View Full Version : Strange Non-Planar Polygons in .obj import

04-26-2006, 09:00 AM
I have been importing .obj files that were generated with Xsi and I am getting polygons that are dead flat showing up as non-planar and showing render errors.
Has anyone had this problem and know how to deal with it? They are black in the open GL view and flicker in the render yet they are completely flat polys.

I have tried deleting and replacing them, and this works, but I don't understand why they show up. Their appearance seems totally arbitrary.

I have attached an example.


04-26-2006, 02:48 PM
Yeah, they're quads, but with extra points in them, n-gons, that's why they are black even though they are planar. I get this too with obj's and worse its arbitrary – some go black, some don't–, like you say, all you can do is re-poly them or subdivide them either manually or through the numeric panel by selecting all of type '>4' and tripling them 'SHIFY+T'

04-26-2006, 03:11 PM
Just like gerry_g said, but with one addition:

1)select all >4 polys in the Poly stats page
2)shift+t (triple)
3)shift+z (with all the tris selected still)

So you can fixed your entire model with just a few keystrokes.

04-27-2006, 11:04 AM
Thanks Lads,

I wonder if this gets fixed with N-gon support in version 9.


04-30-2006, 06:23 AM
Hey All,

Just a little insight on the problem ..

The problem doesn't really originate from ngons .. you can just as well have
this artefact on quads ..

So how comes ? The problem will occur because :

- lw calculates polygon normals by taking the first, second and last point of
the polygon, and calculate the normal from the plane formed by those
- the polys that show the problem have the first, second and last point co-
linear, which makes that you cannot calculate a normal from them. (they
dont form a plane, but a line, which does not have a normal .. )

So, as you can see .. it's perfectly possible to re-create this problem with
just a few clicks, and even on a quad .. (put four points, three of which are
on the same line, select them in the right (wrong) order and you will see..)

It'll occur everwhere where the first, second and last points of the gon are
colinear ..

It's also very unlikely that this gets 'fixed' in any future release as technically
speaking .. it's not a bug .. ;)


05-01-2006, 08:41 AM
In that case, why does it seem to happen so randomly? Thanks for the explanation. It does make sense of what is happening. Couldn't this be fixed by being able to specify the first polygon?


05-01-2006, 08:54 AM
Sorry, I guess you just explained why it seems to happen so randomly.