JPanel repaint from another class

Question

I have a JPanel which displays an image. In a separate class, I'm reading from an xml file points. I am firstly creating an arraylist of triangles from these points. However I need to show the triangles on the image, ie draw them on! (yes this should be simple). But as these points and triangles are created in another class, I do not seem to be able to draw them on the already-displayed image within the GUI class. I have tried creating a ArrayList in the JPanel itself, which I update and then want to repaint, although it will not let me do this as shown below:

Class

triangles = clips.getTriangles();
tempPanel.setTriangles(triangles){

JPanel

 public void settriangles(ArrayList<Point[]> t){
 triangles = t;
 repaint();
}

My only other idea is for the JPanel to have a listener waiting for when triangles are returned, updating the field and hence then repainting.

Any ideas?

Thanks

Edit: Code for Drawing

public void settriangles(ArrayList<Point[]> t){
    triangles = t;
    repaint();
}

public void paintComponent(Graphics g) {

    System.out.println("in paint component");
if (g != null) {
    Graphics2D graphics = (Graphics2D) g;
    try {
        Rectangle back_rect = new Rectangle(0, 0, getWidth(),
                getHeight());
        graphics.setColor(GuiComponentGenerator.GUI_BACKGROUND_COLOUR);
        graphics.fill(back_rect);
        if (image != null) {
            int width = Math.round(image.getWidth() * magnification);
            int height = Math.round(image.getHeight() * magnification);
            Rectangle image_rect = new Rectangle(offset.x, offset.y,
                    width, height);
            graphics.setColor(Color.BLACK);
            graphics.draw(image_rect);
            graphics.drawImage(image, offset.x, offset.y, width,
                    height, null);
            graphics.setRenderingHint(RenderingHints.KEY_ANTIALIASING,
                    RenderingHints.VALUE_ANTIALIAS_ON);
            for(int pos = 0; pos < triangles.size(); pos++){
                Point[] current = triangles.get(pos);                   
                ArrayList<Point> current_triangle = new ArrayList<Point>();
                current_triangle.add(current[0]);
                current_triangle.add(current[1]);
                current_triangle.add(current[2]);
                drawRegion(graphics, current_triangle); 
            }
        }
    }

finally {
        graphics.dispose();
}
}



private void drawRegion(Graphics2D graphics, ArrayList<Point> points) {
    graphics.setColor(trans_grey);
    Area area = getArea(points);
    graphics.fill(area);
    graphics.setStroke(new BasicStroke(2));
    graphics.setColor(Color.BLACK);
    graphics.draw(area);
}

private Area getArea(ArrayList<Point> points) {
    Area area = new Area(getPath(points, true));
    return area;
}

private GeneralPath getPath(ArrayList<Point> points, boolean close_path) {
    GeneralPath path = new GeneralPath();
    Point current_screen_point = calculateScreenPoint(points.get(0));
    path.moveTo(current_screen_point.x, current_screen_point.y);
    for (int point_num = 1; point_num < points.size(); point_num++) {
        current_screen_point = calculateScreenPoint(points.get(point_num));
        path.lineTo(current_screen_point.x, current_screen_point.y);
    }
    if (close_path)
        path.closePath();
    return path;
}

public Point calculateScreenPoint(Point image_point) {
    float h_proportion = (float) image_point.x / (float) image.getWidth();
    float v_proportion = (float) image_point.y / (float) image.getHeight();
    float image_width_in_panel = (float) image.getWidth() * magnification;
    float image_height_in_panel = (float) image.getHeight() * magnification;

    Point on_screen_point = new Point(0, 0);
    on_screen_point.x = offset.x
            + Math.round(h_proportion * image_width_in_panel);
    on_screen_point.y = offset.y
            + Math.round(v_proportion * image_height_in_panel);
    return on_screen_point;
}

Answer 1

Your paintComponent leaves a little to be desired ;)

Firstly, you should never get a null graphics unless the paint method has been called in correctly, which in case they deserve for it to fail.

You should try and use Graphics.create to create a copy of the incoming Graphics context. This allows you to mess with the Graphics properties (such as transforms etc) without adversly effecting the original

I don't know what the image is all about, but basically, if its null , your triangles will never paint (don't know if this is what you want or not).

