---------- Rtrees -- ---------- A wrapper around another model, which provides efficient geospatial queries, like finding the smaller enclosing rectangle for all the items, or all the items within a given region. The items are stored in the base model, and the Rtree model adds an extra data structure on top of it to speed up the queries. When new items are added to the model, Refresh_Layout must be called to refresh the internal cache (but this should always be done in any case to refresh the display of links for instance).
Wrapping a model in a Rtree means that the speed of displaying the canvas (and more importantly scrolling it) now depends on the number of items on the screen, not the total number of items in the model). As a result, it is possible to have models with hundreds of thousands of items.