Creates a graphics port object that can be used for querying fonts and measuring text or images.
The function create-dummy-graphics-port
creates a graphics port object that can be used for font queries, measuring text and images.
graphics-port is a graphics port object associated with screen . graphics-port is never visible on the screen, but can be used to query fonts, measure text and load images to obtain their width and height. Drawing functions are not supported.
CAPI User Guide and Reference Manual (Unix version) - 3 Aug 2017