2.1 Conceptual Overview of Drawing Graphics

2.1.4 Mediums, Sheets, and Streams

Mediums, sheets, and streams are classes of primary importance in the creation of graphics in CLIM.

One of the arguments taken by drawing functions is a medium. A medium keeps track of device-specific information necessary for creating graphics. There are different medium classes to support different devices; thus, there is one medium class for the X Window System and a different one for the Macintosh Common Lisp environment. A medium implements the low-level graphic functions such as drawing a line or displaying a color. A medium also keeps track of its drawing environment, which includes such things as the current transformation, text style, line style, and foreground and background inks.

A sheet specifies the destination for the graphical output of a medium.Whereas mediums are device-specific, sheets are completely portable. Sheets are visible objects that have properties such as a position, a region, a parent, and children. Interface elements such as scrollbars and pushbuttons are subclasses of sheets. For convenience, sheets have also been made to support the graphics protocol. A graphics function call to a sheet object, however, simply results in the same graphics function call being made to the medium object.

Streams are specialized sheets that implement the sheet and stream protocols. A stream is thus a sheet that supports stream methods like write-string and keeps track of additional stream-related state information, such as current cursor position.

CLIM 2.0 User's Guide - OCT 1998

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