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Projection
There are two types of projection. In the perspective projection, the relative size of objects depends on their distance from the viewpoint or eye. Objects further away appear smaller. In the orthographic projection, objects remain at their same size independent of distance from the viewpoint. According to the OpenGL documentation, orthographic projections are used in CAD and architectural work. The projection type can be changed in the 3D context menu and on the Projection property page. The projection page contains a number of other settings that affect the scene.
For both projections, one must define a set of clipping planes that bound the scene. Any objects that lie outside of the planes are clipped. The clipping planes depend on the 3D scale box of the model. How the planes are defined depends on the projection. The perspective clipping planes define a frustum with the smaller end near the viewpoint. The frustum is defined by the position of the near (relative to the viewpoint) and far planes and the opening angle. The near and far planes are set to enclose the 3D scale box. Multipliers on the Projection property page permit changing the near and far plane locations. The page also has a field for the opening angle (the default angle is 45°). The orthogonal clipping volume is a rectangular box that encloses the 3D scale box. The Projection page contains multipliers for each of the clipping volume planes.
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