Scripting Guide - SAS

Chapter 13 Three-Dimensional Scenes 433
Setting the Viewing Space
Figure 13.5 Comparing Projections
Orthographic Scene
viewer
angle
near
far
Perspective Scene
In general, perspective projections give a more realistic view of the world, since it mimics the way an eye or
a camera sees. Orthographic projections are important when it is essential to preserve dimensions, such as an
architectural CAD program.
Setting Up a Perspective Scene
To set up a perspective scene in JSL, send the Perspective command to a display list.
Perspective (angle, near, far)
where angle is the viewing angle, near is the distance to the near plane, and far is the distance to the far
plane, as illustrated in the drawing above. A couple of things need to be remembered when defining the
viewing space.
• Items outside the viewing space (for example, closer than the near plane or farther than the far plane)
are not drawn. They are clipped off.
• The ratio of far to near needs to be small so that the rendering engine can effectively determine which
items should be drawn “on top of” other items, simulating closeness of items. The near parameter must
be greater than zero.
The “Hello World” example contains the line
scene