I don't know what the offset relates to, but just in case, the 0x0 point is always the top, left corner of your component.

public void paintComponent(Graphics g) {

    // This is important, you will to have a very good reason not to call this
    super.paintComponent(g);

    System.out.println("in paint component");
    // Should never need this.  You should never call the paintComponent
    // directly.
    // if (g != null) {
    // Create a copy of the graphics, with which you can play...
    Graphics2D graphics = (Graphics2D) g.create();
    try {
        Rectangle back_rect = new Rectangle(0, 0, getWidth(),
                        getHeight());
        graphics.setColor(Color.GREEN);
        graphics.fill(back_rect);
        // What's this trying to do...
        // Where do you produce this???
        // Because I didn't know where the image was been produced
        // I commented out the code, but you should be aware
        // That if the image is null, you triangles will never paint...
//            if (image != null) {
//                int width = Math.round(image.getWidth() * magnification);
//                int height = Math.round(image.getHeight() * magnification);
//                
//                Rectangle image_rect = new Rectangle(offset.x, offset.y,
//                                width, height);
//                graphics.setColor(Color.BLACK);
//                graphics.draw(image_rect);
//                graphics.drawImage(image, offset.x, offset.y, width,
//                                height, null);
            graphics.setRenderingHint(RenderingHints.KEY_ANTIALIASING,
                            RenderingHints.VALUE_ANTIALIAS_ON);
            for (int pos = 0; pos < triangles.size(); pos++) {
                Point[] current = triangles.get(pos);
                ArrayList<Point> current_triangle = new ArrayList<Point>(3);
                current_triangle.add(current[0]);
                current_triangle.add(current[1]);
                current_triangle.add(current[2]);
                drawRegion(graphics, current_triangle);
            }
        //} // From the image != null
    } finally {
        graphics.dispose();
    }
}

I'd also suggest you have a read through

• 2D Graphics
• Performing Custom Painting in Swing

If you haven't already ;)

Answer 2

This article will give you all the info you need http://java.sun.com/products/jfc/tsc/articles/painting/

but I think you are missing - super.paintComponent(g);

public class MyPanel extends JPanel {
    protected void paintComponent(Graphics g) {
    // Let UI delegate paint first 
    // (including background filling, if I'm opaque)
    super.paintComponent(g); 
    // paint my contents next....
    }
}

Answer 3

I worked out how to draw triangles on the image, when passing through an arrayList, where each Point[] represents the points of the triangle.

Note that this is now in a single entire class which is passed the information, rather than trying to call repaint from another class.

public AnnotatedDisplayTriangles(BufferedImage image, String image_path, ArrayList<Point[]> triangles) {

    this.image = image;
    this.image_path = image_path;
    this.triangles = triangles;

}

protected void paintComponent(Graphics g) {
    super.paintComponent(g);
    // Draw image centered.
    int x = (getWidth() - image.getWidth())/2;
    int y = (getHeight() - image.getHeight())/2;
    g.drawImage(image, x, y, this);

    Stroke drawingStroke = new BasicStroke(2);
    Graphics2D graph = (Graphics2D)g;
    graph.setStroke(drawingStroke);
    graph.setPaint(Color.black);


    for(int p = 0; p < triangles.size(); p++){

        Point[] current_triangles = triangles.get(p);

        for(int triangle = 0; triangle < current_triangles.length; triangle++ ){

            Point current = current_triangles[triangle];
            Point next;
            if(triangle == current_triangles.length -1 )
                next = current_triangles[0];
            else
                next = current_triangles[triangle + 1];

            Line2D line = new Line2D.Double(current.x, current.y, next.x, next.y);
            graph.draw(line);

        }
    }
}

public static void main(String image_path,ArrayList<Point[]> triangles, String panel_name) throws IOException {
    String path = image_path;
    BufferedImage image = ImageIO.read(new File(path));
    AnnotatedDisplayTriangles contentPane = new AnnotatedDisplayTriangles(image, path, triangles);
    // You'll want to be sure this component is opaque
    // since it is required for contentPanes. Some
    // LAFs may use non-opaque components.
    contentPane.setOpaque(true);

    int w = image.getWidth();
    int h = image.getHeight();

    JFrame f = new JFrame(panel_name);
//  f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    f.setContentPane(contentPane);
    f.setSize(w,h);
    f.setLocation(200,200);
    f.setVisible(true);
}