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The first step in the camera-to-screen transformation is to set up the camera projection. RenderMan provides five built-in projections and a plugin interface for user-created projections. The built-in projections are: orthographic, perspective, sphere, cylinder, and torus. The projection is set using RiProjection, and defaults to an orthographic projection.

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The perspective projection takes an optional argument that is the field of view. The perspective field of view moves the screen plane toward or away from the camera, so that the square which falls between (-1,1,-1,1) in screen coordinates covers the specified angle, as seen from the camera. For example, by default the field of view is 90 degrees, and thanks to trigonometry this means the screen plane is 1.0 units in front of the camera. If you were to specify a field of view of 20 degrees, the screen plane would be moved back to 5.6713 units away (1.0/tan(1020.00°/2)).

It is popular to use only the field of view to control the viewing pyramid, in a way similar to the way a physical camera is used. In this scheme, the screen window is allowed to default to its normal value of (-1,1) in the “smaller” direction, and the field of view controls how much is visible through this screen window. The field of view control can then be thought of as the minimum of the horizontal or vertical field of view, because in the “larger” direction you see an even wider angle. This scheme is not as general as using RiScreenWindow, since it gives you only undistorted centered perspective views, but this is usually what you want, anyway.

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