The RenderMan Interface - Paul Bourke

may be nested to any depth, subject to their individual restrictions, but it is never legal for
the blocks to overlap.
4.1 Options
The graphics state has various options that must be set before rendering a frame. The complete
set of options includes: a description of the camera, which controls all aspects of the
imaging process (including the camera position and the type of projection); a description of
the display, which controls the output of pixels (including the types of images desired, how
they are quantized and which device they are displayed on); as well as renderer run-time
controls (such as the hidden surface algorithm to use).
4.1.1 Camera
The graphics state contains a set of parameters that define the properties of the camera.
The complete set of camera options is described in Table 4.1, Camera Options.
The viewing transformation specifies the coordinate transformations involved with imaging
the scene onto an image plane and sampling that image at integer locations to form a
raster of pixel values. A few of these procedures set display parameters such as resolution
and pixel aspect ratio. If the rendering program is designed to output to a particular display
device these parameters are initialized in advance. Explicitly setting these makes the specification
of an image more device dependent and should only be used if necessary. The
defaults given in the Camera Options table characterize a hypothetical framebuffer and are
the defaults for picture files.
The camera model supports near and far clipping planes that are perpendicular to the
viewing direction, as well as any number of arbitrary user-specified clipping planes. Depth
of field is specified by setting an f-stop, focal length, and focal distance just as in a real
camera. Objects located at the focal distance will be sharp and in focus while other objects
will be out of focus. The shutter is specified by giving opening and closing times. Moving
objects will blur while the camera shutter is open.
The imaging transformation proceeds in several stages. Geometric primitives are specified
in the object coordinate system. This canonical coordinate system is the one in which the
object is most naturally described. The object coordinates are converted to the world coordinate
system by a sequence of modeling transformations. The world coordinate system is
converted to the camera coordinate system by the camera transformation. Once in camera
coordinates, points are projected onto the image plane or screen coordinate system by the
projection and its following screen transformation. Points on the screen are finally mapped
to a device dependent, integer coordinate system in which the image is sampled. This
is referred to as the raster coordinate system and this transformation is referred to as the
raster transformation. These various coordinate systems are summarized in Table 4.2 Point
Coordinate Systems.
These various coordinate systems are established by camera and transformation commands.
The order in which camera parameters are set is the opposite of the order in which the
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