XM Magdalena 3D print, GGeorgy (3D)
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View Full Version : why mental ray displacement is so slow?

07-06-2009, 03:22 PM
Why mental ray displacement is so slow? When testing 1-2 objects alone, it's pretty fast, but as long as I render them in a scene, it starts to crawl. With bump the scene is rendered in 30 seconds, with displacement (very subtle on 2 objects) it goes up to 5 mins, and with reflections on those objects up to 16 minutes.

07-06-2009, 04:02 PM
MR uses adaptive feature-based displacement, set it to a lower tessellation factor. Bump is much faster because it is a scalar shift in normal direction, while increasing the number of triangles has more implications, not mention to act of tessellation itself a computational expensive task.

07-06-2009, 04:46 PM
Yea... that's the difference between bump and displace. Displace entails (usually) further subdividing and moving geometry at render time. Once it's displaced, it's got a lot more tris, which leads to more shadow computation (if any), material calculations, etc.

07-06-2009, 04:49 PM
Yeah, you are right... maybe it's shadow casting that slows so much when in a scene. I rendered it without GI, but still, it slows down compared to isolated rendering.
I wasn't using displacement till now, but I want to test it with characters. Damn, it looks so slow. :curious: I know about tesselation and chose the optimal setting, but still.
I remember quite clearly somebody stated mr displacement is fast.

07-07-2009, 05:34 AM
"Fast" compared to.... ?

It depends on a lot of things. If you simply displace an object and thats it, then yes, it's fast. Displacement is slow compared to most other tasks in 3D. I rendered a torch a while back in 720p displacing to about 2 million polys, a 300 frame turntable didn't take much more than an hour or so, including lighting and shadow etc.. Depends on your computer, the number of objects in the scene, lights, render settings, etc.

In conclusion, the displacement itself is fairly quick. If it's very slow, then you might have not chosen the optimal setting. There's too many variables to come to a conclusion in a paragraph on an online forum.

07-09-2009, 09:26 AM
Renderman renders displacement rather quickly :love:.

07-09-2009, 09:58 AM
Renderman renders displacement rather quickly :love:.

I know, but it's not about renderman. :wip:
I will test mr displacement as Phix314 suggested, maybe there are ways to speed it up, like using shadowmaps.

07-09-2009, 11:19 AM
Yes, shadowmaps are definitely going to speed up your renders if you are using raytraced shadows currently. On the other hand, if you are using Zbrush (in windows) then you can use decimation master to reduce the polycount of your model. I haven't tested it yet...but it might reduce render times :shrug:.

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