Transparency in bufferedimage objects.

Question

Hi there. I noted that when i draw something with Color(0,0,0,0), which is over another image, the color shown is the JFrame background, not the image just below it.

Reasons that'd help me to find a solution?

Thanks!!

Edit: See the circles, the grey area (corners) should be transparent but are not, instead, they are the color of the JFrame.

And here is the code to draw the circles:

public void paint(final Graphics g) {
        super.paintComponent(g);
        final Graphics2D g2 = (Graphics2D) g;

        RenderingHints rh = new RenderingHints(RenderingHints.KEY_ANTIALIASING, RenderingHints.VALUE_ANTIALIAS_ON);

        if (unitImage == null) {

            unitImage = (BufferedImage) (createImage(30, 30));
            final Graphics2D gc = unitImage.createGraphics();
            gc.setRenderingHints(rh);
            gc.setColor(outsideColor);
            gc.fillOval(0, 0, diameter, diameter);
            gc.setColor(middleColor);
            gc.fillOval(diameter / 6, diameter / 6, (diameter / 3) * 2, (diameter / 3) * 2);
            gc.setColor(innerColor);
            gc.fillOval(diameter / 3, diameter / 3, diameter / 3, diameter / 3);
        }

        g2.drawImage(unitImage, null, 0, 0);

Been toying with the Alphacomposites, i think its not the solution. So i added all this new info which i believe, will help you guys to give me another tip.

Answer 1

Did you set the AlphaComposite before you draw?

Graphics2D g2d = (Graphics2D)g; //Some graphics object
//Save the original
Composite original = g2d.getComposite();
//Set to semi translucent
Composite translucent = AlphaComposite.getInstance(AlphaComposite.SRC_OVER, 0.5f);
g2d.setComposite(translucent);

//Draw

//Set back to original
g2d.setComposite(original);

Disclaimer: code note tested

Answer 2

@Chuk Lee is right: Unless you change it, the default Graphics2D composite is AlphaComposite.SrcOver. This handy tool displays the composite result for a selected rule a specified pair of color and alpha.

Addendum: One approach is to override paintComponent() and render both map and circles, but you might be able to make the corners transparent by clearing the alpha:

...
gc.setRenderingHints(rh);
gc.setComposite(AlphaComposite.Clear);
gc.fillRect(0, 0, diameter, diameter);
gc.setComposite(AlphaComposite.Src);
gc.setColor(outsideColor);
...

Does createImage(30, 30) relate to diameter? For what component do you override paint() and invoke super.paintComponent(g)?

Answer 3

You are painting with an opacity of 0. If it's black you want, use Color(0,0,0,255).

UPDATE

Sorry, I have misunderstood your question. Having read the code you added to your post, I assume that the little targets are components placed on top of the map which is drawn on a parent component.

To ensure that transparency is used you need two things:

1. you need to call setOpaque(false) in the constructor of the target component

2. Either draw directly the target without backing store, or use an RGBA image:

   unitImage = new BufferedImage(30, 30, BufferedImage.TYPE_INT_ARGB);