  	centerY := bgMinY + bgH/2
  
  	x0 := centerX - img.Bounds().Dx()/2
  	y0 := centerY - img.Bounds().Dy()/2
  
  	return Overlay(background, img, image.Point{x0, y0}, opacity)
  }
docs: // OverlayCenter overlays the img image to the center of the background image and
  // returns the combined image. Opacity parameter is the opacity of the img
  // image layer, used to compose the images, it must be from 0.0 to 1.0.