6.2 Definitions

This section contains definitions of terms that will be used in this chapter.

Drawing plane

A drawing plane is an infinite two-dimensional plane on which graphical output occurs. The drawing plane contains an arrangement of colors and opacities that is modified by each graphical output operation. It is not possible to read back the contents of a drawing plane, except by examining the output-history. Normally each window has its own drawing plane.

Coordinates

Coordinates are a pair of real numbers in implementation-defined units that identify a point in the drawing plane.

Mediums

In this chapter, we use a medium as a destination for output. The medium has a drawing plane, two designs (called the medium's foreground and background), a transformation, a clipping region, a line style, and a text style. There are per-medium, dynamically scoped, default drawing options. Different medium classes are provided to allow you to draw on different sorts of devices, such as displays, printers, and virtual devices such as bitmaps.

Sheets

Many sheets can be used for doing output, so the drawing functions can also take a sheet as the output argument. In this case, drawing function "trampolines" to the sheet's medium. So, while the functions defined here are specified to be called on mediums, they can also be called on sheets.

Streams

A stream is a special kind of sheet that implements the stream protocol; streams include additional state such as the current text cursor (which is some point in the drawing plane).

By default, the "fundamental" coordinate system of a DUIM stream (not a general sheet or medium, whose fundamental coordinate system is not defined) is a left handed system with x increasing to the right, and y increasing downward. (0,0) is at the upper left corner.

For more general information about DUIM streams, you should refer to the manual Library Reference: System and I/O.

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Functional Developer Library Reference: DUIM - 3 Dec 1998