hide object

[Dalibor Toth]

Hi!
If I draw, say, a circle, how do I make it hidden and, of course, visible again.
Thanks!

[Manfred Leonhardt]

Hi Dalibor,
You can do it 2 different ways.
1. Draw it with some color (not background) to show. Draw it with background to hide it.
2. Set XOR graphics mode. Draw it with some color will now toggle it from shown to hidden.
Regards,
Manfred.

[Dirk Schreckmann]

Hello,
There are lots of ways to do what you are asking. Here are a few of them:
You could just draw a new circle on top of the original, this time using the background color.

g.setColor(Color.black);
g.drawArc(50, 50, 200, 200, 0, 360);
g.setColor(getBackground());
g.drawArc(50, 50, 200, 200, 0, 360);

You could use clearRect(x, y, width, height) to clear a rectangle (draw a filled rectangle of the background color) surrounding the circle.

g.setColor(Color.black);
g.drawArc(50, 50, 200, 200, 0, 360);
g.clearRect(50, 50, 201, 201);

Note: The cleared rectangle width and height are set to be one pixel wider and taller than one might think is necessary. Play with it to convince yourself that this is necessary.
You could use an image file containing a circle and another blank image file and display the blank image on top of the circle image.

Image circleImage = getImage(getCodeBase(), "o.gif");
Image blankImage  = getImage(getCodeBase(), "blank.gif");

g.drawImage(circleImage, 50, 50, 100, 100, this);
g.drawImage(blankImage,  50, 50, 100, 100, this);

You could use double-buffering techniques to first draw to an offscreen image buffer and then display that image when the time is right. This is especially useful when you need smooth animation effects.

import java.applet.Applet;
import java.awt.*;

public class DeleteMe extends Applet {

private Graphics offScreenGraphics;
private Image offScreenImage;

public void paint(Graphics g) {

int width  = getWidth();
int height = getHeight();

if (offScreenImage == null) {
offScreenImage = createImage(width, height);
offScreenGraphics = offScreenImage.getGraphics();
}

// draw the circle off screen
offScreenGraphics.drawArc(50, 50, 200, 200, 0, 360);

// show the offScreenImage
g.drawImage(offScreenImage, 0, 0, this);

// then, if you're doing animation, you could sleep for a bit
try { Thread.sleep(5000); }// this will pause for 5 seconds
catch (InterruptedException e) { System.err.println("sleep exception"); }

// erase your circle on the offScreen graphics object
// using one of the previous techniques
offScreenGraphics.clearRect(0, 0, width + 1, height + 1);

// and again show your offScreenImage
g.drawImage(offScreenImage, 0, 0, this);
}

}

Note: There are cleaner implementations of Double-Buffering, but this one is a beginning.
Hopefully something here helps you.
Good Luck,
-Dirk Schreckmann
[ January 31, 2002: Message edited by: Dirk Schreckmann ]

[Dalibor Toth]

Guys, all I can comment is thank you!