Using max 2009, when rendering using a daylight system (mr sun and mr sky) with global illumination and final gather enabled, I can’t find a way to get mental ray to handle reflected skylight properly—it only includes the direct illumination from the sky and not any skylight that has, say, been reflected off the ground and onto a ceiling. The same applies to light from any object with self illumination. Disabling global illumination and increasing the final gather “bounces” value renders the skylight properly but then omits any reflected light from the sun, which is just as unhelpful. I’ve managed to get a decent result by rendering with each light separately, using different settings, and combining the renders afterwards, but obviously this doubles the rendering time. Is there a better way? Or does max 2010 resolve this problem?

See the attached image for an example. The top image was rendered normally; the bottom image was combined from two separate renders, one with GI turned on and just the sun enabled, and the second with GI off and just the skylight enabled (both using FG).

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Have you tried to use “mr Sky portal” to get more light in to the interior of the box?

Thanks Jorge—good idea but the box is only a test to show the problem. Most of my scenes are exterior renderings so I’m actually more interested in areas such as the shadows under the spheres. Also, the sky portal still needs the final gather diffuse bounces to work properly, and enabling GI still disables this.

Typically, for an exterior scene, we don’t use GI.  Just FG with 2 or 3 bounces.  Most of your Photons in an exterior just bounce once and disappear into the sky anyway.

3DS Max Design 2011 64-bit - Advantage Pack
Dell Precision T5500, Dual Six Core Xeon X5650 @ 2.67GHz, nVidia Quadro 5000, 24 GB RAM, Win 7 Enterprise 64-bit
Minneapolis, MN, USA

Chris - don’t you find that you scenes lose some “warmth” without GI? When I’ve tried using FG alone my renders always seem to have a blue cast, and trying to correct for that in Photoshop turns the sky grey. OK so I could render with an artificially blue sky but surely the point of mental ray is that it’s meant to be physically correct so we shouldn’t have to sort out this kind of thing afterwards!

The reason it gets so blue is that the skylight is using the Sky color in it’s light.  When you add bounces to the FG, you’re bouncing more of that blue tinted light around.  You can change what the skylight uses in the Daylight System.  Under the Non-Physical Tuning, there’s a spinner for Red/Blue Tint and Saturation.

3DS Max Design 2011 64-bit - Advantage Pack
Dell Precision T5500, Dual Six Core Xeon X5650 @ 2.67GHz, nVidia Quadro 5000, 24 GB RAM, Win 7 Enterprise 64-bit
Minneapolis, MN, USA

Surely the blue tint of skylight bouncing around ought to be balanced by some yellow-tinted sunlight bouncing around too. Look at the inside of the tube in the images I posted; with GI the sunlight tints it yellow; in the bottom image the skylight and sunlight combine to make white. I think the question I’m trying to ask is, is the software CAPABLE of rendering both reflected skylight and reflected sunlight at the same time? Where do you go to get an answer from Autodesk on things like this… anyone know?