I've been looking for a solution for like three days straight. I haven't been a part of a CG community before, so please ask if I omit a setting or spec while I get the hang of things. I've found numerous threads in several forums about dielectric rendering problems, and I've tried everything I could in most of them, but nothing has worked. I have a few ideas of why my scene is different. Let me lay it out for you all.
I'm revamping an old scene that I had done with all Maya materials and rendering, now in mental ray with all mental ray shaders. I left the geometry and stripped all the materials away and started over. I also redid the lighting.
I'm rendering a snow globe. I have a little scene inside of a clear glass shell with a wooden base. The base is set up with a DGS with a texture lookup on the Diffuse, and just a touch of glossy (0.05), no specular. It is connected to both lights (below).
I spent a while tweaking the lighting on the wooden base, and it's the only thing outside of the dielectric in the scene. There are two lights in the scene, both Maya point lights set to be mental ray area lights. I came up with the follow mental ray settings for the key:
X Emit Photons
Photon Intensity 200
Cosine Exponent 1
and the fill:
X Emit Photons
Photon Intensity 50
Cosine Exponent 1
The photon intensity is significantly (like, order of magnitude) lower than I've seen in tutorials and stuff. However, if I have it at say, 2000 instead, the scene is totally bleached out by the light. The lights are situated roughly 4 units away from the scene, the globe is exactly two wide. The physical light value and the photon intensity values are the same as suggested.
I will be introducing caustics into the scene later, but right now I need to get the shaders inside the dielectric to respond to the light. This is how the scene should look, the glass shell has been removed for this render. All the shaders "inside" have only diffuse, again mapped to a mental ray texture lookup.
My mental ray render stats for that are
Min/Max Sample Level 0/2
X Ray Tracing
Max Trace Depth 200
Shadow Trace Depth 2
No Global Illumination
No Final Gather
No Auto Volume
I think everything else is at its default setting.
I've tried a number of things for the glass shell, all of which have resulted in the same basic idea
As you can see, none of the shaders below the dielectric are functioning correctly. Let me detail what I've tried so far.
I've done everything with NURBS. The glass started as a single sphere, normals facing out. I assigned it a Maya lambert, set the transparency to 0.98 so I can see what's going on, moved to the shading group, and created a Material Shader that was a mental ray dielectric_material. Here are the settings
Color (HSV) 230 0.07 0.97
_ Ignore normals
and connected to both lights. I later checked Ignore_normals and it didn't change anything.
I hypothesized that everything was literally "inside" the glass, that it was acting as a solid ball of glass. I started trying various modeling tricks to get a single-mesh NURBS shell. What I have now has a hole at the bottom so it's not two meshes (like if I used a boolean). It still came out black. The normals on the outside face out, the normals on the inside face in. That's what is actually in the screenshot above.
I added the dielectric to the Photon Shader in the glass Shading Group.
I tried setting Col_out.Value to 0 and Ior_out to 0, this made some crazy stuff happen, it was very distorted, still black, so I put it back.
I then tried the single sphere again, same settings as before. Then I put a duplicate sphere just inside the outer surface close, but hopefully not intersecting the snow mesh. I assigned this a second dielectric_material with an Ior of 1.0 since the inside is just air. I got this idea from the fruit thread
which gave me a lot of insight into the "interface" idea of the dielectric physics. The difference in the final render for this one was the black "edge" of the glass that is visible above is not when I have the dielectric interface.
I haven't clocked them, but the render times are about 5-15 minutes, I'm on a P3 2.4GHz with 512MB RAM, and a NVidia Ti4600. Not bad, huh?
Okay! That's all I can think of that would be helpful for my diagnosis, if I left anything out, please ask, I'll be checking frequently. I know it was a long post. Thanks for reading!