How to save canvas as png with  transparent background


I want save canvas as png photo.
But the background is always black. 



```
                fDst, err := os.Create("out2.png")
		if err != nil {
			log.Fatal(err)
		}
		defer fDst.Close()

		err = png.Encode(fDst, cv.GetImageData(0, 0, cv.Width(), cv.Height()))
		if err != nil {
			log.Fatal(err)
		}
		wnd.Close()
```