Question about backfacing polys...

[Short_Fuse]

Good question. Since i mostly model lower poly objects for games all the backfacing faces are deleted on my models. If there not goin to be seen they have no use. But there are a few things you should think about before deleting them.

*Will that part ever be seen during animations or other movements.
*Are there any transperant objects near it which could see right through the back face.
*Does it have any use.

Also deleting these lowers the poly count which is all good for game modellers.

[Tylak]

I think Short_Fuse covered things pretty well. I just want to second the suggestion that you need to think about how the model will be animated, cause its a nasty suprise to find later on.

[TheWriter]

The only modeler i've used that doesnt have backface culling was Blender.

Luckily, those days are over :P

[gaggle]

There's a remarkable difference between backface culling, and deleting unseen faces.

"If there not goin to be seen they have no use" works for me.

Theoretically if you're doing collision-detection or something really wacky I suppose it could become relevant to have properly closed meshes and such..? Though any modern game featuring dynamics to any serious extent should have separate (simplified) collision-hulls, so that's probably a non-issue anyway.

[Tylak]

well put gaggle

[OttovonDrone]

If you just make models, it's alright. However, most of the game engines (especially those that use dynamic lighting) require CLOSED geometry, which means no open objects. That means -- like it or not -- these polygons must stay unless you want to blend that separate object with the body of the character. However, you must make the connection nicely -- you have to cut the face on the body, and weld vertex to vertex with the external object after you do the attachment/collapse/combine/whatever. This in addition brings the polycount up, and sometimes makes your mesh difficult to manage. Bottom line -- sometimes it's just better to keep them as they are as long as they are closed.

No easy way here :)

Best,
O_v_D

[Dargon]

Originally posted by OttovonDrone
If you just make models, it's alright. However, most of the game engines (especially those that use dynamic lighting) require CLOSED geometry, which means no open objects.

Where did you get that information??

Ever heard of Alpha flats?

I'd have to say, it would be a pretty sh*t poor engine that wouldn't allow double-sided polys, and would require you to make everything water-tight.

Not only is it a good idea to shy out polygons that you do not see, but it is industry standard that you do. An art director would give you an exact poly count that you would have for a model, and it is up to you to make that model as good as possible for that poly count. Which means, deleting faces you do not see, to use them somewhere else instead.

Most of the models for the game I work on have open surfaces - in fact have many things like necklaces, and hair, which only have one triangle total for the item. No backside, nothing.

The ONLY type of modeling I have ever seen where you would have to have a model totally closed, would be to have plaster casts made out of them.

[OttovonDrone]

I probably overstated a bit when I said "MOST" engines. Alright, some of the engines using Vertex Lighting do need convex geometry, but most of them indeed do allow you to delete the back faces.

However, if the engine uses Dynamicc Lighting, then no way in hell can you delete the back faces because the non-convex geometry will not cast shadows. When calculating the real shadows of Dynamic Lighting (and mind you, most game engines move in this direction) the light vector needs to intercect both the "entry" face, and the "exit" face in order to provide information for the shadow. Of course, if you need low poly objects that should not cast shadows, it's OK to save a few faces. Cutting off all back faces on everything is a big no-no with Dynamic Lighting, though.

Best,
O_v_D

[Tahl'eN]

As another thought to the original question, a lot (if not all) of the original UT 2003 models simply use a cylinder for belts, arm bangles, etc. The caps close in the middle of the character's bodies. The same is true for boots and gloves. This saves not only on the inside of the belt, but saves 2 extra polygons per face of the belt, as the top and bottom of each segement is just a tri, rather than a quad.

[jimcoldt]

Originally posted by Tahl'eN
As another thought to the original question, a lot (if not all) of the original UT 2003 models simply use a cylinder for belts, arm bangles, etc. The caps close in the middle of the character's bodies. The same is true for boots and gloves. This saves not only on the inside of the belt, but saves 2 extra polygons per face of the belt, as the top and bottom of each segement is just a tri, rather than a quad.

Why do the caps even need to be kept instead of deleted?

[Poopinmymouth]

Dargon, what ottovon was trying to say, is that if you want the shading to be correct, it needs to be water tight. because lighting is PER VERTEX. that means if there isnt a vertex, its not gonna be shadowed. there are times when you dont care, and use planes, or other such objects, but if you have a belt, and its just floating on top of another plane, its not gonna shade it. however if the plane is cut, and the belt welded to the plane, you would get accurate shading. its optional, but prefered to have most things on a model watertight. this is excluding double sided planes and such.

[Dargon]

Now, I can't say what happens with dynamic self-shadowing, because our engine, being a PS2-based engine, doesn't do self-shadowing. Of course, any system that can handle self shadowing, would of course, be able to handle quite high poly counts, as it is incredibly processor-heavy.

That being said, we do both pre-lit and dynamically lit objects and characters in our engine, and it has no problem doing per-vertex lighting on objects that aren't complete.

I can't even understand why it would have problems with it on a dynamic lighting type of level.

After all, what about alpha flats? I would think you would be hard pressed to find a game that did not use these anywhere at all. (not impossible, just really rare) If dynamic lighting could not affect these, your characters could not have hair, your ground could not have grass, your trees could not have leaves being lit by lightsources at all.

As far as Tahl'eN was saying - that technique works well for solving the problem of bangles, and so on, and definatly helps reduce polys - no reason to not do it. For jimcoldt, you keep the caps so the bangle or belt appears to have thickness. Otherwise, you'd just have a paper-thin belt, and it would look a bit wierd, and possibly give you problems when animating. By giving it some thickness, you can have it a little farther away from the body. Basically, if you were going to make it that thin, you might as well just do it with texture.

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