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Author Topic: Hidden Geometry and Performance  (Read 6938 times)

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Derrek3D

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Hidden Geometry and Performance
« on: December 23, 2007, 12:23:22 PM »

In order to improve the way the 3D actor's shadow falls on objects I tried a more complex hidden geometry for one of my scenes and although it really looks better now, I'm experiencing a serious drop in performance. My FPS for that scene dropped from 100 to 70. Is this normal? I read that a more detailed hidden geometry affects its loading time and pathfinding time (if 3D pathfinding is used) but I didn't read anything about performance. On the other hand I read that the hidden geometry is actually rendered so I believe it can have some impact on FPS.

Any info and advice on this issue would be greatly appreciated.
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Mnemonic

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Re: Hidden Geometry and Performance
« Reply #1 on: December 23, 2007, 01:00:15 PM »

When using stencil shadows, the hidden geometry needs to be rendered. So in this case it indeed affects rendering performance.
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Derrek3D

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Re: Hidden Geometry and Performance
« Reply #2 on: December 23, 2007, 02:39:39 PM »

I was afraid this would be the answer... but thanks :)
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Jyujinkai

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Re: Hidden Geometry and Performance
« Reply #3 on: December 24, 2007, 01:29:43 AM »

70 frames per second? is that bad? Films is 25 frames and TV is 25 frames (pal) and 30 (ntsc).... so anything over 30 frames a second is not needed.
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Derrek3D

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Re: Hidden Geometry and Performance
« Reply #4 on: December 24, 2007, 12:08:01 PM »

The FPS values that I mentioned are using my current development machine. I can't count on everyone having the same specs that I have. But anyway, I mentioned the values just to emphasize the performance hit and I just wanted to know if losing 30 FPS is normal for a slightly more complicated geometry.

Theories aside though, my scene starts stuttering when the FPS goes below 60 or 50 and not 30. At 30 FPS my scene is half dead...
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nuclear_winter

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Re: Hidden Geometry and Performance
« Reply #5 on: December 27, 2007, 09:13:57 PM »

Derrek, just out of curiosity, what is your hidden geometry and your character polycounts?
and what are your test PC specs?

thanks
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sybixsus

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Re: Hidden Geometry and Performance
« Reply #6 on: December 30, 2007, 01:46:31 AM »

Films is 25 frames and TV is 25 frames (pal) and 30 (ntsc).... so anything over 30 frames a second is not needed.
This is false logic. I won't bore you with technical information you can easily find with Google, but suffice it to say that motion blur, interlacing, low resolutions and large pixel pitch are all among the reasons why TV is smooth at 24FPS and videogames are not. If you can't sustain 60FPS then you'll really struggle to get playable results, particularly on TFT's where you pretty much *have* to VSync in order to eliminate flickering.

So if Derrek has a fairly powerful system, then yes, this is bad, because people with a less powerful system will be getting some pretty poor framerates. If, on the other hand, Derrek has a pretty old system that is giving him 70FPS, then it's not too bad.
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Jyujinkai

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Re: Hidden Geometry and Performance
« Reply #7 on: December 30, 2007, 03:39:27 AM »

hmm not sure if i agree on this.
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Derrek3D

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Re: Hidden Geometry and Performance
« Reply #8 on: December 30, 2007, 10:31:45 AM »

Derrek, just out of curiosity, what is your hidden geometry and your character polycounts?
and what are your test PC specs?
The geometry is something like 7000 polys and the actor has some 4800. My system is a Pentium 4 3GHz with 1GB RAM and a GeForce 7600 GT GPU running Windows XP. Do these numbers make any sense or are they too high/low (I'm referring to the polycounts and FPS)?
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Re: Hidden Geometry and Performance
« Reply #9 on: December 30, 2007, 12:28:20 PM »

7000 polys for hidden geometry is brütal! I never go above 100.
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Derrek3D

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Re: Hidden Geometry and Performance
« Reply #10 on: December 30, 2007, 02:37:43 PM »

Well, it's just a test for now. The overall geometry is fairly simple but I have three objects in it that are about 2000 polygons each. In fact I took these three objects straight from the actual 3D scene that was used for the rendering of the background image. I know that it's not good practice to do that but I have a problem in this scene that the actor walks behind these objects and if I place simplified objects instead of the original detailed ones, their outlines don't perfectly match their equivalent rendered images and it's clearly visible in the game that the actor is obstructed by something transparent. Also, if the simplified and the detailed objects don't match perfectly, the shadow on the objects look weird because it either doesn't cover the entire object or falls also on the air near the object.

Bottom line, I don't know how to make the shadow look accurate when it falls on objects without using the original detailed 3D objects. Any ideas?
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nuclear_winter

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Re: Hidden Geometry and Performance
« Reply #11 on: January 01, 2008, 02:07:39 PM »

Derrek i too believe that 7000 + 4500 is way too much... what are your 2K poly objects? maybe they can be optimised in a way that they still look hi-poly from the angle of the camera making the shadows look smooth.
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Derrek3D

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Re: Hidden Geometry and Performance
« Reply #12 on: January 02, 2008, 02:05:04 PM »

We're currently doing exactly that. We're simplifying the detailed objects while leaving their outline intact. For now, we've managed to gain back 12 of the 30 lost FPS. I hope we'd reach even better results soon. Thanks for the advice and the support.
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