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Mental Ray - large interior scene
show user profile  Mr.Burns
I need to do an animation of a factory hall of sorts. The hall is rather large (about 120 x 40 m) and there are loads of animated objects in there that are very thin (about 1 cm). The length of the animation will be a few thousand frames, so render times are crucial.

So far whenever I had to do this kind of animation it was on extremely short notice, so I couldn't put too much effort into it - I'd just leave out the hall, put a daylight system in there, place the sun at a right angle to the ground and give it a low intensity and soft shadows. This would give me a reasonable quality at low render times.

However, my boss has asked me to do as nice an animation as possible, and I'll have lots of time to work on it, no specific deadline or anything. Now I'm wondering what would be the best way to go about this. (I'm using Max 2010 and have 8 rendering machines at my disposal)

If I were to just do what they do in these actual halls I'd place lots of overhead spotlights in there, but wouldn't it be better maybe to just use mrSky and a huge sky portal that covers the entire hall?
I've read that using GI and FG in combination is a good idea for interior scenes, but I'm not sure how well this will work with all those thin objects in that large space.

If anyone could point me in the right direction I'd be very grateful.

This is the kind of hall I'm talking about btw, it'll need to look nicer than those images though:

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5/1/2015 1:06:29 PM (last edit: 5/1/2015 1:06:29 PM)
show user profile  khamski
How about planning your shots.
I mean making animatic and show the boss what the montage will be.
And then work shot after shot.

This way you won'y need to lit the whole warehouse.
Just one scene shot at a time.

Hope i got the problem right.
Not sure though.

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5/1/2015 1:39:40 PM (last edit: 5/1/2015 1:40:06 PM)
show user profile  herfst1
If I was going all out and using Vray and I'd use a dome light with custom made HDRI (think the program's called HDRStudio) and projecting it through the walls (with backface cull on), then I'd use a load of GI. Also with indirect illumination I'd turn the hemispherical interpolations up to catch the fine details. Then you can fake the "real" lights by using self illuminating textures.

Sorry, don't use MR.

Oh, and I've never made a custom HDRI map, just had an idea to do that recently, should work well, and would be pretty darn easy to set up and tweak the lighting afterwards.
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5/1/2015 3:06:58 PM (last edit: 5/1/2015 3:07:43 PM)
show user profile  mike_renouf
I've done a few interiors with mental ray. Yes - GI (photonmapping) gives a broad-brush lighting solution and FG gives more detail to the lighting. If you have many moving objects though you might consider cheating a little and doing some compositing, or faking some of the lighting, otherwise you'll end up calculating the GI for every frame which will lead to heavy render times. You might consider rendering the hall with any camera moves as one pass and then your moving objects afterwards. I add the caveat that I've never tried doing separate passes myself, but I gather some vfx guys round here are experts.
I last used mental ray seriously in max 2010, so hopefully the whole unified sampling system has been cleaned up, it was a bit clunky before. You can use Image Based Lighting now I gather - which is what herfst was referring to - and whack HDRIs in to get good lighting. Hopefully someone more up to date can help with specifics, but give me a yell if you need. I'm more a Vray fan now, but I'll take a look if need be.

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5/1/2015 11:26:11 PM (last edit: 5/1/2015 11:26:11 PM)
show user profile  Mr.Burns
Thanks guys!

herfst: That would be a good solution to give me the kind of lighting I need. What kind of a speed difference does it make in V-Ray using a HDRI map vs using just a skylight (no sun)?

mike: I've stumbled across a blog that described this method. The guy used GI for all the static objects and FG for the animated ones. It seems to get a bit complicated though if you need reflections :S
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5/2/2015 12:26:09 PM (last edit: 5/2/2015 12:26:09 PM)
show user profile  herfst1
I really wouldn't know the speed difference. All I know is you can control your lights from multiple angles and it will make your reflections nice.
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5/2/2015 1:20:56 PM (last edit: 5/2/2015 1:20:56 PM)
show user profile  Coxy
First make sure you use the Arch & Design shader for basically everything.

If you are using just a HDRI to light the scene, you possibly want to use the environmental blur shader to help smooth the reflections (Although I'm not 100% sure if this has been solved with the new skylight IBL option in the GI) . If you are having trouble with inter-material reflections you can limit ray distance in the reflections to help with that, but sometimes that can create odd results (I've had it rendering out pure white before). Master Zap has a good section about Mental Ray and this stuff on his blog, I would check it out!

Also the thing to remember with Unified Sampling is that although it unifies everything you still need a few samples on the materials/lights to give it some leeway. 4 for lights and 8 for the materials samples usually works fine and gives the sampler enough room to work with. I tried multiple different ways to work with it and had to visit some pretty obscure places online to find this out. MR isn't very good at telling you exactly how things work under the hood, just what things do.

I'd try and make as much use of the 'Highlights and Final Gather only' options for very low reflected surfaces, it really can be a life saver time wise and often you won't notice much of a difference. I recently did a very extreme marble room with unified sampling and it took me a while to get it looking correctly, I had to basically spend a while working out why it was giving me problems. Out of memory I found there to be problems with the Light Portal and Unified Sampling at an extreme level (i.e when the majority of light is coming only from a light portal). It gave horrible reflective noise, so I think out of memory I ended up relying just on either just the Final Gather pass with a lower exposure..

For animated objects you'll need a Final Gather map along camera path, rather than one individual file. I read somewhere that the camera path division should be total length of frames /fps. But I can also imagine you might need more depending on how fast the objects move.

One more thing to remember about Photons is that they work in 3D space, so if it hits a wall and the sampling distance is too large it might interact with another Photon on the outside of the wall and produce off lighting, so you sometimes need to limit the radius to stop this happening when you have thin geometry. I would personally use GI+FG, or just FG for the interior room using one Photon Map and a FG map and then composite in the moving images using FG only and the camera path option using the initial pass as the matte.
- A good thread about doing this for Maya. I believe the shader they are talking about in there is the Render Subset of Scene/Masking one in the 3DS Max Lens Camera Shaders. Hope some of this has been helpful!
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5/4/2015 3:07:58 AM (last edit: 5/4/2015 3:19:15 AM)
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