I've just discovered that the Ray Trace setting for shadows in the Renderer will sometimes black out the object texture.
Can you reproduce this reliably? Does it happen consistently in specific circumstances, or randomly? If you can provide a .tgd and object combination that reproduces it I'm sure we'll be able to track down the source of the problem.
Here's the TGD, though it has two textures not included here (of course), as well as the one object texture file since it is 3500x3500. I used a displacement file of 3500X3500 created using a Nvidia plugin for Photoshop. The TER file (not included) for the loaded Heightfield can be found here -
http://www.joshushund.com/frames.html -- Scroll down to get the TER file koyaanisqatsi.
If you have another way I can get the texture file to you, let me know. It's about a $25 object soon to appear on DAZ.
I tested this without GI in various stages from maximum to all the partial combinations in between on the way to almost zero and never going above 2 for the GI. I tried lowering all of the render settings to no avail. The only setting that made a difference was the Ray Traced Shadows.
Here is the object and the result of the TGD with Ray Tracing Shadows turned off.
The issue is probably due to the use of displacement which as you know can cause problems on imported objects. Did you try the render without displacement? If not please test that and let us know if it makes a difference. We can't really test here without the object in question.
I'll try it this weekend.
[edit - I didn't have time. There's a render going on at +70 hours now and it isn't halfway finished. As soon as it's finished, I'll look into this.]
I tested without the displacement maps and when using the Ray Tracing found no problems. I can do a little more testing to make sure. Anything else?
The displacement maps were made with this - http://developer.nvidia.com/object/photoshop_dds_plugins.html (see specifically the Normal Map Filter plugin).
Thanks for the update. This is a known problem then. The solution is to either not use displacement on imported objects, or to use more judicious settings. Generally very subtle settings should work. Using a negative or positive offset is *not* recommended with imported objects at this time as it greatly increases the likelihood of encountering a problem.
I use .05 to .06 on the displacement and .0125 on the displacement offset...unless it just doesn't look right and then I go to lower (not negative, though) settings.
As I said avoid the offset. There should be little or no need for it. All it does is set the displacement positively or negatively away from the object surface origin by the specified amount. It should not particularly help with the appearance of the displacement.