public void saveAsTransparentPng(String outputFileName) throws IOException {
        if (this.img == null) {
            throw new IllegalStateException("File can not be converted.");
        }

        BufferedImage bi = getColorTransparentImage();
        File out = new File(outputFileName);
        ImageIO.write(bi, "PNG", out);
    }
Makes the background of the qaf transparent and saves it under the given name

@param outputFileName
@return
@throws IOException
plugins/image-plugin/src/test/java/com/github/hui/quick/plugin/test/BgFilter.java