       Various  data types are defined by Tcl SIPP.  They are all
       built as Tcl lists, so standard  list  operations  can  be
       used  to  construct  and access them.  The "tcl_precision"
       variable is used when formatting all floating  point  num-
       bers.   The  value  of  this  variable  is  set to the Tcl
       defined maximum less one when TSIPP  is  initialized.   In
       many  cases,  the  precision  can  be  reduce for a slight
       increase in performance without altering the generation of
       images.   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.
