在Java 2d / AWT中淡出效果

Question

I would like to display an image for 6.5s in total and fade it out in the last 1.5s. I'm getting a NPE at line:

g2d.setComposite(AlphaComposite.getInstance(AlphaComposite.SRC_OVER, transparency));

I read some, and I think it is because gameLogo is not a BufferedImage, just a simple Image. I made it with paint.net so it should have an alpha channel by default. I searched some forums about making BufferedImage but still confused.

The code snippet below worked perfectly before I added the extra lines to chage the last 1.5s.

Thank you for your help in advance!

EDIT:

with adding cast (Graphics2D) NPE if fixed but it still doesn't fade out totally...

Should I use offG.dispose(); somewhere?

offG.setColor(Color.BLACK);
offG.fillRect(0, 0, 800, 480);
counter += deltaTime;
// start fade out after 4 seconds
if (counter < 4000)
    offG.drawImage(devLogo, 0, 0, this);
else {
    transparency = (float)(1 - (counter - 4000) / 50 * 0.02);
    if (transparency < 0)
    transparency = 0;
    ((Graphics2D) offG).setComposite(AlphaComposite.getInstance(AlphaComposite.SRC_OVER, transparency));
    offG.drawImage(devLogo, 0, 0, this);
}
// wait 6,5 seconds
if (counter > 6500) {
    gameState = 2;
    counter = 0;
    transparency = 1;
    ((Graphics2D) offG).setComposite(AlphaComposite.getInstance(AlphaComposite.SRC_OVER, transparency));
}

Answer 1

You should provide more code for that. In you case g2d is null . According to your comment you are holding Graphics objects in fields. Graphics object is passed to the drawing method by AWT, and you probably assigning it to your field, but you forgot to assign new value to your g2d field as well. I think there is no need to keep single object in 2 different typed fields. If I were you I would operate only on Graphics2d type object. That would put end to the confusion in your code that you have made. You are threating the same Graphics object as 2 separate entities but your fields referring to the same object. Why for? No need for that.

  ((Graphics2d)offG).setComposite(AlphaComposite.getInstance(AlphaComposite.SRC_OVER, transparency));

and NPE is gone right?