Intersecting and open Geometry, go or no go?

AlexLegg

Hi guys,

Just wondered what the opinion is on intersecting and open geometry? Is it an acceptable practice to save polys and time?


Can I delete back faces etc that are hidden?


Will game engines not happily accept these things, or does it cause problems and best avoided?

Would it be ok to show this in a portfolio? Would it be seen as effcient modelling or sloppyness?


Thanks for any advice you can give,

Alex

bugo

INTERSECTING is fine if they are near 90 degrees, if it goes 30 degrees or lower then that, there might be a problem with the Zbuffer trying to render it on lower bits. Better avoid lower angles for intersection.

If BACK FACES are hidden, and for some reason you are not going to use them, yes, definately needs to be deleted or will count on poly count for no need. And that will be bad for you mister!

If a mesh it's opened any game engines nowadays will be alright to render that. In some cases, for an example Unreal, it's preferred to NOT make a backface deleted for lightbaking purposes, but that doesn't apply everywhere. Someone might be able to explain you that better.

Chris Bruin

Delete every face that you can not see from your camera. You don't need them, games won't crash without them, and it just keeps your scene clean. Intersecting geometry is ok, just make sure that the seam that they make is on purpose and doesn't stick out. An Ambient Occlusion pass usually makes these seams non-existent.

Say for example you are making a low poly cardboard box that has it's top open and no flaps. Having a single sided polygon isn't bad at all, however, keep in mind that what appears on the outside will also appear on the inside of said box. It is an open geometry box at this point, and will (should in 99% of all cases) not cause any problems.

Efficiency is key when making objects, getting the most for as little as possible without sacrificing quality. Use whatever tricks you learn to make your project awesome. If you keep your scene clean and delete unseen objects, it will be seen as efficient.

rooster

if there's grass on the pitch, lets play! aai

(yes)
but in addition to whats been said, be careful with animated meshes, because sometimes what's not visible in the bind pose can be seen during animation

Hazardous

rooster wrote: »

if there's grass on the pitch, lets play! aai

(yes)
but in addition to whats been said, be careful with animated meshes, because sometimes what's not visible in the bind pose can be seen during animation

+1 to this - especially around common deformation areas. A common one is bands or cloth around shoulders, or gun holsters around the groin etc etc verts weighted around deforming areas rotate around the bones pivot and split influences can make it more difficult for your animators to balance things out, causing some things to pop out further than what you might think!

Also you dont want to piss off your animators by giving them more stuff to worry about/ sort out..... do you ?

J-K

And just to add to what everyone else has already said: Don't forget that in many cases, intersecting geometry will cause overdraw, which can be a bit nasty if you're dealing with large objects (e.g. environment stuff) and there's a lot of it. It's definitely something to keep in mind if your target resolution is native 1080p on a console!

ZacD

J-K wrote: »

It's definitely something to keep in mind if your target resolution is native 1080p on a console!

There's native 1080p on consoles?

EarthQuake

This can be very engine specific, some engines require "water-tight" meshes that have no open edges/deleted backfaces.

Ott

And I would never accept a job at a studio working with an engine like that. Oh, the horror....

But yes, some Engines DO require them, but (thankfully) many nowadays are moving away from it, as it is a complete nightmare to have to work under those sort of restrictions.

cryrid

I've worked with at least two engines over the last year that require water-tight models for proper lighting. I don't really see it as a nightmare (keeping the few hidden faces didn't seem to kill the performance, and its at least one-less button press to not delete them)

Mark Dygert

Well there's a difference between meshes that need to be built for vertex lighting and water tight meshes. The games that require actual water tight meshes are a nightmare. The games that use vertex lighting aren't so difficult to work with unless they never tell you how to deal with vertex lighting. I would never want to work at a place that used vertex lighting but told its artists to create water tight meshes...

For example you have a box with a cylinder thats meant to look like its part of the box. You can float the cylinder and delete the end cap that would be inside the box. Now for vertex lit pieces you want to float the verts just above the surface of the box. If you put them inside it will create dark shadows across your cylinder mesh, unless you put a row of verts just above the surface to contain the shadows. This isn't water tight but it will light correctly.

Water tight would be welding the cylinder and box together and so they're one continuous piece, maddening to work on and needlessly wasteful in most cases. There are times when it is smart to do one continuous mesh but those cases can be rare and its not worth punishing yourself 90% of the time.

cryrid

I had meant water-tight as in not one continuous piece (a hand going into a sleeve it wasn't connected to, eyes, etc), but still not being able to delete the end caps and faces that never get seen. Even if faces/verts were left as padding for the lighting, it still wouldn't light properly until every island on the mesh was closed up (it also seemed like you could cap the ends, leave them intersecting without added verts around the point of intersection, and it would fix the lighting in C4). VBS2 also seemed to require closed edges as well, but again has no problem with islands or intersecting geometry (floating ammo pouches, misc details on armored vehicles, etc) as long as everything was at least capped. Not the greatest of engines to be sure, but they've put me in the habit of leaving every face until the project allows for their removal.

Water-tight as you described does sound like much more of a pain.

Mark Dygert

Well yea it is very engine specific, most of the engines I've dealt with have used vertex lighting and open ended pieces don't matter as long as you have verts in the right spot to catch lighting.

http://www.hourences.com/book/tutorialsvertex.htm

Ghostscape

what engines require "water-tight" meshes that are all merged into one solid mesh (with no intersecting geo?)

Engines utilizing stencil shadows (which require water-tight meshes) just required you to cap the intersecting meshes so that you had no open borders, you could still intersect all you wanted.

Valandar

Getting back to the 'removed caps can sometimes be seen' bit, I have seen many a free MMO being played by a friend where certain angles let you see that a hidden face has been removed, which becomes visible in animation, meaning you now have a hole - and since backfaces usually aren't rendered, that means they now have a hole through the mesh.

AlexLegg

Thanks for all the advice, its a big help.

Should give me more confidence in what I'm doing now!

albino_goldfish

AlexLegg wrote: »

Hi guys,



Will game engines not happily accept these things, or does it cause problems and best avoided?

Would it be ok to show this in a portfolio? Would it be seen as effcient modelling or sloppyness?


Thanks for any advice you can give,

Alex

I know atleast in the UDK, when making collision meshes, there can be no open edges, etc. models need to be "perfect" and simple

mLichy

Yeah, I've worked with some engines that require water tight meshes, but that was a long time ago.

Now a days like others said, you can get away with deleting hidden stuff, and it's usually recommended. However, I've had to make water tight collisions for certain engines for it to not have errors, and also for vertex lighting, float things or cut in extra geo to fix nasty shadows.

So I would say just get used to deleting backface/hidden stuff, and if you happen to work at a place that you can't do that, no big deal.

Mark Dygert

Ghostscape wrote: »

what engines require "water-tight" meshes that are all merged into one solid mesh (with no intersecting geo?)

Engines utilizing stencil shadows (which require water-tight meshes) just required you to cap the intersecting meshes so that you had no open borders, you could still intersect all you wanted.

I worked on an indie project where the collision mesh and detail mesh where the same and they could contain one sub-object, one smoothing group and the meshes needed to be water tight. It was my first intro into water tight meshes and wow did it leave a bad taste in my mouth.

Ever since then I've been able to delete hidden faces in just about every major engine I've worked with and only run into a few easily fixed problems with shadows and floating geometry.

They used some engine that was made by someone else. I don't remember the name off hand, it was 2-3 years ago. It was a bit of a nightmare to work with. Pretty low poly but even then it was nuts. Big surprise it went no where.