The RASTRAC Map Refresh Algorithm


The RASTRAC Map Refresh Algorithm

Each time RASTRAC draws the vehicle crosshair and icon on a map image, it must first somehow erase any previous vehicle (so you only see one vehicle on the screen at a time). The easiest and most robust way to do this is for RASTRAC to simply redraw that entire image before it draws the new vehicle location. This works quite well for BMP images, and somewhat well for 256-color or monochrome GIF images. This works efficiently because the image formats do not require a large amount of processing to load, and thus load relatively quickly.

However, images having large file size, and images with high color resolution (e.g., 24 bpp images) can take as long as 10-15 seconds to load and display (depending on the speed of the computer and graphics accelerator card being used). Of course, if RASTRAC is to redraw the vehicle once per second or faster, it clearly cannot redraw such an image with every redraw operation of the vehicle. Therefore, RASTRAC only redraws a small portion of the image each time the vehicle is drawn. You can set the size of the small update rectangle by changing the Icon Update Rect Size in the "Tools... Options... System... Map" property sheet.

It is possible that the small redraw rectangle will not completely cover some pieces of the track history. To compensate for this possibility, RASTRAC periodically performs a complete image refresh. The interval between complete image refreshes is set by the Auto-Refresh Interval in the "Tools... Options... System... Map" property sheet.

You may disable the auto-refresh function by setting the Auto-Refresh interval to 0 seconds.

If you notice extremely slow image update rates or the hourglass cursor stays on the screen too long, you may need to segment your map images into smaller sub-images. Large images take longer to display than small images. Zooming in on an image will also cause it to require more time to refresh.

Slow image drawing and update rates can be alleviated, in many cases, by upgrading to a faster computer and/or adding a graphics accelerator card to your system.