Transcript Dynamic Grphx

Java Dynamic Graphics
In Java
• Repaint event:
• Java Swing components catch repaint event,
and call their paintComponent( ) method
• Default paintComponent( ) method paints component
– E.g. panel background color
• Sub-class the component (typically JPanel)
• Over-ride paintComponent( ) method
• Custom graphics here
• Can call repaint( ) to invoke paintComponent( )
Code
public class MyPanel extends JPanel
{
public void paintComponent(Graphics g)
{
super.paintComponent(g);
// erases background
Graphics2D g2 = (Graphics2D)g; //cast for java2
}
}
// my graphics:
g2.setColor(new Color(255,150,0));
g2.fillRect(10,10,200,50);
g2.setColor(new Color(0,0,0));
g2.drawString("Hello World", 10, 10);
Hello World
Typical program structure for
Dynamic Graphics
• Store window contents in data structure
• E.g. user drawn picture in paint program
• Repaint event:
• Erase window (draw background color)
• Draw window contents using data structure
• Other event that alters window contents:
• modify the data structure
• send repaint event
Storing window contents
2 approaches:
– Store logical contents in a data structure(s)
• e.g. drawing program: lines, shapes, colors, …
• Then draw contents of data structure
– Store visual contents as an off-screen image
(bitmap)
• e.g. pixels
• Then use g2.drawImage( ) in paintComponent( )
Problem: Flashing
• Ugly flashing when repaint:
• Paint background
• Erase
• Redraw shapes
Solution: Double buffering
Solution: Double buffering
• Double buffered repaint:
• Draw all graphics in temporary off-screen image
• Paint background color
• Paint shapes
• Then paint image to JPanel
• Bonus: Swing does this for you!!!
• Draw graphics on JPanel
• JPanel maintains off-screen image
What Subwindows Do
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Directs mouse input to correct component
Determines repaint events
Own coordinate system
Don’t need repaint when moving
Clipping: hides drawing behind a subwindow
• Some subwindows remember what’s under them:
• Popup menus