View Full Version : Intersecting Mesh

11 November 2005, 07:53 PM
I just watched some of the 3D Max Games Development DVD and was a bit suprised. I noticed the artist seemed to have some intersecting meshs wish he doesn't seem to explain. I thought this was a big No-no.

Here is a clip illustrating what I am talking about:

For months I have been struggling with keeping my objects as one solid mesh. Have I been doing it wrong this whole time? Is this ok?


11 November 2005, 08:05 PM
Its allowed and commonly used in game related modeling to keep poly counts down. Altho keeping things as one mesh is good and right in some cases in others you can save tri counts with intersecting meshes.

11 November 2005, 09:02 PM
first of all, you realy save poly counts of making one mesh from many.
the second thing is that you have bether performance whe you have one object instead of many!.
why ?
because the video card doesnt have to compute the position of every objects polys.
it computes it as one object at a time ;)
for example, if you have vehicle that has a tires separete from the body of the car.
the VGA myst compute the position in every frame for the 4 tires and the body...
instead it computes one object only... this saves time a.k.a. performance ?! :)

11 November 2005, 11:07 PM
Well it is possable to have the poly's from the tire not touching the polys of the car body, but still be one object. Is this ok?

I understand what your saying and it makes sense. But I thought intersecting poly's can create artifacts. Am I wrong?

11 November 2005, 12:26 AM
This is about intersecting, right? Not the whole one/multiple meshes stuff (wich is pretty straightforward).

Intersecting can indeed increase poly's, it also might not if done strategicly. Let's say we have a cube, than you "fuse" a small cilindre so it goes through and through the cube. Now, the top surface of the cube has to break up and make all sorts of tri's so taht the cilindre can be there. It about doubles the poly's.

Blah, explaining it is pretty difficult for a Dutchy like me.

Anyways, intersecting isn't bad, it can cause lighting issues.

11 November 2005, 12:36 AM
Intersecting geometry is generally ok.

I believe the performance hit is pretty much zero on the later PC engines, but that it may cause performance issues on older engines and on handheld platforms (can anyone confirm/deny this?).

The main points are to try and make the intersecting area as perpendicular (at right angles) to the main mesh as possible. This helps to prevent "z-fighting", which is where the engine is unsure which poly should be drawn on top of the other and it looks pretty damn ugly. It's most noticable - if at all - when the two polys are at similar angles to each other.

Also try to keep intersecting geometry from spanning areas that will/can deform through animation. It can create a few problems there by not moving completely with the main mesh and therefore looking odd or looking obviously not joined to the main mesh (sticking out of or through the main mesh).

11 November 2005, 12:47 AM

i've also noticed that using 32-bit textures instead of 24 can help cut-out "z-fighting" issues.
well, at least in World of Warcraft.
not sure about anything else :)

11 November 2005, 08:48 AM
Well it is possable to have the poly's from the tire not touching the polys of the car body, but still be one object. Is this ok?
yes ?!?
you can make your car from 10 objects, but connect them into one! its not problem! its the right way to do it actualy ;)
even, it will be easy for you to rig it ;)
even in modern games youll notice that theare is a lot of intersecting mesh,

check this dudes arms. ugly! :argh:

11 November 2005, 02:35 PM
Rob, some graphics cards (at least mine gf5600) will use 16bit depth buffering in 16 and 24bit colour depth, and will only use 24bit depth buffering when running in 32bit colour depth.
Thatīs probably the reason you see less to 0 z-fights in 32bit.
Not sure about other models.

As far as I know thereīs nothing wrong with intersecting meshes when polies must be saved. As mentioned, just donīt if it results in z-fights.

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