17.2 Coordinates for Graphics

Each graphics device has two different coordinate systems associated with it: device coordinates and virtual coordinates. Device coordinates are generally defined by low-level characteristics of the device itself, and often cannot be changed. Most device coordinate systems are defined in terms of pixels, and usually the upper-left-hand corner is the origin of the coordinate system, with x coordinates increasing to the right and y coordinates increasing downwards.

In contrast, virtual coordinates are more flexible in the units employed, the position of the origin, and even the direction in which the coordinates increase. A virtual coordinate system is defined by assigning coordinates to the edges of a device. Because these edge coordinates are arbitrary real numbers, any Cartesian coordinate system can be defined.

All graphics procedures that use coordinates are defined on virtual coordinates. For example, to draw a line at a particular place on a device, the virtual coordinates for the endpoints of that line are given.

When a graphics device is initialized, its virtual coordinate system is reset so that the left edge corresponds to an x-coordinate of -1, the right edge to x-coordinate 1, the bottom edge to y-coordinate -1, and the top edge to y-coordinate 1.