When an object is seen through a portal, the portal can communicate with the object during rendering to change how the object appears.
The portal can also filter user events (mouse clicks, etc.) which pass through it - either passing these events on unchanged, blocking them, or communicating with the object to decide how to interpret the event. So objects can behave differently when seen through portals.
By using these features, developers can create portals which act as magic lenses - lenses which change the nature of the data in unique ways. (the concept of portal filters and magic lenses was developed independently by Ken Perlin and David Fox in the original version of Pad and by Eric Beir, et. al. at Xerox PARC).
Lets look at some examples.
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