Re: Graphics - how to show partial progress.

I am writing a graphics program that calculates individual colour
values for every point, hence it runs rather slowly as the
calculations for each point are somewhat complex. What I want to be
able to do is to show progress-so-far every so often, so the user gets
to see something, rather than a long wait with a blank screen. I know
I can do an updating % progress bar, but I would rather actually show
the progress on the screen so they can see a partial picture while the
next part is being calculated.

I am new to Java graphics, so I am probably missing some obvious
technique here; my google-fu has not found anything relevant to what I
want. In other languages I have used a technique with two Graphics
objects, updating one while displaying the other, but in this case I
cannot find a Component.setGraphics() method. Would it be possible to
do a similar thing with two separate JPanel objects?

The partial code below shows what I am trying, I need a "display and
continue calculating" function that I can call at intervals while
calculating points.

Does anyone have any suggestions?

Thanks in advance,

rossum

// --- Partial Code Starts ---

    class GraphPanel extends JPanel {
        
        @Override
        public void paintComponent(Graphics g) {
            super.paintComponent(g);
            
            // Resize
            Container parent = this.getParent();
            this.setSize(parent.getWidth(), parent.getHeight());
            
            // Draw points on screen
            for (int x = 50; x < (250); ++x) {
                for (int y = 60; y < (300); ++y) {
                    g.setColor(pickColour(x, y));
                    g.drawLine(x, y, x + 1, y + 1);
                } // end for
                if (x % 50 == 49) { showProgressSoFar(); }
            } // end for
        } // end paintComponent()
        
        private Color pickColour(int x, int y) {
            // Slow calculation of colour for point
            switch ((x + y) % 3) {
                case 0: return Color.CYAN;
                case 1: return Color.GREEN;
                case 2: return Color.MAGENTA;
                default: return Color.BLACK;
            } // end switch
        } // end pickColor()

        private void showProgressSoFar() {
            // ###########################
            // ### What do I put here? ###
            // ###########################
        } // end showProgressSoFar()
        
    } // end class GraphPanel

// --- Partial Code Finishes ---