       SippShaderStrauss ambient smoothness metalness color
              [opacity]
              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.
