Page 1 of 1

Let's check your depth buffer precision!

Posted: Sat Aug 19, 2017 8:06 am
by bkd
Some time ago I read here (but can't find now where exactly) about rendering quality of various systems. I made simple scene in Maya 6.5 (attached, originally on Windows XP, but IRIX Maya opens it without problems) for this purposes. I want that people here simply open this scene, rotate and examine for Z-fighting errors. At best I want that Dolly tool (i.e. zoom) was used and measure distance (left buttom angle of Maya) at which depth precision errors disappears during rotation. I mostly interested in IMPACT and RE2/IR+ tests. What I made myself: on my Quadro XGL it produces HUGE depth errors (see attachments). On my V10 there are ALMOST NOTHING green bleedings but at high distance (> 500) there are some grid errors (MANY less then on PC).

PS: all attachments - this file on Quadro XGL (Windows XP). As you can see in High Quality mode quantity of errors even more.

PPS: please also write on which graphics system you tested.

Re: Let's check your depth buffer precision!

Posted: Sun Aug 20, 2017 9:01 pm
by bkd
So far no one post results. Something wrong with test scene? On my Maya 6.5 (IRIX 6.5.22) it definitely opens with no problems.

Re: Let's check your depth buffer precision!

Posted: Sun Aug 20, 2017 10:48 pm
by Dodoid
I am currently "obtaining" Maya 6.5 for IRIX. Will try it on my Onyx with RealityEngine2 graphics and my Octane with SE the day after tomorrow, when I get home. I have no clue how to use Maya, so I may have to learn a bit before I can follow your instructions, but I will try my best.

Re: Let's check your depth buffer precision!

Posted: Sun Aug 20, 2017 11:56 pm
by bkd
Simply load a scene into Maya and then rotate the planes (there are two of them) - hold Alt and move your mouse while holding left button. And watch for some glitches. In perfect situation you must see only grey color on top and only green below (+ grid) but NOT that you can see on my screens. It means that precision of depth buffer is not sufficion (24 bit on Quadro). Later I want to check this on Wildcat card with 32 bit Z-buffer. On your Onyx I expect no such errors but we will see.)

Re: Let's check your depth buffer precision!

Posted: Mon Aug 21, 2017 1:41 pm
by Dodoid
My O2 isn't working right now, but it would also be interesting to see this done on CRM as well as perhaps the VW320/540's Cobalt. Does anyone have Maya 6.5 on their VW or O2?

Re: Let's check your depth buffer precision!

Posted: Mon Aug 21, 2017 2:14 pm
by spiroyster
I have Maya 6.5 and a 540 that I have been wanting to setup so might do this sometime this weekend. I can do one better! write an OpenGL app to exploit this directly! You'll also find that if you move the object closer to the far clipping plane it will get a lot worse.

Z-Fighting is undefined behavior though... whats correct? If you expect to see a green quad, then this implies that the green quad is closer to the viewer than the grey one, but they are at the same distance. There are a few things that can be done to mitigate this including using the stencil buffer, re-ordering drawing of your primitives, also by reducing the depth of your frustum. Ultimately it needs to be decided what is to be drawn on a per-frame basis

Re: Let's check your depth buffer precision!

Posted: Mon Aug 21, 2017 8:35 pm
by bkd
spiroyster wrote:You'll also find that if you move the object closer to the far clipping plane it will get a lot worse.

spiroyster wrote:If you expect to see a green quad, then this implies that the green quad is closer to the viewer than the grey one, but they are at the same distance.

Of course, on standard OpenGL Z-buffer (Z/W depth buffer). But on VPro this is NOT the case (read again my first post). On VPro there is NO green artifacts at all up to far clipping plane (when two scene planes disappear).

"The depth buffer maintained by Buzz is also rumored to be rather atypical. Namely, it reportedly remembers not the 1/w (W-buffer) or Z/w (Z-buffer, OpenGL standard) but the object Z. This approach is expensive, but it allows the depth buffer to have an uniform resolution in the whole view volume."

Let's see how SGI implemented this on old high-end graphics.

Re: Let's check your depth buffer precision!

Posted: Tue Aug 22, 2017 3:13 am
by rooprob
Hi,
I tried this out for you, for what it's worth:

Zoom < 150 zero artefacts
Zoom >150 < 160 start to get green artefacts
Zoom beyond 200 lots of green artefacts.

I have two machines (O2 w/6.5.30, Indigo2 Max Impact w/6.5.22). They produced pretty close (almost exactly the same) results running Maya6.5.

I took screenshots, but they're the same as your first two pics (the noisy green fragments, not the obvious green square) all the way to clipping distance 1000.

Re: Let's check your depth buffer precision!

Posted: Tue Aug 22, 2017 4:50 am
by bkd
rooprob wrote:Hi,I tried this out for you, for what it's worth:

Thanks, pretty interesting. Wait for someone with MXE try this but I think results will be the same as yours.

Re: Let's check your depth buffer precision!

Posted: Tue Aug 22, 2017 6:01 pm
by Dodoid
outfile.gif
outfile.gif (4.78 KiB) Viewed 379 times


Screenshot is from Maya 6.5, IRIX 6.5.30, Octane with SE graphics. Appears to be z-fighting. I don't know how to zoom, so I can't tell if it still does when I go closer or farther. Haven't been able to test on my Onyx yet, it's -12VDC power is acting up.

Re: Let's check your depth buffer precision!

Posted: Thu Aug 24, 2017 9:30 pm
by bkd
Thanks, Dodoid. It seems that MXE will show the same result. Wait for your Onyx to test it.)

Re: Let's check your depth buffer precision!

Posted: Thu Aug 24, 2017 9:37 pm
by HurricaneJames
I'll test on my MXE when I get the machine back online.

Re: Let's check your depth buffer precision!

Posted: Fri Aug 25, 2017 7:22 pm
by Intuition
This could be the a classic Maya z-fighting display bug that is simply due to the default camera clip settings. Depends on default scene scale. I'll test tonight. I bet I can get your settings to make it go away.

This is something that has never really been solved due to it not being a true bug per say but the way the gl optimizes itself for interactive update. It's a min max clip setting.

Go to the attribute panel with the camera selected. You'll see a near and far clip attribute.

Set the far like 100000 and the near to 1.0 instead of the 0.01 default. This may relieve the hardware of z fighting itself.

EDIT---

Yeah I was able to address the z-fighting. This is a maya feature/bug that is still even present in the current maya 2017 which I use at work daily.

I attached the scene file but it is saved in maya ascii. I am not sure if you can open it in Maya 6.5 irix. Usually there is an option to ignore version so it will open. I need to set up my o2 again ( I am on the road at the moment) and I'll just try the same with the Maya 6.5 on my o2 and make a new attachment. But overall yes. If I set the near clip to 1 it fixes it until another length of zoom. Set the near to 10 it can go further. 100 etc etc. But the higher you go with near clip the sooner it will start to clip away the near zoom visibility. Its all for scene view refresh capability.