System and method for efficient computer workstation screen updates

DETAILED DESCRIPTION The invention is based on the need in collaborative or cooperative processing environments to allow updates to the screen of a local computer to be shared or mirrored as quickly as possible on the screens of one or more remote terminals connected to the local computer via some form of network.
Rather than having to transmit the whole contents of the screen, only those areas of the screen that have been modified are transmitted, resulting in much quicker updating of the remote screens.
In a preferred embodiment, the computer workstation further includes means for storing a list of the updated portions of the screen, means for deleting a portion from the list when its contents have been transmitted, means for detecting overlap if any part of the screen is updated twice prior to transmission, and means for modifying the list of stored portions to avoid duplication.
Thus if the same area of the screen is repeatedly updated, as would happen for example if a window was slowly dragged from one location to another, then the list still only contains a single reference to each modified area.
This avoids having to repeatedly send the same area of the screen, even although it may have been updated several times.
Note that the store only contains a list of the updated portions of the screen.
The actual contents of the updated portions are not stored, since they may well be updated again before transmission.
In a preferred embodiment each identified portion of the screen is represented by a rectangle, and it is then the contents of this rectangle that is transmitted to the other computers in the network.
The use of a rectangle is computationally very simple, and turns out to correspond to a large majority of updates.
In a few cases the update has a more complicated shape, so that possibly a large proportion of the rectangle transmitted has not been updated.
This could be avoided by using other shapes, perhaps based on more sophisticated calculations to determine very accurately the updated area of the screen for transmission