Re: Graphics - how to show partial progress.

Lew <>
Fri, 21 Dec 2007 19:59:43 -0500
rossum wrote:

// --- Partial Code Starts ---

    class GraphPanel extends JPanel { final int
biXpos = 30;
        final int biYpos = 60;
        final int biHeight = 100;
        final int biWidth = 150;
        volatile BufferedImage mOffScreenImage;
                /** To run the pixel clculations */ Thread
                volatile boolean mStopCalcRequested = false;
                        public GraphPanel() {

D'you think you could do something about all this wacky indentation?

            // Set up off screen image
            mOffScreenImage = new BufferedImage(biWidth,
                    biHeight, BufferedImage.TYPE_INT_BGR);

            // Start pixel calculation thread
            PixelCalc pc = new PixelCalc();
            mCalculator = new Thread(pc);
        } // end constructor
           /** Calculates colours for pixels */
        class PixelCalc implements Runnable {
                        public void run() {
                Graphics2D g2d = mOffScreenImage.createGraphics();

                final int xLimit = mOffScreenImage.getWidth();
                final int yLimit = mOffScreenImage.getHeight();
                for (int x = 0; x < xLimit; ++x) {
for (int y = 0; y < yLimit; ++y) {
                        g2d.setColor(pickColour(x, y));
                        g2d.drawLine(x, y, x + 1, y + 1);
                    } // end for
                                        // Check for termination request
                    if (mStopCalcRequested) { return; }
                                        Thread.yield(); // Play nice
                                        // Show partial progress
                    if (x % 50 == 49) {
                    } // end if

                    // Artificial Delay
                    try {
                    } catch (InterruptedException iex) {
                    } // end try/catch
                } // end for // Show completed image
            } // end run()

            private Color pickColour(int x, int y) {
                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()
        } // end class PixelCalc
        public void paintComponent(Graphics g) {
            // Resize
            Container parent = this.getParent();
            this.setSize(parent.getWidth(), parent.getHeight());
                        // Show image
            g.drawImage(mOffScreenImage, biXpos, biYpos, null);
        } // end paintComponent()

    } // end class GraphPanel

// --- Partial Code Finishes ---

Daniel Pitts wrote:

Still got problems. you can't access a buffered images graphics from a
different thread without synchronization. You might be better off
queuing up *blocks* of calculations into a ConcurrentQueue, and then
during a repaint, use queue.poll, and build out your BufferedImage at
that time.

I'll bet he thought making the buffered image 'volatile' would do the trick,
and indeed, it would do part of it. I still have enough trouble reasoning
about concurrency to be unsure of the pitfalls of that approach, though.

Don't forget to call Graphics2D.dispose() when you're done with the
graphics object you created.

Isn't it dangerous to call repaint() from off the EDT?

It also makes me nervous to see a thread kicked off from the GraphPanel
constructor, but I don't see any egregious actual danger from it yet. Just
that queasy feeling ...


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(Defender Magazine, October 1933)