Abstract: When clipping a graphics element defined by its element border to a viewing window having a window border within which the graphics element is to be displayed, it is necessary to plot closure lines along the window border between the points where the element border exits and enters the viewing window. This closure line may be plotted along one of two paths between the points and along the window border. The system detects which of these paths the closure line should take. An effectively infinitely extending line from a point on the window border is tested to see whether it crosses that portion of the element border outside the viewing window between the exit point and the entry point an odd or an even number of times. If it crosses an odd number of times then the closure line is plotted to pass through the predetermined point, and if it crosses an even number of times then the closure line is plotted to not pass through the predetermined point.

Abstract: A multiple window display system includes a display device and a screen ownership area pointing to the identity of the window which is to contribute the data for each display area of the display device. An ordered list is maintained of the active windows in the priority order thereof. Means are provided to regenerate the screen ownership area from the ordered list, on each change made to the list, in terms of list position per device display area, by overwriting, progressing through the list in order of increasingly significantly priority order, the list indicating, in each position thereof, the identity of the window having the respective priority. The list contains the addresses of the windows in storage and the type thereof. The screen ownership area is reset to the lowest potential priority list position value and is overwritten.

Abstract: A data display system in which input-output display devices are connected to a central processor, and users select application programs that are run on the central processor. The control system of the central processor includes a display manager control system and a windowing control mechanism which allows a plurality of tasks to be performed concurrently and the results displayed in areas of a display screen. The windowing control mechanism includes, a task manager control program which runs as an application in the display manager control system and includes means to interact, via the display manager, with the operator to allow applications to be initiated, means to create tasks to control the processing of the application in such a way that the applications can be suspended or resumed according to whether the operator is ready for them, and means to identify to the display manager a coordination controller that the display manager can call to allow the task manager to suspend and resume applications.