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( )

Codepublic 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

• 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