Java Graphics

Review 3 kinds of elements in components API? Layout managers Events Extend vs. Implement

Useful Java DataStructures Vector (dynamic array) V = new Vector( ); V.add(item); V.elementAt(5); HashTable (maps keys to items) H = new HashTable( ); H.add(key, item); H.get(key); Iterators (automatic FOR loops) I = V.iterator( ); While (I.hasNext( )) dosomething(I.next( ));

Graphics Window is like a painter’s canvas App must paint its window contents Java components paint themselves Anything else: Programmer When to paint? How to paint? JButton

How to Paint?

Pixels

Coordinate System Upside-down Cartesian Y window = height - Y cartesian (0,0) (width,0) (0,height)(width, height)

Component Hierarchy Each component has its own subwindow Subwindow = rectangular area within parent component Has own coordinate system Clipping: Can’t paint outside its subwindow Can’t paint over child components? (0,0) (w p, h p ) (0,0) (w b, h b ) JPanel JButton

Painting Components Can paint any component JPanel is good for custom graphics area JButton JPanel

Painting in Java import java.awt.Graphics import java.awt.Graphics2D// Java2 1. Get the “graphics context” of component Graphics g = myJPanel.getGraphics( ); Graphics2D g2 = (Graphics2D) g; 2. Paint in it g2.drawLine(x1,y1, x2,y2);

Graphics Primitives DrawFill Point (x,y) Line (pt1,pt2) PolyLine (pt list) Arc Oval (pt, w,h) Rectangle (pt, w,h) RoundRectangle Polygon (pt list) Image (file, x,y) Text (string, x,y) label

Graphics Attributes Color Font Stroke attributes: –Line width, dash, end caps, joins, miter Paint attributes: –Color, gradient, texture Composite: –Blending Transforms: –Translate, rotate, flip, shear, scale

Color Combinations of Red, Green, Blue Each [0, 255] Java: new Color(255, 150, 0) Hokie Orange

in Java Drawing primitives: g2.drawLine( ),.drawRect( ), … g2.fillRect( ), … Object oriented: 1.Create shape: import java.awt.geom.* shape = new Point2D.Float(x, y); Line2D, Rect2D, CubicCurve2D, … 2.Draw the shape: g2.draw(shape); g2.fill(shape);

in Java Color and font: g2.setColor( new Color(r,g,b) ); g2.setFont( new Font(…) ); Advanced: g2.setStroke(…); g2.setPaint(…); g2.setComposite(…); g2.setTransform(…); 1.Set graphics attributes 2.Draw graphics

When to Paint?

Re-Paint Screen is like a painter’s canvas All windows paint on the same surface! Windows don’t “remember” whats under them Need to re-paint when portions are newly exposed Receive Repaint events Open, resize, bring to front When other windows in front move, resize, close

MyApp

Open WinExp, Notepad

Close WinExplorer Repaint event sent to: MyApp, Desktop

Desktop gets repaint event

MyApp gets repaint event MyApp JPanel gets repaint event

MyApp gets repaint event MyApp JPanel forwards repaint event to JButton

Repainting Static Graphics Repaint event: Erase subwindow (fill with background color) Draw subwindow contents

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(255,0,0)); 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 data structure of window contents 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

In JBuilder Subclassing JPanel Overriding paintComponent( ) Using subclassed JPanel in a JFrame

Storing window contents 2 approaches: Store logical contents in a data structure »E.g. drawing program: lines, shapes, colors, … » 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 Redraw shapes

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