Have you ever been in a situation where you would like an application to know about graphics drawn by some other programme? For instance, you display an image of the sky, then later you want to obtain the co-ordinates of the stars within the image via the cursor. There are two main approaches to achieving this functionality. The first is to duplicate the display code in the CURSOR application. This is wasteful and inflexible. The second is to store information about `pictures' in a database that can be accessed by subsequent graphics programmes. This is the technique used by KAPPA.
Each graphics application that creates a display on a graphics device, also stores information describing the display in the graphics database. This is a file, which usually resides in the user's home directory, and is often referred to as the AGI database.9 Displays are described in terms of pictures. A picture is basically a rectangular area on the graphics device within which an application produces graphical output. Each time an application creates a picture, the dimensions and position of the picture (together with other ancillary information) are stored in the graphics database. Subsequent applications can then read this information back from the database, and use it (for instance) to align new graphics with previously displayed graphics.
The best way to demonstrate the use of the graphics database is to give
some illustrated examples.
KAPPA --- Kernel Application Package