The main features of graphics ports are:
Each port has a "graphics state" which holds all the information about drawing parameters such as line thickness, fill pattern, line-end-style and so on. A graphics state object can also be created independently of any particular graphics port.
The graphics state contents can either be enumerated in each drawing function call, bound to values for the entirety of a set of calls, or permanently changed.
The graphics state includes a transform which implements generalized coordinate transformations on the port's coordinates.
Off-screen ports can compute the horizontal and vertical bounds of the results of a set of drawing function calls, thus facilitating image or pixmap generation.
CAPI User Guide (Unix version) - 30 Aug 2011