To save memory, VisBio only keeps one dimensional position of the dataset (e.g., one timestep) in memory at a time, thus allowing visualization of huge datasets quickly. Unfortunately, when switching positions, VisBio needs to load the new position into memory from disk, which takes time and slows down animation.
To sidestep this problem, VisBio reads in all data when it first imports the dataset. It creates a low-resolution thumbnail of every image, keeping all these thumbnails in memory at once. It does this in the background, so that you can begin visualizing your data even before all thumbnails have been computed. You can see how far along the computation is from the progress bar in the Tasks panel. You can stop the computation process by clicking the "Stop" button next the progress bar, if you wish to conserve memory or CPU use.
When visualizing data, VisBio has a "lo-res" mode and a "hi-res" mode. In lo-res mode the thumbnail is displayed, whereas in hi-res mode the data is loaded from disk. When you switch positions, VisBio uses lo-res mode by default to avoid the slowdown of going to disk for the data, keeping animation quick.
VisBio loads hi-res data for the current dimensional position in the background. If it finishes loading the hi-res data, it displays it. Thus, the hi-res data will eventually "burn in" if you leave the displays idle for a moment.
If there is no thumbnail for an image plane in lo-res mode (e.g., if
thumbnails are not finished loading, or if you pressed the "Stop" button), the
plane will be missing. Once the hi-res data for that plane is loaded, its
thumbnail will be computed automatically and cached on disk.
The default thumbnail resolution is 96 x 96 pixels. You can adjust these
values in VisBio's Options (Preferences on Mac OS X). Making these numbers
smaller will result in lo-res mode looking blockier but thumbnails taking less
memory, whereas larger numbers will require more memory to store the thumbnails
but look crisper while animating.