       SippShaderStrauss ambient smoothness metalness color
              [opacity]
              NOTE: This shader is currently has a  serious  bug.
              As  smoothness and metalness get larger, the object
              gets darker.   This  will  be  fixed  in  a  future
              release.
              This  is an implementation of a shader described by
              Paul Strauss in IEEE CG&A Nov. 1990. In his article
              he  explains that most shading models in use today,
              i.e. Phong, Cook-Torrance, are difficult to use for
              non-experts,  and  this  for  several  reasons. The
              parameters and their effect on a surface  are  non-
              intuitive  and/or  complicated.  The  shading model
              Strauss designed has parameters  that  is  easy  to
              grasp and have a reasonably deterministic effect on
              a surface, but yet produces very realistic results.

              ambient is a value between 0 and 1 which determines
              how much of the base color of  a  surface  that  is
              visible  when  it  is  not illuminated by any light
              source.

              smoothness  is  a  value  between  0  and  1   that
              describes how smooth the surface is. This parameter
              controls both diffuse and specular  reflections.  0
              means  a  dull  surface while 1 means a very smooth
              and shiny one.

              metalness is also a  value  between  0  and  1.  It
              describes how metallic the material is. It controls
              among other things how much of  the  surface  color
              should  be  mixed  into the specular reflections at
              different angles. 0 means a non-metal while 1 means
              a very metallic surface.

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

              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.  Optional, default is 1.0.
