Where a single 4x4 matrix is expected -- as in the
INST transform field, the camera's camtoworld transform
and the Geomview xform* commands -- use a transform object.
Note that a transform is distinct from a TLIST, which is a type
of geometry. TLISTs can contain one or more 4x4 transformations;
"transform" objects must have exactly one.
Why have both? In many places -- e.g. camera positioning -- it's only meaningful to have a single transform. Using a separate object type enforces this.
Syntax for a transform object is
<transform> ::=
[ "{" ] (curly brace, generally needed to make
the end of the object unambiguous.)
[ "transform" ] (optional keyword; unnecessary if the type
is determined by the context, which it
usually is.)
[ "define" <name> ]
(defines a transform named <name>, setting
its value from the stuff which follows)
<sixteen floating-point numbers>
(interpreted as a 4x4 homogeneous transform
given row by row, intended to apply to a
row vector multiplied on its LEFT, so that e.g.
Euclidean translations appear in the bottom row)
|
"<" <filename> (meaning: read transform from that file)
|
":" <name> (meaning: use variable <name>,
defined elsewhere; if undefined the initial
value is the identity transform)
[ "}" ] (matching curly brace)
The whole should be enclosed in { braces }. Braces are not essential if exactly one of the above items is present, so e.g. a 4x4 array of floats standing alone may but needn't have braces.
Some examples, in contexts where they might be used:
# Example 1: A gcl command to define a transform
# called "fred"
(read transform { transform define fred
1 0 0 0
0 1 0 0
0 0 1 0
-3 0 1 1
}
)
# Example 2: A camera object using transform
# "fred" for camera positioning
# Given the definition above, this puts the camera at
# (-3, 0, 1), looking toward -Z.
{ camera
halfyfield 1
aspect 1.33
camtoworld { : fred }
}
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