       SIPP  creates  shadows with a technique called depth maps.
       A detailed description of this technique can be  found  in
       the  article Rendering Antialiased Shadows with Depth Maps
       by Reeves, Salesin and Cook in the Proceedings of SIGGRAPH
       1987.

       In principle, a depth map is generated for each light that
       should cast shadows. The depth map is simply an  image  of
       the  scene, as seen from the light, but instead of a color
       we store the depth (Z-buffer value) in each  "pixel".  The
       finished map will contain the distance to the object clos-
       est to the light in each point.

       When the scene is rendered we transform each point we  are
       shading  into  depth  map coordinates and if it is further
       away from the light than the value stored  in  the  corre-
       sponding  point  in the depth map, the point is in shadow.
       The actual implementation is of course a bit more  compli-
       cated  with  some  sampling  and filtering but we won't go
       into that.

       The reason we describe this algorithm at all is that it is
       easier  to  understand how to get good looking shadows and
       why shadows sometimes look weird if  one  have  an  under-
       standing of the underlying process.

       First of all: The shadows are generated by sampling in the
       depth maps.  Sampling usually means we are  in  danger  of
       aliasing and this is very true in our case. SIPP automati-
       cally fits the depth map for a spotlight so that it covers
       all  area lit by the spotlight's light cone.  If this area
       is large and the depth map resolution is low, the  shadows
       will get very jagged.

       Also,  if we have a large surface that is close to perpen-
       dicular to the depth map plane,  the  depth  map  "pixels"
       will be projected as long stripes on that surface, so even
       if the depth map resolution is high, a shadow cast on such
       a surface will suffer from aliasing (be jagged).

       So,  if  the  edges of a shadow look weird, try increasing
       the size of the depth map (the depth map size is set  with
       `SippShadows').   If they still look weird, or you run out
       of memory, try changing the position  of  the  lightsource
       that  generate the shadow.  After some tweaking it is usu-
       ally possible to get fairly decent shadows.

       There are two ways to generate depth maps in SIPP,  either
       automatically  for  each new rendering, or explicitly on a
       command which will then keep the depth maps  around  until
       they are explicitly deleted.

       See  the  help  file tsipp/primitives/Texture-Map for more
       details.
