Addendum to QRT Version 1.3
        
        
        
        The following are changes to QRT made since version 1.2
        
        Additions and Enhancements
        
           'Remove' Command Added
               
               The 'REMOVE' command  was  added  to  allow  sections of a
               planar primitive to be removed.  This makes it much easier
               to create flat  objects  that  are  not  shaped like QRT's
               planar primitives.  For  example,  I  had a QRT input file
               which created the image of a grand  piano.  The top of the
               piano, because of its complex shape, initially was created
               from 11 planar primitives.  Now, it is only 
               1   PARALLELOGRAM   primitive,   and   a  section  of  the
               parallelogram is removed to  create  the  shape of a piano
               top.
               
               Before the REMOVE command  can  be used, a pattern must be
               created using any of QRT's  pattern  primitives (the piano
               uses a polygon). 
               This pattern need not be given any surface characteristics
               - it simply defines  the  region  to  be  removed from the
               planar  primitive.   The   pattern   is  attached  to  the
               primitive in exactly  the  same  way  as a normal pattern,
               except that the  REMOVE  command  is  used  instead of the
               PATTERN command:
               
                   PARALLELOGRAM ( loc = (0, 0, 0)
                                   v1  = (10, 0, 0)
                                   v2  = (0, 0, 10)
               
                                   diff    = (1, 0, 0)
                                   remove  = SECTION_TO_REMOVE
                                   pattern = ANY_PATTERN
                                 )
               
               This example shows that the PATTERN command can be used on
               the  same primitive as the REMOVE command.
               
               Several things should be  noted  about the REMOVE command.
               First, the pattern defines the sections to remove, not the
               sections which will  remain.    In  the piano example, the
               pattern mapped all of the  parallelogram  except  the part
               that composed the actual  piano  top.  Second, even though


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               the remove command can  be  used  with  the curved surface
               primitives (SPHERE,  QUADRATIC),  the  results will not be
               quite right.  To use the  sphere  as an example, the front
               surface of  the  sphere  would  have  the  correct pattern
               removed from it, but the back surface of the sphere would 
               be completely gone.  This is because  the SPHERE intersect
               routine ignores completely the back surface of the sphere.
               I will probably fix this in a future version.
               
           Nested Comments
               
               Comments may  now  be  nested.   This  makes  it easier to
               comment out blocks of QRT  code  which  themselves contain
               comments.
               
               
        Planned Extensions for 1.4
        
           I had planned to create an "#INCLUDE" command for version 1.3,
           but instead I worked on  the  REMOVE  command  (it seemed more
           important).  #INCLUDE will probably be coming next time.
              































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