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javaswingdebuggingnetbeansgraphics2d

How can I effectively find a Graphics2D rendering error?

Can you suggest a method of identifying the source of a rendering error from a debugger?

Misko Hevery classifies bugs into three categories:

  • Logical
  • Wiring
  • Rendering

Its clear to me that my problem is a rendering bug.

I have a Swing application with a Panel that contains multiple layers. Rendering all the layers can take a significant amount of time so the application uses a thread pool to render layers and tiles from layers into BufferedImages. When it comes time for the Event Dispatch Thread to render the panel the most-recently rendered BufferedImages are drawn to the screen.

This setup has performed adequately.

A new feature requires that a certain layer type support transparency. Something, somewhere isn't preserving the transparency. The error could be in a number of places, possibly in the implementation of the objects to be rendered, the error could possibly be in the offline rendering thread implementation. Its possible that the many BufferedImages aren't combined together correctly in the EDT rendering code.

I'm not asking anyone to look at the code and tell me where the error is.

What I want to know is what techniques people have found particularly effective in troubleshooting a Graphics2D rendering issue.

I'm a big supporter of unit tests but I'd prefer to start with another technique.

Is there a method or trick to visually inspect a BufferedImage or Graphics2D object from the debugger?

In the Netbeans Variables and Watches windowsExample Watch window Netbeans sometimes uses PropertyEditors to display variable values. In this example image the value of foregroundColor and backgroundColor are shown as small swatches of the Color's value.

Is there an easy way to add/enable a Netbeans PropertyEditor which would display the contents of a BufferedImage?

I could temporarily sprinkle the code with method calls to write the various BufferedImages encountered to the disk such that they could then be inspected offline. It might work but it would be tediious to match the file on disk to the source code.

What would you do?

Solution

  • You might compare your approach to the one shown here with regard to clearing the buffer:

    g2d.setComposite(AlphaComposite.Clear);
g2d.fillRect(0, 0, w, h);

    In the worst case, you can break at a point in which your image is accessible and set a watch on the expression image.getRGB(0,0) with the display set to hexadecimal. The high order byte is the alpha value: FF is opaque, and 00..FE represents varying transparency.