3-D visualization in GRASS

[Bill Brown]

In response to Jeffrey Copeland:

> Does grass allow for  viewing of data  that occurs at an elevation other
> than the surface?  What I mean is, say I had topography with vegetation
> overlayed as one surface and I also wanted to see a surface of vertical 
> velocity at 1km above ground level, or does GRASS only allow one 2-D
> surface?

There is currently one program module in GRASS called SG3d
that you can use to display multiple surfaces.  It only runs on
Silicon Graphics workstations, so it is not automatically compiled
when installing GRASS.  (In the GRASS source tree, it's under
src.contrib/CERL/SGI )

The multiple surface capability is only available in the latest release
of GRASS 4.1.  SG3d is mainly a single surface display program, using
one GRASS raster file for topology and one GRASS raster file for color.
It also allows you to drape 2D vector & site data over the surface.
With the latest release, the user is allowed to display multiple
surfaces by defining one surface at a time as above and displaying them
all in the same 3D space.  

SG3d uses a graphical user interface to allow easy real-time viewer 
positioning, quickly drawing the surface with a wire frame while the user 
adjusts vertical exageration and viewer position.  For full rendering, 
there is also an interactive lighting adjustment panel.  But when
displaying multiple surfaces, SG3d only holds one surface at a time in
memory, so it's best to make all exaggeration and positioning adjustments,
then render the first surface, then immediatley load & render subsequent
surfaces.  If you try to readjust viewer position after drawing a few 
surfaces, the display is erased and only the last surface defined remains
in memory, so you have to reload the other surfaces.  

SG3d also allows 3D querying.

Better multiple surface capabilities is a high priority for the next
upgrade of SG3d.

There is also a module called d.3d that similarly displays a raster file
for color draped over a raster file for elevation & runs on any GRASS
platform, but it does not have a graphical user interface for interactive
positioning, does not use lighting, and does not drape vector data or
support multiple surfaces.

> What about  3-D isosurfaces?
  
There is currently no GRASS module for displaying 3D isosurfaces.


Bill Brown 
brown at zorro.cecer.army.mil