Q:how to use MESH_OBJ

[Rick Towler <rtowler(at)u.washington.edu>]

Hi all,

First a big thanks to everyone who responded to my post a few weeks back
on z-buffer ordering.  Paraphrasing Emeril, I have been forced to kick
my IDL programming up a notch and your comments have helped.

Today I am wrestling with the MESH_OBJ routine.  I am trying to generate
a surface that represents a 3d fish.  The closest example I can come up
with is the teapot in the IDL demos except that the code to actually
generate the teapot mesh isn't included (or I haven't found it.  The
program that displays the teapot loads a file that contains the mesh and
polygon data.)

I have an array of x,y,z points that make up the fish body.  I have had
success manipulating the data a little and using the "extrusion" surface
type to generate a collection of thin oval cylinders that when viewed
from the side look o.k.  The problem with this approach is that these
individual extrusions aren't connected to their neighbors so when the
polygon normals start to point away from the eye you can start to see in
between them.  And, when viewed head or tail on, the object virtually
disappears (the extrusions have no thickness).  The problem here seems
to be the polygon list.  If I omit the polygon list from the call to
IDLgrPolygon the resultant object will be interconnected and I will not
have the problems describe above. But, I do introduce another problem in
that it takes far to long to generate the IDLgrPolygon object without
the polygon list information.

Since the problem seems to be with the polygon list (the mesh points are
fine, they just aren't connected), I tried using a polygon list
generated from a separate call to MESH_OBJ where I chose the
triangulated surface.  When I pass the mesh data from the MESH_OBJ
"extrusion" call and the polygon data from the MESH_OBJ "triangulated"
to my IDLgrPolygon call I get a ton of errors (invalid connectivity list
detected(invalid vertex reference)) and finally an object that is
interconnected.  The only problem here are the errors, and what seem
like overly complex meshes (hard to explain).

I thought that the "triangulated" surface type would be the ticket but
taking this approach yields unexpected results.  I must be missing
something.  Even if I define 6 vertices for a simple 3d diamond and pass
that to MESH_OBJ,1 what I get when I pass that data to IDLgrPologon is
an object that sort of looks like you took the diamond and cut along the
edges and laid it out.  This isn't exactly what it looks like, but it
certainly isn't a closed object.

What very basic thing am I missing here?  Is this the wrong tool?  How
did the RSI people generate the surface mesh and ploy list for the
teapot?  Is there a way to fix the polygon list generated by the
MESH_OBJ "extrusion" routine so all the little guys are connected?

And lastly, am I just screwed and do i have to generate the polygon list
manually?

Thanks!

-Rick Towler