Graphics Reference
In-Depth Information
Figure 1.2.
3d graphics coordinate systems and terminology.
coordinate system
. Looking from the viewpoint along the positive z-axis of the camera
coordinate system specifies the
view direction
. A
window
in the view plane specifies
the area of interest. The
view volume
or
view pyramid
is the infinite volume swept out
by the rays starting at the viewpoint and passing through points of the window. To
limit the output of objects one often uses a
near
(or
front
or
hither
) and
far
(or
back
or
yon
)
clipping plane
. The volume inside the view volume between these two planes
is called the
truncated view volume
or
truncated view pyramid
. Only those parts of
objects that lie in this volume and project into the window will be displayed. Finding
those parts of an object is referred to as
clipping
. In principle, the three coordinate
systems - the world, the camera, and the view plane coordinate system - could be dis-
tinct. In practice, however, one assumes that the coordinate axes of the camera and
view plane coordinate system are parallel and the z-axes are perpendicular to the view
plane. One also assumes that their x- and y-axes are parallel to the sides of the window.
The final step in mapping an object to a graphics device involves a map that trans-
forms view plane coordinates to physical device coordinates. This is usually thought
of as a two-stage process. First, an initial map transforms the window to a
viewport
that is a subrectangle of a fixed rectangle called the
logical screen
, and a second map
then transforms logical screen coordinates to physical device coordinates. See Figure
1.3. Sometimes the logical screen is already defined in terms of these coordinates, so
that the second map is not needed. Other times, it is set equal to a standard fixed rec-
tangle such as the unit square [0,1] ¥ [0,1], in which case we say that the viewport is
specified in
normalized device coordinates
(NDC). The basic 3d graphics pipeline can
now be summarized as shown in Figure 1.4. Chapter 4 will discuss it in great length
and also fill in some missing details.
