绘制透明PNG图像 Graphics2D

Question

所以我的目标是有一个 window 打开，您只需单击即可在白色背景上绘制一些线条。 问题是，当它尝试保存时，它总是以 png 的形式返回，但它以方形图像的形式出现。 如果我用我的线条画一个三角形，我会在一个白色正方形内得到一个三角形，但我只想要三角形。 我真的很感激任何帮助！

我尝试了我在 stackoverflow 上遇到的所有解决方案，我试图深入了解 Graphics2D，但遗憾的是失败了

class PadDraw extends JComponent{
Image image;
//this is gonna be your image that you draw on
Graphics2D graphics2D;
//this is what we'll be using to draw on
int currentX, currentY, oldX, oldY;
//these are gonna hold our mouse coordinates
int firstX;
int firstY;

//Now for the constructors
//will draw from tail to head
public PadDraw(){
    setDoubleBuffered(false);
    addMouseListener(new MouseAdapter(){
        public void mousePressed(MouseEvent e){
            if (currentX == 0 && currentY == 0) {
                firstX= e.getX();
                firstY = e.getY();
                oldX = e.getX();
                oldY = e.getY();
            }
            currentX = e.getX();
            currentY = e.getY();
            graphics2D.drawLine(oldX, oldY, currentX, currentY);
            repaint();
            oldX = currentX;
            oldY = currentY;
        }
    });
}

public void paintComponent(Graphics g){
    if(image == null){
        image = createImage(getSize().width, getSize().height);
        graphics2D = (Graphics2D) image.getGraphics();
        graphics2D.setRenderingHint(RenderingHints.KEY_ANTIALIASING, RenderingHints.VALUE_ANTIALIAS_ON);
        graphics2D.setComposite(AlphaComposite.Clear);
        graphics2D.fillRect(0, 0, getSize().width, getSize().height);
        graphics2D.setComposite(AlphaComposite.Src);
        clear();
    }
    g.drawImage(image, 0, 0, null);
}
//this is the painting bit
//if it has nothing on it then
//it creates an image the size of the window
//sets the value of Graphics as the image
//sets the rendering
//runs the clear() method
//then it draws the image

public void clear(){
    currentX = 0;
    currentY = 0;

    graphics2D.setPaint(Color.white);
    graphics2D.fillRect(0, 0, getSize().width, getSize().height);
    graphics2D.setPaint(Color.black);
    repaint();
}
//this is the clear
//it sets the colors as white
//then it fills the window with white
//thin it sets the color back to black

public void save(){
    repaint();
    if (currentX != 0 && currentY != 0) {
        graphics2D.drawLine(oldX, oldY, firstX, firstY);
        currentX = 0;
        currentY = 0;
    }

    try {
        BufferedImage bfrdImage = new BufferedImage
    (image.getWidth(null), image.getHeight(null), BufferedImage.TYPE_INT_ARGB);
        // Draw the image on to the buffered image
        Graphics2D bGr = bfrdImage.createGraphics();

        bGr.setComposite(AlphaComposite.Clear);
        bGr.fillRect(0, 0, getSize().width, getSize().height);
        bGr.setComposite(AlphaComposite.Src);
        bGr.drawImage(image, 0, 0, null);

        javax.imageio.ImageIO.write(bfrdImage, "PNG", new File("Drawing.PNG"));
        bGr.dispose();
    } catch (Exception ex) {
        Logger.getLogger(PadDraw.class.getName()).log(Level.SEVERE, null, ex);
    }
}
//saves and also comes back to the first point to finalize the shape
}