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(BACK-DISTANCE value) |
(COORDINATE-SYSTEM-HANDEDNESS (LEFT|RIGHT)) |
(DISTANCE value) |
(FRONT-CLIPPING (OFF|ON)) |
(FRONT-DISTANCE value) |
(IMAGE-TRANSFORMATION-MATRIX matrix-by-rows) |
(PLANE-NORMAL x y z) |
(PROJECTION (PARALLEL|PERSPECTIVE) x y z) |
(REFERENCE-POINT x y z) |
(UP x y z) |
(WINDOW-CLIPPING (OFF|ON)) |
(WORLD-MODELLING-MATRIX matrix-by-rows)
The VIEW command provides for the specification of all of the CORE
viewing parameters relevant to 3-D graphing, as well as explicit
specification of the image transformation matrix and world modelling
matrix. These two matrices may also be constructed from stepwise
geometric operations using the TRANSFORMATION command. This command
should normally not be necessary in pure 2-D applications.
The COORDINATE-SYSTEM-HANDEDNESS option can be used to select either a
right-handed or a left-handed system. The default is a right-handed
system.
The REFERENCE-POINT option defines the (x,y,z) coordinates of the view
reference point which is required by several other subcommands. It
will generally be a fixed point on the object being plotted, and by
default is the origin (0,0,0).
The window containing the object view which is displayed in the current
viewport on the device surface lies in a view plane defined by a normal
vector given by the PLANE-NORMAL option. This plane is positioned at
the view distance defined by the DISTANCE option. The view distance is
positive measured from the view reference point in the direction of the
view plane normal, and its default value is 0, making the reference
point lie in the view plane. The intersection of the view plane normal
with the view plane when it points to the view reference point is the
(0,0) window coordinate origin.
The up direction in the window coordinate system is parallel to the
projection of the view up vector in the view plane, and the UP option
defines this view up vector.
The view projection is selected by the PROJECTION option. For a
parallel projection, projection lines are parallel to the vector from
the view reference point to the specified projective point (x,y,z).
For a perspective projection, the center of projection is at the
projective point (x,y,z).
Normally, every point in space which has a projection in the current
window will be visible. To make a finite view volume, it is possible
to introduce back and front clipping planes, parallel to the view
plane, with the BACK-DISTANCE and FRONT-DISTANCE options. The numeric
value provided with these options is the distance of the given plane
from the view reference point, and is positive in the direction of the
view plane normal.
Clipping against the window and the back and front planes may be turned
on or off with the WINDOW-CLIPPING, BACK-CLIPPING, and FRONT-CLIPPING
subcommands. By default, window clipping is on, and back and front
clipping are off. If the latter two are turned on, then back and front
distances should also be specified, because they otherwise default to
0. This generally makes the entire plot invisible since the back and
front planes then coincide.
Finally, the IMAGE-TRANSFORMATION-MATRIX and WORLD-MODELLING-MATRIX
options allow the user to provide an explicit 4 by 4 matrix, entered in
row order, to be used for the indicated purpose. In <PLOT79>, matrix
transformations of 4-D homogeneous coordinates are always written in
the form (x',y',z',h') = (x,y,z,h) T, where T is a 4 by 4 matrix. The
4-D point (x,y,z,h) is projected onto the 3-D point (x/h,y/h,z/h) for
plotting.