Very Basic Hard Surfacing Polygon Question

[mak]

I'm more of a generalist and have always been more of a nurbs modeler. I've noticed at several studios that when polygonal modelers are creating architecture, that they break up planar faces (say the facade of a builging) so that edges extend in a horizontal and vertical direction from every single vertex through the planar face, even when those verticies are on the outside edge of the planar surface. For lack of a better term, their models seem as if they are building parameterization into them.

For example, let's say you're building an extruded letter L. Instead of the front and back faces, which are planar, each consisting of a single face, they break them up instead into three faces each(the vertical part, the horizontal part, and the square in the corner between them). The also would extend edges through the planar sides to join up the extra verticies.

My question: What is the advantage of this? And secondly, what techniques would you use to do this in the first place - it seems like a lot of extra work to me.

Thanks.

[rival2000]

L as a single face? Faces shouldn't be concave, that's bad. And when poly modeling it is best to keep quads cause it makes it much easier to texture. Mainly it's just easier to handle in most all aspects.

Techniques? uh, i guess everytime you want to add an edge you just use the cut face tool and cut it where you want it across the whole plain.

I hope i answered your question, it was kinda hard to follow.

[mak]

Thanks for your reply.

Please explain to me what the problem with concavity is. As long as a surface is planar, it will tesselate at render time just as well whether concave or convex, or so I thought. I've never encountered any render artifacts (popping or flickering) with concave polygons.

I'm also not sure why quads are easier to texture.

The downside as I see it is that you are creating much more geometry that on a very complex scene will slow down load/save times and interactivity.

As an example, I am working on an ancient city scene which is surrounded by a turreted wall. One face of the wall, as it was modeled, has hundreds if not thousands of polygons breaking up what is otherwise a single planar surface.

As another example, if you used a boolian difference operation to put a hole through a cube, the faces of the cube with the holes in them are not subdivided (without any further work), but render just fine. To create all the subdivision seems like a lot of extra work.

Thanks again.

[failure]

All complex faces are stored internaly as triangles. You can preview this by enabling triangles in custom polygon display options. Sometimes automatic triangulation of complex/convex faces tends to produce not so pretty results - i.e. long and thin triangles.

This is not a big deal for static objects but if you plan to animate via nonlinear deformers incuding skining you normaly don't want this kind of uncontroled triangulation. It deforms poorly.

Keeping the mesh in quads is implicite way to make triangulation clean, consistent and obvious.

[joojaa]

And theres no reason to work in a unclean manner just because its for rigds.

But yeah the arbitrary triangulation can cause the normals/ texture interpolate badly. So even for stills, nondeforming surfs this can cause problems down the line.

[mak]

OK, so I get it, it's basically "good form" to make everything quads. You could potentially have render issues but your millage may vary.

But I did come across this in the Maya Help regarding doing Polygon Cleanup:

"You can also tessellate faces that may be valid in Maya, but not in your game engine, such as concave faces, or faces with holes."

Is it possible that some of these modeling techniques migrated from modelers whose background was working on games?

[joojaa]

not realy, thise kinds of modelling technicues came to be because of how polygons work. you can do this as long as you

a ) dont exprect things to interpolata reighr, it can cause all sorts of artifaccts form bad highlights to totaly bonked uv textures.

b ) deont ever expect to deform the surface.

the first is not a idle thing. but you can tackle it when it comes along. but you must remeber that even smoothing algorthms are cocerned of quads so in the event you need more detail alter use quads.