chapter  4
Projections and Viewing Transformations
Pages 26

The rationale behind these coordinate systems is the following: All objects are initially defined in their own local coordinate system which may, for example, be the result of a digitization or design process. These objects are unified in WCS where they are placed suitably modified; the WCS is essentially used to define the model of a three-dimensional synthetic world. The transition from WCS to ECS, which involves a change of coordinates, is carried out in order to simplify a number of operations including culling (e.g., the specification of the clipping bounds by the user) and projection. Finally, the transition from ECS to CSS ensures that all objects that survived culling will be defined in a canonical space (usually ranging from−1 to 1) that can easily be scaled to the actual coordinates of any display device or viewport and that also maintains high floating-point accuracy.