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Coordinates and Transformations

GKS defines three coordinate systems:

  1. WORLD Coordinates (WC)
  2. NORMALIZED DEVICE Coordinates (NDC)
  3. DEVICE Coordinates (DC)
The application program specifies the coordinates of points in primitive calls using a cartesian coordinate system with whichever scale is the most convenient. This system is called the World Coordinate System. The WC range can go from negative to positive infinity along both axes, which gives the application an unlimited choice of coordinates. The definition of the WC space is independent of any graphics output device and can be different for each application as the requirements dictate.

All world coordinates are transformed by GKS to a 'virtual' device space, the Normalized Device Coordinate (NDC) space. The NDC space ranges from 0 to 1 in both x and y. This space is device independent and appears identical for all workstations in the system. A transformation from WC to NDC is called a Normalization Transformation. GKS allows more than one such transformation to be defined, and so an application may draw each part of a picture in its own WC system, and then map them into a single NDC space. See gif and gif.

The third coordinate system is that of the hardware device. GKS uses specific information about the graphics output device to transform normalized device coordinates into coordinates that the device itself understands. Usually the units in the device dependent space will be metres or raster units. The translation from NDC to DC is called the Workstation Transformation.

Janne Saarela
Mon Apr 3 17:00:12 METDST 1995