CGTalk - HDRI workflow tips?

Couture

05-25-2008, 03:35 AM

Hey everyone. Lately I've started doing a lot of work creating and using HDRI environment maps with Mental Ray, and I had a few question on workflow efficiency tips...

First off, RENDER TIMES. If I'm using MR's environment map preset slot, and use the "emit light" option, it's giving really nice results. The render times though are unreal. I've been using an mia_exposure_simple lense shader node instead of using "emit light," but it's just been washing my scene out. Take a look at these two examples; Both have the same render settings (GI, FG, caustics, ect all added). The scene is just an HDRI, glass shader, and a single spotlight.

With emit light: About 6.5 hours
http://i8.photobucket.com/albums/a50/saber_holder/School/EL_caustics.jpg?t=1211686210

Using the mia_exposure_simple node: About 15 minutes.
http://i8.photobucket.com/albums/a50/saber_holder/School/NEL_caustics.jpg?t=1211686263

Does anyone know of any good workarounds or tips to acheive the look of the first render without the crazy render times, or is it simply acheived through additional lighting and compositing? I'm still pretty new, and I'm trying to learn as much about rendering as I can.

Thanks!

Jozvex

05-25-2008, 04:25 AM

Hi!

The 'Emit Light' and 'mia_exposure_simple' features aren't the same thing at all, so using one instead of the other isn't helpful. You should actually use both at the same time, but if you're new to rendering then perhaps leave the mia_exposure_simple out altogether until you're up to speed on other areas of rendering and MR.

When you use the Maya Image Based Lighting feature, the 'Emit Light' option turns the environment into a big area light (more or less) which is why it's so slow. There are settings to control the number of shadow samples (lower = faster but the shadows are grainier) and to control the quality of the overall effect, in U and V directions. Lowering the U and/or V quality of the effect greatly speeds it up, but greatly changes the look of the lighting. It's a bit like making your dome have less polygons, if it were a polygon sphere.

Combining Final Gather with the Emit Light feature will be really slow and you don't need to do it. Use FG 'or' Emit Light. Both are doing the same thing really, only Emit Light is more accurate.

The mia_exposure_simple doesn't have anything to do with HDR lighting per se, it controls the gamma/contrast/etc of the final render regardless of how you lit it.

Sorry I'm not actually at Maya right now (it's having issues!) so I might be off on my descriptions of the settings.

:thumbsup:

Couture

05-28-2008, 01:29 AM

Thanks for all of the info. I messed with it some more this weekend, and got some way more render time happy results. I added my map, and upped its gain to a value of 2.0, and used final gathering and GI in my render, and got this in about 15 minutes:

http://i8.photobucket.com/albums/a50/saber_holder/School/helix_2.jpg?t=1211938094

I was wondering if anyone had any more general HDRI workflow guidelines or tips that they could share for a huge noob like me? Thanks! :-)

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