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### cameras

A camera object specifies the following properties of a camera:

position and orientation
  specified by either a camera-to-world or world-to-camera transformation; this transformation does not include the projection, so it's typically just a combination of translation and rotation. Specified as a transform object, typically a 4x4 matrix.
"focus" distance
  Intended to suggest a typical distance from the camera to the object of interest; used for default camera positioning (the camera is placed at (X,Y,Z) = (0,0,focus) when reset) and for adjusting field-of-view when switching between perspective and orthographic views.
window aspect ratio
  True aspect ratio in the sense <Xsize>/<Ysize>. This normally should agree with the aspect ratio of the camera's window. Geomview normally adjusts the aspect ratio of its cameras to match their associated windows.
near and far clipping plane distances
  Note that both must be strictly greater than zero. Very large <far>/<near> distance ratios cause Z-buffering to behave badly; part of an object may be visible even if somewhat more distant than another.
field of view
  Specified in either of two forms.
  ``fov'`
    is the field of view -- in degrees if perspective, or linear distance if orthographic -- in the shorter direction.
  ``halfyfield'`
    is half the projected Y-axis field, in world coordinates (not angle!), at unit distance from the camera. For a perspective camera, halfyfield is related to angular field: halfyfield = tan( Y_axis_angular_field / 2 ) while for an orthographic one it's simply: halfyfield = Y_axis_linear_field / 2
  This odd-seeming definition is (a) easy to calculate with and (b) well-defined in both orthographic and perspective views.

The syntax for a camera is:

    <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)

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