12.4 Transforms

The AffineTransform class can transform a shape (or coordinate system) by translating, scaling, rotating, or shearing it, using any combination of these individual transformation types. Figure 12-5 illustrates the results of various types of coordinate-system transformations on the appearance of one shape, drawn multiple times. Example 12-7 shows the Java code that generates the figure.

Figure 12-5. Transformed shapes

An affine transformation is a linear transformation that has two important properties: all straight lines remain straight, and all parallel lines remain parallel. The AffineTransform class can perform a great variety of transformations, but it can't produce nonlinear effects, such as distorting a figure as through a fisheye lens. That kind of effect can be achieved only with image-processing techniques. For a further discussion of transformations and the linear algebra behind the AffineTransform class, see Chapter 4 of Java Foundation Classes in a Nutshell.

Example 12-7. Transforms.java

package je3.graphics;
import java.awt.*;
import java.awt.geom.*;

/** A demonstration of Java2D transformations */
public class Transforms implements GraphicsExample {
    public String getName( ) { return "Transforms"; } // From GraphicsExample
    public int getWidth( ) { return 750; }            // From GraphicsExample
    public int getHeight( ) { return 250; }           // From GraphicsExample

    Shape shape;                   // The shape to draw
    AffineTransform[  ] transforms;  // The ways to transform it
    String[  ] transformLabels;      // Labels for each transform

    /** 
     * This constructor sets up the Shape and AffineTransform objects we need
     **/
    public Transforms( ) {
        GeneralPath path = new GeneralPath( );   // Create a shape to draw
        path.append(new Line2D.Float(0.0f, 0.0f, 0.0f, 100.0f), false);
        path.append(new Line2D.Float(-10.0f, 50.0f, 10.0f, 50.0f), false);
        path.append(new Polygon(new int[  ] { -5, 0, 5 },
                                new int[  ] { 5, 0, 5 }, 3), false);
        this.shape = path;  // Remember this shape

        // Set up some transforms to alter the shape
        this.transforms = new AffineTransform[6];
        // 1) the identity transform
        transforms[0] = new AffineTransform( );  
        // 2) A scale tranform: 3/4 size
        transforms[1] = AffineTransform.getScaleInstance(0.75, 0.75);
        // 3) A shearing transform
        transforms[2] = AffineTransform.getShearInstance(-0.4, 0.0);
        // 4) A 30 degree clockwise rotation about the origin of the shape
        transforms[3] = AffineTransform.getRotateInstance(Math.PI*2/12);
        // 5) A 180 degree rotation about the midpoint of the shape
        transforms[4] = AffineTransform.getRotateInstance(Math.PI, 0.0, 50.0);
        // 6) A combination transform
        transforms[5] = AffineTransform.getScaleInstance(0.5, 1.5);
        transforms[5].shear(0.0, 0.4);
        transforms[5].rotate(Math.PI/2, 0.0, 50.0);  // 90 degrees

        // Define names for the transforms
        transformLabels = new String[  ] {
            "identity", "scale", "shear", "rotate", "rotate", "combo"
        };
    }

    /** Draw the defined shape and label, using each transform */
    public void draw(Graphics2D g, Component c) {
        // Define basic drawing attributes
        g.setColor(Color.black);                                   // black
        g.setStroke(new BasicStroke(2.0f, BasicStroke.CAP_SQUARE,  // 2-pixel
                                    BasicStroke.JOIN_BEVEL));
        g.setRenderingHint(RenderingHints.KEY_ANTIALIASING,        // antialias
                           RenderingHints.VALUE_ANTIALIAS_ON);

        // Now draw the shape once using each of the transforms we've defined
        for(int i = 0; i < transforms.length; i++) {
            AffineTransform save = g.getTransform( );    // save current state
            g.translate(i*125 + 50, 50);                // move origin
            g.transform(transforms[i]);                 // apply transform
            g.draw(shape);                              // draw shape
            g.drawString(transformLabels[i], -25, 125); // draw label
            g.drawRect(-40, -10, 80, 150);              // draw box
            g.setTransform(save);                       // restore transform
        }
    }
}