Creating Output from GDL Object Hierarchies
      ===========================================

Output formats can be used in conjunction with the "with-format" macro
to produce customized output from GDL objects, as shown in the
examples below.

If you create new GDL objects, you can also create output-functions
for your new object for the various formats by using the define-view
macro, for example:

 (define-view (pdf my-widget)()
   :output-functions
   ((cad-output 
     ()
     (pdf:move-to (the start))
     (pdf:line-to (the end)))))

Please see the reference documentation for define-view for more
details.


1 PDF
=====

 PDF is the native graphics and document format of GDL and as such is
 the most developed. Most graphical and textual objects in GDL have
 PDF views defined for them with at least a "cad-output" method. The
 term "cad-output" is somewhat dated, as the output could refer to a
 paper report with charts and graphs as much as a drawing of
 mechanical parts, so these names may be refined in future GDL
 releases.

 Here is an example of producing a PDF of the built-in robot assembly:

 (with-format (pdf "/tmp/robot.pdf" :page-length (* 6.5 72) 
		   :page-width (* 8.5 72)
		   :view-transform (getf *standard-views* :trimetric)
		   :view-scale 37)
   (write-the-object (make-object 'gwl-user::robot-assembly) (cad-output-tree)))



 Available slots for the PDF writer and their defaults:

   (page-width 612) 
   (page-length 792) 
   (view-transform (getf *standard-views* :top)) 
   (view-center (make-point 0 0 0)) ;; in page coordinates
   (view-scale 1)


2 DXF
=====

 DXF is AutoCAD's drawing exchange format. Currently most of the
 geometric objects and drawing and text objects have a cad-output
 method for this output-format. The GDL DXF writer currently outputs a
 relatively old-style AutoCAD Release 11/12 DXF header. In future GDL
 versions this will be switchable to be able to target different
 release levels of the DXF format.

 Here is an example of producing a DXF of the built-in robot assembly:

  (with-format (dxf "/tmp/robot.dxf"
		    :view-transform (getf *standard-views* :front)
		    :view-scale 37 :view-center (make-point 0 0 0))
    (write-the-object (make-object 'gwl-user::robot-assembly) (cad-output-tree)))     

 Note that the DXF writer is currently 2D in nature and therefore
 takes a :view-transform to specify a view plane onto which all 3D
 entities are projected.

 A 3D version of the DXF writer will be available in a future GDL
 release.

 Available slots for the PDF writer and their defaults:

   (view-transform (getf *standard-views* :top)) 
   (view-center (make-point 0 0 0)) ;; in page coordinates
   (view-scale 1)


3 vrml
======

 GDL currently contains a rudimentary VRML writer which maps GDL
 primitive geometric objects (boxes, spheres, etc) into their VRML
 equivalents. Future GDL releases will include more object types and
 mechanisms to specify viewpoints, lighting, textures, etc.


  (with-format (vrml "/tmp/robot.wrl")
    (write-the-object (make-object 'gwl-user::robot-assembly) (world)))


4 html-format
=============

 The HTML output format is used extensively throughout GWL for
 generating dynamic web page hierarchies corresponding to GDL object
 hierarchies. Please refer to the GDL Tutorial, gwl-usage.txt, and
 gwl-reference.txt for extensive examples of using this output format.



5 iges
======

 5.1 overview
 ============

 We supply an IGES output-format with the optional GDL NURBS Surfaces
 Facility. The IGES format will also convert certain GDL wireframe
 primitives into appropriate curve and surface objects for IGES output
 (e.g. l-lines into linear-curves, arcs into arc-curves). 

 Here are some examples of simple use:

   (with-format (iges "/tmp/try.iges")
     (write-the cad-output-tree))

   (with-format (iges "/tmp/try.iges")
     (write-the cad-output))


 5.2 brep solids representation in iges
 ======================================

 For breps, you can output them as individual breps or as a bag of the
 faces (trimmed surfaces) making up the brep.

 This is done with the format variable :breps-format, which can have
 the value :breps (the default) or :surfaces, e.g.

   (with-format (iges "/tmp/try.iges" :breps-format :surfaces)  ...)  ;; or

   (with-format (iges "/tmp/try.iges" :breps-format :breps)  ...) 


 5.3 units in iges
 ==================

 The iges units can be specified with the format variable :units, e.g:

  (with-format (iges "/tmp/try.iges" :units :millimeters) ...)  ;; or
  (with-format (iges "/tmp/try.iges" :units :feet) ...)  ;; or


 The allowed values for :units are the keyword symbols in the
 following list:

   (:inches :millimeters :feet :miles :meters :kilometers
    :mils :microns :centimeters :microinches :no-units-specified)

 The default is :inches.