Motion Blur refraction depth?

[Seraph135]

I'm not sure if such a thing is possible or not, but I'm working on a project that has a LOT of reflections and refractions. With lots of moving pieces behind layers of glass.

I'm noticing that without motion blur turned on my renders are quite fast, but with motion blur everything crawls. If all settings are the same except turning on and off motion blur (with default moblur settings) the difference in render time is 20 times longer with motion blur as opposed to without it. In many of the shots most of the motion blur would only be seen on the beveled edges of some of the glass objects and really isn't contributing anything noticable to the final render, but yet those areas are just as slow to render as the areas where we are more directly seeing motion blurred objects. All objects are behind at least two layers of glass and we have upwards of 50 layers of glass.

So what I'm wondering is if its possible to set a reflection/refraction depth for motion blurring to be visible?

Like if motion blurred objects are being refracted through 50 layers of beveled edges and not really contributing anything to the final shot, it would be cool if there was a way to tell Vray that if a motion blurred object is seen through more than 5 layers of refractions to treat it as though there was no motion blur on them for those refractions.

Is it possible? If it was would it actually speed up my renders? and of course....If it is possible how soon can I get that feature?

I'm working with Maya/Vray btw. No GI. Just a dome light which lights only the non-glass objects. The glass objects are not affected by lighting at all and is reflecting/refracting an HDR. I'm open to hearing other ideas for speeding up these renders if anyone has other ideas.

Tim J

[Morbid Angel]

No there is no way to make refraction depth to have no motbion blur. The way vray works, its either all or none, same thing where specific objects cant get different gi attributes then others.

You can however limit the number of times objects refract by controling global reflection / refraction depth. Quite honesltly though, unless you can share the settings of your AA, I dont think there is much more help we can give you. Motion blur typically slows down the rendering only at places where objects motion blur. If you are experiencing significant slowdown accross the board, perhaps there could be something else at work.

You may experiment with motion vectors, they surely can speed things up, but it may be tricky to get them to work, as refractions/reflections is its weakest area.

This is a vray for 3ds max section fyi.

[vlado]

You can also disable moblur for objects moving relatively slowly that you do not wish to have motion blur (you need to set the geometry moblur samples to 1 for these objects). If you are using more geometry samples, it will help to enable them only for objects that really need them. Switching from "static" to "dynamic" default geometry may also help.

Best regards,
Vlado

[MasterBercon]

The default motion blur subdivs is 6. That means huge amount of more sampling by default. If you have only slow moving objects something like 3 would probably do and render a lot faster.

[Seraph135]

You can however limit the number of times objects refract by controling global reflection / refraction depth. Quite honesltly though, unless you can share the settings of your AA, I dont think there is much more help we can give you. Motion blur typically slows down the rendering only at places where objects motion blur. If you are experiencing significant slowdown accross the board, perhaps there could be something else at work.

You may experiment with motion vectors, they surely can speed things up, but it may be tricky to get them to work, as refractions/reflections is its weakest area.

Thanks Dmitry. As it stands I've already lowered my global reflections/refractions to the minimum I can get away with and still be able to see through all the layers of glass. In my tests so far I've been using basically no AA...Its set to DMC min 1 max 1 and threshold of 1.0. These are of course not final settings, just the fastest I can get and still see the result. Even with settings this low it takes 4 minutes a frame without motion blur and with it its takes 81 minutes. I can't use motion vectors since they only see the geometry in front and not the objects moving behind the glass.

You can also disable moblur for objects moving relatively slowly that you do not wish to have motion blur (you need to set the geometry moblur samples to 1 for these objects). If you are using more geometry samples, it will help to enable them only for objects that really need them. Switching from "static" to "dynamic" default geometry may also help.

Unfortunately, everything is going to be moving quickly and fairly randomly at some point during each shot. It just isn't all moving at the same time so there are large areas of the frame that aren't moving at the beginning of a shot, but do move at the end of it. So I don't think I can easily turn down settings for individual objects since everything will move. I will try switching to dynamic memory and see if that helps. Thanks Vlado!

If you have only slow moving objects something like 3 would probably do and render a lot faster.

Unfortunately, when objects move they move fast. The default motion blur setting is too low for final renders, AND if that wasn't enough all the parts are spinning so I'll need to turn up geometry samples too. This project is a raytracers worst nightmare. Its going to look cool when its done though.

Thanks for the ideas guys!

Tim

[Morbid Angel]

is there any displacement involved? what is the frame size you are rendering at? and finally, perhaps you could share a part of your secene for a study.

[Seraph135]

No displacement, no GI, and at the moment no lighting even. This is purely just reflecting/refracting an HDR which is projection mapped onto a very lowres model of our CG environment. At the moment I'm rendering 1024x576. Some shots may need rendered at 1920x1080 for final. I wish I could share my scene but I can't. Imagine a ton of hollow glass boxes and put an object inside each one. Then have groups of them spinning around at random times and try to look through the whole lot of them.

Tim

[MasterBercon]

Well there is only so much you can do within the limits set by mathematics. CPU's simply aint that suited for this kind of thing. You'll have to wait for hardware thats desgined for raytracing or just buy a whole lot of render slaves.

One thing you might try (asuming everything in the scene is motion blurred) is to disable anti-aliasing and use only motion blur subdivs to get rid of the noise. I'm not certain but it might be faster. If vlado could give us a little hint about the motion blur sampling. How does it differ from DMC anti-aliasing?

[Morbid Angel]

well i had run a quick test. Not sure what you have but I didnt get too bad of render times. This was in 3ds max however:

Here is 2000 refractive cubes with another set of cubes spinning inside farely fast. Lighting is a domelight + hdr as both refractive/reflective and illuminating source.
Motion blur is set to 5 geom samples.
Adaptive DMC 1/4 (default)

At 720p on my 2 4 core machines without motion blur it rendered in 3m 28 sec.


with motion blur 8 m 11 sec (about twice as fast)


another angle, i took the whole thing and put it inside another refractive box and rotated it,
4 m 51 sec.

[Seraph135]

That's interesting. By the looks of it your doing almost exactly what I'm up against but your render times are far better. Are the corners of your boxes beveled? Does the glass have reflections also? Are the boxes hollow on the inside? As though you took 6 square sheets of glass and made a box out of them.

I have a few ideas I'm going to try next. Thanks for taking the time to set up a test like that and give me something to compare to. I appreciate the help.

Tim J

[Morbid Angel]

so i made some adjustements.
Yes my boxes are with a shell modifier so they have thickness. They are beveled with 3 devisions on the edge. I added reflections and here are the result:

7m 57s for non mb one


18m 24 sec for mb. Keep inmind it had 5 mb geometry samples.


roughly 2.5 times increse in render time. Not nearly as bad as what you have...imo.