AlphaComposite.CLEAR not working

问题

I have two JPanels on a JLayeredpane. One of them displays a pdf and the overlapping one has a transparent background (I have used setOpaque(false)). Now I can add drawings to the transparent panel such that it seems I'm actually annotating the pdf. I want to have a eraser tool to erase these annotations. I tried using the following code

@Override
public void draw(Graphics2D g2) {
    g2.setPaint(Color.WHITE);
    g2.setComposite(AlphaComposite.getInstance(AlphaComposite.CLEAR));
    g2.setBackground(new Color(255, 255, 255, 0));
    g2.setStroke(new BasicStroke(thickness, BasicStroke.CAP_ROUND, BasicStroke.JOIN_ROUND));
    g2.draw(path);
}

where path is the shape constituted from a number of lines. Now instead of drawing a transparent line over the earlier drawings a black line is being drawn. Where am I going wrong?

回答1:

Note that an instance of AlphaComposite.CLEAR, equivalent to AlphaComposite.Clear, is a composite mode that clears both the color and the alpha of the destination. "Neither the source nor the destination is used as input." In effect, you can't draw with CLEAR. This example illustrates a common usage. To get the effect you want, keep a copy of the unaltered image and draw() an eraser-sized sub-image over the destination BufferedImage as the mouse moves.

来源：https://stackoverflow.com/questions/17414933/alphacomposite-clear-not-working