       The following data structures are used by Tcl-SIPP:

              o color - A list containing the red, green and blue
              values  for  the  surface.   Each  being  a  number
              between 0.0 and 1.0.

              o opacity - Describes the opacity of a surface. 0.0
              means  totally  transparent,  1.0   means   totally
              opaque.  May be a list of red, green and blue opac-
              ity values or a single value to  be  used  for  all
              color channels.

              o  vertex  - A vertex is defined by a Tcl list of 3
              floating point numbers in the form: {x y z}.

              o vertex/texture pair - A vertex and  corresponding
              texture  coordinates  are  defined by a Tcl list of
              two list, each having 3 floating point  numbers  in
              the form: {{x y z} {u v w}}.

              o  vector  - A vector is defined by a Tcl list of 3
              floating point numbers in the form: {x y z}.

              o boolean - Boolean arguments may be  specified  as
              `0' or `1', `true' or `false', `t' or `f' (upper or
              lower case).  Only `0' or  `1'  are  valid  in  Tcl
              expressions  and commands that return booleans only
              return `0' or `1'.

              o angle - An angle may be expressed in  degrees  or
              radians.   If  the  number is prefixed with an "R",
              then it is taken as radians, if it is prefixed with
              "D",  then it is taken as degrees. (e.g. 1.2, R1.2,
              D10.5).  If no prefix is specified,  some  commands
              assume degrees, some assume radians.  See the docu-
              mentation of the specific command.

              o matrix - A 4x4 transformation  matrix  is  repre-
              sented  as a list of 4 elements, each element being
              a row.  Each of the row elements is  a  list  of  4
              floating  point  numbers, representing the columns.
              The values in column 4 must be 0,  0,  0,  1.   For
              example:

                      {{1.2  1.4  0.1  0}
                       {2.1  2.3  1.6  0}
                       {2.4  6.1  2.6  0}
                       {2.9  1.1  3.6  1}}

       Be  careful,  many  commands  take lists of the above data
       types, which are themselves lists.
