🚀 go-pugleaf

Direct mail to hayes%valkris@hac2arpa.hac.com was undeliverable,
so ...

In article <24830@hacgate.SCG.HAC.COM>, you write:
|> I'm having the geometry viewer display a GEOM_POLYHEDRON geometry of a
|> double headed arrow:
|>   /      \
|>  /|______|\
|> /          \       (wow, with ascii graphics like this, why use AVS?;^)
|> \  ______  /
|>  \|      |/
|>   \      /
|> 
|> When I set the render mode to "lines", I observe the expected result in 
|> the geometry viewer window: I see an outline of a double headed arrow.
|> 
|> HOWEVER, when I set the render mode to "no_light", I don't get a solid 
|> double headed arrow like I want.  I get something like this:
|>            
|>   /     /\
|>  /|___ /  \
|> /          \ 
|> \    ____  /
|>  \  /    |/
|>   \/     /
|>   
|> 
|> How can I get a solid double headed arrow like I want?  I've tried other
|> rendering modes but I don't get a solid double headed arrow.

AVS has problems triangulating concave polygons in a GEOM_POLYHEDRON
geometry when they have multiple concave regions.  We've reported
the problem to AVS support, and are in the queue, but according to the
last contact with them, this is a low priority item.  Perhaps,
if you report it also, the priority will get bumped, and the problem
will be fixed sooner.

Our fix was to break the regions into convex polyhedra, which
AVS cannot possibly mess up.  You lose the ability to show clean
outlines, but at least the surface representation is correct.  The
algorithm is

    begin:
	project the polygon onto a plane
	pick a starting point
	progress around the polygon in a fixed direction (CW or CCW)
	foreach point
	    check the direction of the turn 
	      (needs last, current, and next point)
	    if the direction is incorrect (e.g. left when going CW) then
		mark current point
		continue progressing around the polygon, searching
		  for a next point that
		    1. has the correct turn direction
		    2. creates a line segment from the current point 
                       that does not intersect any other line segments 
                       in the polygon
		break the polygon into two polygons by drawing a
		  line between the current and next point
		apply the algorithm recursively on each polygon
	    end if
	    if current point is the starting point then
		congratulation, the polygon is convex!
	    end if
	end foreach
    end:			  

It may not be the most efficient algorithm possible, but it works.

If you intelligently work with their GEOM file representation, you could
even triangulate the regions yourself, generating a consistent 
GEOM_POLYTRI object with the correct outline data.  


Dan Yergeau
yergeau@gloworm.Stanford.EDU
#include <std.disclaimer>