The RenderMan Interface - Paul Bourke

Coordiante System
”object”
”world”
”camera”
”screen”
”raster”
”NDC”
Description
The coordinate system in which the current geometric primitive is
defined. The modeling transformation converts from object coordinates
to world coordinates.
The standard reference coordinate system. The camera transformation
converts from world coordinates to camera coordinates.
A coordinate system with the vantage point at the origin and the
direction of view along the positive z-axis. The projection and
screen transformation convert from camera coordinates to screen
coordinates.
The 2-D normalized coordinate system corresponding to the image
plane. The raster transformation converts to raster coordinates.
The raster or pixel coordinate system. An area of 1 in this coordinate
system corresponds to the area of a single pixel. This coordinate
system is either inherited from the display or set by selecting
the resolution of the image desired.
Normalized device coordinates — like ”raster” space, but normalized
so that x and y both run from 0 to 1 across the whole (uncropped)
image, with (0,0) being at the upper left of the image,
and (1,1) being at the lower right (regardless of the actual aspect
ratio).
Table 4.2: Point Coordinate Systems
imaging process was described above. When RiBegin is executed it establishes a complete
set of defaults. If the rendering program is designed to produce pictures for a particular
piece of hardware, display parameters associated with that piece of hardware are used. If
the rendering program is designed to produce picture files, the parameters are set to generate
a video-size image. If these are not sufficient, the resolution and pixel aspect ratio
can be set to generate a picture for any display device. RiBegin also establishes default
screen and camera coordinate systems as well. The default projection is orthographic and
the screen coordinates assigned to the display are roughly between ±1.0. The initial camera
coordinate system is mapped onto the display such that the +x axis points right, the +y
axis points up, and the +z axis points inward, perpendicular to the display surface. Note
that this is left-handed.
Before any transformation commands are made, the current transformation matrix contains
the identity matrix as the screen transformation. Usually the first transformation command
is an RiProjection, which appends the projection matrix onto the screen transformation,
saves it, and reinitializes the current transformation matrix as the identity camera transformation.
This marks the current coordinate system as the camera coordinate system. After the
camera coordinate system is established, future transformations move the world coordinate
system relative to the camera coordinate system. When an RiWorldBegin is executed,
the current transformation matrix is saved as the camera transformation, and thus the world
coordinate system is established. Subsequent transformations inside of an RiWorldBegin-
RiWorldEnd establish different object coordinate systems.
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