Mental Ray Sky Portals

Although the use of the sky portals is a nice and recommended workflow for getting outside skylight into your interiors, it seems the Soft Shadow quality and Shadow Samples can be a killer for render times if you have a bunch of these in your scene. My latest scene required 16 of these, and that was with a lot of overlap on the exterior walls to minimize the number of Portals. Likewise, if I remember correctly, the Sky Portals render black from the backside if you are veiwing an exterior with the sky portals active (think of a transition from exterior to interior where the interior is lit primarily by these sky portals). I could be mistaken about this and need to double check the scenario in which I saw it. However, in scenes with large amounts of windows in a lot of different places, I used a workaround, creating Plane objects that mimic the sky portals but have a self illumiated Arch and Design Material assigned. Place these planes just inside the window and uncheck "Visible to Camera" in their properties. Tweak the Illumination settings to your liking. Although they don't cast shadows, their illumination seems to render much faster and gets that "outside" light look into your interiors.

Mental Ray Sampling

Having spent some more time with the sampling settings (will be posting images soon) I wouldn't attempt a final render with Min/ Max set to anything lower than 4 and 64. Actually 4 and 256 is my recommendation for minimum "safe" quality to avoid flickering pixels in small details. However, using Fast Rasterizer in the same scene with Samples set to 64 (or even as low as 25 or 36) can yield equivilent results at much lower render times. Definitely worth testing as you approach final render time estimates in projects.

Mental Ray and Forest Pack Pro

Noting that it seems "Fast Rasterizer" is generally a speed improvement for a lot of Mental Ray render scenes, it does not work with Forest Pack Pro, even with some adjustments to the settings and Forest Material. Of course you are warned when trying to render with settings other then those recommended by Forest. Still, a little tweaking and testing with different settings or non-conventional workflows can often lead to a better understanding of the conventional process.  So disabling "Scanline" and hence the Fast Rasterizer is required for the Forest Pack trees to render correctly with the Forest Material.
Although replacing the Forest Material with a conventional 'Standard" Material works, the render times do increase. Obviously the Forest Material shader is optimized by itoo for Mental Ray and works very well.
In addition, testing with Mental Ray Proxies vs the Forest Pack's Planes (using the same forest but the proxies as 'Custom Objects") shows that the Planes render faster. Surprise there.
Obviously Forest still suffers from the dreaded Top view scenario, but I have a few ideas for workarounds that I'll be testing.

More to come soon on Mental Ray Sampling settings

Will be posting some images and hopefully short clips to show the results of the different sampling settings I'm obsessed with right now. Support for how to use these settings correctly is woefully lacking, but their impact on your render times and image quality is dramatic.

UVW Unwrap and texture maps

I did not kow that the UVW Unwrap Modifier retains the images it uses in its editor as maps that are used in the scene, even when you've assigned a different material to the object. So again, any map that was used in the UVW Unwrap editor stills shows up in the Materials Editor as being used in the scene, even when its Material has been removed. You have to collapse the object to get rid of the map in the scene. Something new everyday...

Mental Ray Sampling

The fast rasterizer seems to work better and faster than the "min/max" and 'spatial contrast" method for fine details. Not sure how this will work in a large scene with full frames though. More testing needed to determine that.