Triangulate and smoothing

General discussion about the development of the open source Blender

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ionte
Posts: 7
Joined: Mon Jan 20, 2003 7:06 pm

Triangulate and smoothing

Postby ionte » Mon Jan 20, 2003 8:37 pm

Hi,

I'm new to Blender and 3D-modelling in general as I'm primarily a programmer.

I've written an export tool in python to my personal 3D format. I use only triangles so I have to triangulate the mesh first. Now, if I use ctrl-t in blender or if I create triangles from quads my self I have the same problem. To compute normals I do for each vertex take the average of all connected surfaces normals. This works great most of the time, but there is one problem.

Imagine the following four quads:

Code: Select all

+-+-+
| | |
+-+-+
| | |
+-+-+


Triangulated:

Code: Select all

   A B
  +-+-+
1 |\|\|
  +-+-+
2 |\|\|
  +-+-+


This means that the vertex in the middle will get 2 average values from the quads at 1A and 2B, but only 1 value from 1B and 2A.

Do you have any comments on this problem?

Is there any good way to control normal generation in blender? Is there anyway to monitor normals (such as small lines sticking out of the vertices)?

K-Zimir
Posts: 1
Joined: Mon Jan 20, 2003 10:03 pm

Postby K-Zimir » Mon Jan 20, 2003 10:07 pm

Hello,


Unforunately, I can only answer to the last question of your post : you can monitor faces normals as lines by selecting the "Draw Normals" option in the Edit Panel (F9). The normals will appear in blue in each face when you're in Edit Mode. I hope I've understood what you meant (I'm not English-speaking).

Hope this helps you. Good luck with your project. :wink:

ionte
Posts: 7
Joined: Mon Jan 20, 2003 7:06 pm

Postby ionte » Tue Jan 21, 2003 12:03 am

Ah, how could I miss that... Thanks...

It only displays the face normals though, not vertex normals.

LethalSideParting
Posts: 83
Joined: Mon Oct 21, 2002 12:53 am
Location: Bucks, England

Postby LethalSideParting » Tue Jan 21, 2003 3:04 pm

Hmmm. A peculiar problem, but I see where it's coming from. The way I see it, you have 2 ways to solve this problem, and both of them involve getting involved in the triangulation stuff itself. In short, you're probably not going to be able to do this inside Blender (unless you write a python script to do it....).

Anyway, back to my ideas. The first is a bit of a cheat - instead of sampling both triangles from 1A and 2B, you only sample one from each quad (since they came from a quad , their normals should be almost identical, so you shouldn't get any loss of accuracy.

The second idea....well, I'll let you decide how easy it would be to code it. You change your quads so that they look like this:

Code: Select all

     +-+-+
     |\|/|
     +-+-+
     |/|\|
     +-+-+


This way, all your vertices will either be able to take 4 or 8 triangle normals for the average. But they'll all be right next to the vertex, so you should be OK?

Hope that was clear, and helped a little. By the way, are you sure that vertexes actually need normals for whatever you're exporting this stuff for? It's only faces that need normals (I think...) so that OpenGL knows whether to display them or not - I can't think of a reason you'd need vertex normals unless you were displaying stuff in 'points only' mode and wanted to remove hidden verts...

Or have I just missed the point completely? ;)

LethalSideParting

ionte
Posts: 7
Joined: Mon Jan 20, 2003 7:06 pm

Postby ionte » Tue Jan 21, 2003 4:19 pm

Well, I'm starting to think that the only way to solve this problem in all cases is to never, ever, never use quads when modelling!

You are right that a quad converted to two triangles should have almost identical normals (not quite, but yet). The problem though is that blender does not mind creating non-planar quads. That should not be allowed! It's *wrong*! :evil:

When it comes to true planar quads I'll let it be up to the artist to create triangulated surfaces in a manner such as your example. It's too bad that Blenders triangulate-command doesn't do that by itself...

Oh, and yes, you do need vertex normals to create an illusion of a smooth, curved surface.

ionte
Posts: 7
Joined: Mon Jan 20, 2003 7:06 pm

Postby ionte » Tue Jan 21, 2003 4:21 pm

Ehm, as a matter of fact, a triangulated quad *should* have identical normals for the two triangles.

eskil
Posts: 141
Joined: Tue Oct 29, 2002 10:42 pm

Postby eskil » Tue Jan 21, 2003 4:59 pm

Here is my take on it:

I will divide the problem in to 2 different problems: triangulation and normal generation.

To do the triangulation there is 2 different approaches, the first one is to take each N-sided polygon and split it to triangles so that all triangles have as close to as possible the same corner angle (ideally all triangles generated would have 60 degree corners). This avoids very stretched triangle corner angles close to 0 and 180 degrees.

this is good for flat polygons, how ever if the N-sided polygons are non flat, there is an other way. depending on how you split a non flat N-sided polygon it will become concave or convex. witch one is best depends on if the surface is concave of convex. this can done by computing a vector that starts in the middle of the polygon, and ends at a the average of all mid point of the neighboring polygons. do a dot product, between this vector and the surface normal, if the output is positive it is concave surface, and if it is negative if is concave.

Ok so how do you compute the normals? for each vertex you average together the normals of the polygons connected to it, and (here comes the trick) you weight their values depending on the angle of the corner connected to the vertex.

That's it!

This may not be the easiest or fastest way to do it, but its the way I think its the best if you have the cycles to spare.

E

ionte
Posts: 7
Joined: Mon Jan 20, 2003 7:06 pm

Postby ionte » Tue Jan 21, 2003 5:50 pm

Thanks, that was some good ideas! Especially that last trick.

I'm thinking about an easier solution though: If I calculate all normals (vertice and surface normals) before triangulation I will avoid the problem. The downside is that I won't be able to recalculate them afterwards if i need to. But then I could use your method!

Thanks for the help!


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