Selective repaint() and Graphics2D transform?

Question

Hi all,
I am programming a graphic 2D simulation consisting of moving circles in the plane. I am trying to go for efficient repaint() invocations by repainting only those areas that need refreshing, i.e., the previous and new location (bounding box, to be exact) of each circle. This is similar to what is demonstrated somewhere in the Java Tutorial, where a small red square is dragged by the mouse and repainted efficiently. So far so good.
However, I would like to support transforming the view, to enable zooming and panning (as well as auto-fitting) in the simulation window (or panel or whatever). Transforming is typically also straightforward - just concatenate the desired transform onto the Graphics2D object in my paintComponent() override and that's it.
The typical flow in Java is to issue a repaint() somewhere in the application code (e.g., mouse listener, or in my case - when updating the simulation, panning, or zooming). This request is later addressed by a Swing thread in a call to paintComponent() which also passes the Graphics2D object on which the actual drawing and transforming is performed.
Now to the thing that puzzles me: What coordinates should I pass to repaint()? At that moment I do not possess the Graphics2D object, so I don't know the transform from user coordinates (in the graphics context) to the component's coordinates. Do I have to somehow query the current transform assigned to the control, add my pan/zoom transform to it, and then manually transform the coordinates of my circles before feeding them to my repaint() calls? This feels inelegant and inefficient to me: Basically all transformation will be calculated twice: Once by me for the repaint calls, and again by paintComponent() when actually rendering the circles. Perhaps I could save the redundant calculations by keeping
the manually transformed coordinates (or transformed copies of my Shapes), and then draw them in paintComponent() without applying the transform to the Graphics2D object.
These solutions don't look right to me. Does anybody have a better idea? It seems to me like a fairly common scenario: Efficient painting of a 2D animation supporting view transformation (e.g. zoom/pan). There must already exist a known "correct" way to do this!
Hopefully...
Noam

Answer 1

You can drag the frame sides/bottom to see what happens to the clip and transform of the graphics context of the white panel. The clip seems to be in white panel coordinates and the transform translation values are relative to the parent component.
Each parent component passes its graphics context on to its children to use to paint themselves.
import java.awt.*;
import java.awt.geom.*;
import javax.swing.*;
public class GraphicsTest extends JPanel {
    public GraphicsTest() {
        setBackground(Color.white);
    protected void paintComponent(Graphics g) {
        super.paintComponent(g);
        Graphics2D g2 = (Graphics2D)g;
        inspect(g2);
        Shape clip = g2.getClip();
        Rectangle r = clip.getBounds();
        g2.setPaint(Color.red);
        g2.drawRect(r.x, r.y, r.width-1, r.height-1);
    public Dimension getPreferredSize() {
        return new Dimension(360, 300);
    private void inspect(Graphics2D g2) {
        Shape clip = g2.getClip();
        Rectangle r = clip.getBounds();
        System.out.printf("clip = [%d, %d, %d, %d]%n",
                           r.x, r.y, r.width, r.height);
        //    [  m00  m01  m02  ]
        //    [  m10  m11  m12  ]
        //    [   0    0    1   ]
        AffineTransform at = g2.getTransform();
        double[] matrix = new double[6];
        at.getMatrix(matrix);  // { m00 m10 m01 m11 m02 m12 }
        int[] loRow = { 0, 0, 1 };
        for(int i = 0; i < 2; i++) {
            System.out.print("[ ");
            for(int j = i; j < matrix.length; j += 2) {
                System.out.printf("%5.1f ", matrix[j]);
            System.out.print("]\n");
        System.out.print("[ ");
        for(int i = 0; i < loRow.length; i++) {
            System.out.printf("%3d   ", loRow);
System.out.print("]\n");
System.out.println("---------------------");
private JScrollPane getContent() {
// Show the graphic component at a smaller size
// so we can investigate space issues.
JPanel panel = new JPanel(new GridBagLayout());
panel.add(this, new GridBagConstraints());
return new JScrollPane(panel);
public static void main(String[] args) {
GraphicsTest test = new GraphicsTest();
JFrame f = new JFrame();
f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
f.add(test.getContent());
f.setSize(500,500);
f.setLocation(100,100);
f.setVisible(true);