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12.14 Custom Paint

Figure 12-8 showed a variety of shape-filling techniques; it included a large letter A filled with a complex pattern defined by the GenericPaint class. Example 12-18 shows the implementation of this class. You may want to take another look at Example 12-10 to see how the GenericPaint class is used, before you dive into the code listed here.

The GenericPaint class itself is pretty simple: it defines both the abstract color computation methods that subclasses implement and a createContext( ) method that returns a PaintContext. The implementation of PaintContext does all the hard work. This is pretty low-level stuff, so don't be dismayed if you don't understand everything. The code should at least give you a basic idea of how painting works in Java 2D.

Example 12-18. GenericPaint.java
package je3.graphics;
import java.awt.*;
import java.awt.geom.*;
import java.awt.image.*;

 * This is an abstract Paint implementation that computes the color of each
 * point to be painted by passing the coordinates of the point to the calling
 * abstract methods computeRed( ), computeGreen( ), computeBlue( ) and
 * computeAlpha( ).  Subclasses must implement these three methods to perform
 * whatever type of painting is desired.  Note that while this class provides
 * great flexibility, it is not very efficient.
public abstract class GenericPaint implements Paint {
    /** This is the main Paint method;  all it does is return a PaintContext */
    public PaintContext createContext(ColorModel cm,
                                      Rectangle deviceBounds,
                                      Rectangle2D userBounds,
                                      AffineTransform xform,
                                      RenderingHints hints) {
        return new GenericPaintContext(xform);

    /** This paint class allows translucent painting */
    public int getTransparency( ) { return TRANSLUCENT; }

     * These three methods return the red, green, blue, and alpha values of
     * the pixel that appears at the specified user-space coordinates.  The return
     * value of each method should be between 0 and 255.
    public abstract int computeRed(double x, double y);
    public abstract int computeGreen(double x, double y);
    public abstract int computeBlue(double x, double y);
    public abstract int computeAlpha(double x, double y);

     * The PaintContext class does all the work of painting
    class GenericPaintContext implements PaintContext {
        ColorModel model;  // The color model
        Point2D origin, unitVectorX, unitVectorY;  // For device-to-user xform

        public GenericPaintContext(AffineTransform userToDevice) {
            // Our color model packs RGB values into a single int
            model = new DirectColorModel(32, 0x00ff0000,0x0000ff00,
                                         0x000000ff, 0xff000000);
            // The specified transform converts user to device pixels
            // We need to figure out the reverse transformation, so we
            // can compute the user space coordinates of each device pixel
            try {
                AffineTransform deviceToUser = userToDevice.createInverse( );
                origin = deviceToUser.transform(new Point(0,0), null);
                unitVectorX = deviceToUser.deltaTransform(new Point(1,0),null);
                unitVectorY = deviceToUser.deltaTransform(new Point(0,1),null);
            catch (NoninvertibleTransformException e) {
                // If we can't invert the transform, just use device space
                origin = new Point(0,0);
                unitVectorX = new Point(1,0);
                unitVectorY = new Point(0, 1);

        /** Return the color model used by this Paint implementation */
        public ColorModel getColorModel( ) { return model; }

         * This is the main method of PaintContext.  It must return a Raster
         * that contains fill data for the specified rectangle.  It creates a
         * raster of the specified size, and loops through the device pixels.
         * For each one, it converts the coordinates to user space, then calls
         * the computeRed( ), computeGreen( ) and computeBlue( ) methods to
         * obtain the appropriate color for the device pixel.
        public Raster getRaster(int x, int y, int w, int h) {
            WritableRaster raster = model.createCompatibleWritableRaster(w,h);
            int[  ] colorComponents = new int[4];
            for(int j = 0; j < h; j++) {      // Loop through rows of raster
                int deviceY = y + j; 
                for(int i = 0; i < w; i++) {  // Loop through columns
                    int deviceX = x + i;
                    // Convert device coordinate to user-space coordinate
                    double userX = origin.getX( ) + 
                        deviceX * unitVectorX.getX( ) + 
                        deviceY * unitVectorY.getX( );
                    double userY = origin.getY( ) + 
                        deviceX * unitVectorX.getY( ) + 
                        deviceY * unitVectorY.getY( );
                    // Compute the color components of the pixel
                    colorComponents[0] = computeRed(userX, userY);
                    colorComponents[1] = computeGreen(userX, userY);
                    colorComponents[2] = computeBlue(userX, userY);
                    colorComponents[3] = computeAlpha(userX, userY);
                    // Set the color of the pixel
                    raster.setPixel(i, j, colorComponents);
            return raster;

        /** Called when the PaintContext is no longer needed. */
        public void dispose( ) {  }
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