5- How the Display Works

The JadeDisplay component asynchronously requests tiles as they are needed for the image display. The tiles are loaded and computed in the background using JAI worker threads, and are painted onscreen when the computation is complete. This has the following advantages:

• User interface remains active and responds to events even while the image is loading... no waiting for the computer. Experience has shown that this is a prime factor in usability.
  
  
• Viewing of enormous images is practical because the user does not have to wait for them to load in their entirety. The display has been tested with images larger than 2 gigabytes.
  
  
• Scrolling is instantaneous, even for slow-loading images.
  
  
• Computations of tiles which are in the queue, but become unnecessary because of subsequent scrolling, are cancelled, thus improving overall throughput.
  
  
• Threaded computation allows for greater throughput for compute-intensive operations on multiprocessor machines (e.g. rotation, filtering).
  
  

Tile Computation Sequence of Events

When Java sends a repaint event, the affected area is saved in a "damage list". Tiles affected by the repaint are queued with JAI for computation (unless the tile has already been queued). When JAI sends a tile completion notification, the part of the tile intersecting the damage list is immediately painted, and the tile's area is subtracted from the damage list. Multiple worker threads can be active simultaneously (omitted from the drawing for clarity).