View Full Version : indoor/outdoor rendering

03 March 2011, 10:37 PM
I'm kind of new to Mental Ray and the whole architectural side of things and I'm having some trouble finding a balance in my lighting. It's a large interior scene with huge windows along part of the space that lets in a lot of daylight. I can adjust my exposure control on the camera paths that are mostly indoor and for the ones that have mostly daylight lighting, but some of the cameras go from one to the other, so it either goes from well lit to really blown out or well lit to really dark, and I can't find the balance. I'm calculating GI and emitting an enormous amount of photons but it still ends up being significantly darker inside than it is when you're near the windows.

Does anyone have any techniques or approaches in how to handle this? In a couple of the camera's I can render the outside stuff separately, as it's own pass, but on a couple of the shots it spills in on the interior geometry, so I can't really do a separate pass for that. One thought I had was to set my exposure control to make the daylight light look right, then just pump up my interior lighting to brighten the dark interior areas, but the GI map takes half a day to calculate so trial and error on those settings is not an option, really.

I'm using Max Design 2011, a daylight system, photometric lights, Mental Ray and the Arch and Design material.


03 March 2011, 12:41 AM
GI takes forever to calculate? Photons? They are generally fast. They may be shooting into space and not storing. That can be solved by using an invisible container around the scene with a photon shader to store them if they miss. Not an elegant solution.

But I would suggest using portal lights for the interior and rebuild your final gather map for the camera animation using FG set to off and give it a file name. (This is if rendering on one machine)

The extreme change in exposure is correct, that naturally happens in the real world. You can keyframe your lens shader to account for the effect. But honestly, in the interest of photorealism, a camera that takes a moment to readjust to the lighting difference might be a nice effect.

03 March 2011, 06:51 AM
I concur as well. Portal Lights are really the key to making naturally-lit interiors, and this also means a Sun/Sky. If you're not already using the Sun/Sky, that would also explain your long GI calculations. The photons need to have a distance limit; if they're firing off into null space it takes mental ray much longer to return this result - a real killer for photon GI alone.

The Sun/Sky will do this automatically, but if you don't wanna use it a big sphere (with normals reversed, pointing in) surrounding your scene will also work.

I haven't had a 12-hour photon map for years... Usually only takes a few minutes, even with a dozen different lights, several portal lights, and a sun/sky. Depends on your settings of course though, too.

03 March 2011, 03:44 PM
Well, by half a day I mean half a work day, so it's around five hours. I am using a daylight system and It's using the radius thing so it doesn't emit more than it needs too. I've used sky portals on smaller spaces before, when it's just a room with a window, but one space is a corridor with windows along one side and the other is a huge rotunda entry room with a two story wall of windows. And although this exposure issue is physically accurate, the client wants the interior to be bright and welcoming, but when I expose for that, any area near a window is terribly blown out.

I'll try the sky portal thing I guess and see if I get better illumination inside.

03 March 2011, 08:31 PM
Clients are always going to ask for this kind of thing. There is nothing wrong with breaking the physical accuracy to please the client. I have to do this all the time.


03 March 2011, 02:20 AM
I've used sky portals on smaller spaces before, when it's just a room with a window, but one space is a corridor with windows along one side and the other is a huge rotunda entry room with a two story wall of windows.

The key to using portal lights isn't that they're an area light so much, nor that they are physical lights that show up in physical reflections, but that they are "FG concentrators". This is explained very well in the Maya help, but I don't know about Max's help files at all.

Basically, when using FG inside a room and with a sun/sky, most of your FG rays get lost outside the room. This makes for dark, splotchy, and generally hideous renders. The Portal Light tells the FG rays regarding the sun/sky to concentrate through the opening, so you don't waste a bunch of math on FG stuff outside that isn't even visible. It's faster and provides better lighting. That's the theory anyway.

Going from outside to inside in an animation? Never tried it with portals. Should work. But the portal's visibility may be a factor too - when looking at them from their back side, all you'll see is a pass-through to the physical_sky node, and no geometry renders in that case. There are workarounds for this as well.

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