<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)

# 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 }
}

<camera> ::=

   [ "camera" ]			(optional keyword)
    [ "{" ]			(opening brace, generally required)
	[ "define" <name> ]

	"<" <filename>
      |
	":" <name>
      |
				(or any number of the following,
				 in any order…)

	"perspective"  {"0" | "1"}		(default 1)
					(otherwise orthographic)

	"stereo"       {"0" | "1"}		(default 0)
					(otherwise mono)

	"worldtocam" <transform>	(see transform syntax above)

	"camtoworld" <transform>
				(no point in specifying both
				 camtoworld and worldtocam; one is
				 constrained to be the inverse of						 the other)

	"halfyfield" <half-linear-Y-field-at-unit-distance>
				(default tan 40/2 degrees)

	"fov"		(angular field-of-view if perspective,
			 linear field-of-view otherwise.
			 Measured in whichever direction is smaller,
			 given the aspect ratio.  When aspect ratio
			 changes -- e.g. when a window is reshaped --
			 "fov" is preserved.)

	"frameaspect" <aspect-ratio>	(X/Y) (default 1.333)

	"near"  <near-clipping-distance>	(default 0.1)
	
	"far"	<far-clipping-distance>		(default 10.0)

	"focus" <focus-distance>		(default 3.0)

	
     [ "}" ]				(matching closebrace)

window ::=

	[ "window" ]			(optional keyword)
	  [ "{" ]			(curly brace, often required)

	    			(any of the following, in any order)

		"size"  <xsize> <ysize>
				(size of the window)

		"position"  <xmin> <xmax> <ymin> <ymax>
				(position & size)


		"noborder"
				(specifies the window should
				 have no window border)

		"pixelaspect"  <aspect>
			    (specifies the true visual aspect ratio
			     of a pixel in this window in the sense
			     xsize/ysize, normally 1.0.
			     For stereo hardware which stretches the
			     display vertically by a factor of 2,
			     ``pixelaspect 0.5'' might do.
			     The value is used when computing the
			     projection of a camera associated with
			     this window.)

	  [ "}" ]			(matching closebrace)