View Full Version : My dielectric_material glows !

06 June 2007, 02:08 PM
Hi there,
I've been reading this forum for a pretty long time now, but this is my first post ever. I'm pretty new to Maya, yet I used to use Blender in the past so I'm not a total newbie.
Anyway, as suggested in various tutorials I read on the net, it sounds like dielectric_material is the way to go when you want to texture transparent materials like water and glass. In the following scene :

I made a "glass" material, which uses :
-> A "dielectric_material" as a Material Shader
-> An "mib_shadow_transparency" shader for the Shadow shader
-> And a "dielectric_material_photon" for the photon shader

The room is light by two kinds of lights : a sun, which is outside, and a area-light photon-emitting light in each window.

In the scene, the vase, the table, and even the windows all use the same "Glass" material. Even the lamp in the corner of the room has small pieces of glass.

And while it's not visible on the windows, the vase seems to be emitting more light than it receives. This is also slightly visible under the table, and near the lamp in the corner. This yelds to a weird "glowing" effect. Do you have any idea as of _why_ it's behaving like this ? And do you know what I could do to try to fix it ?

Thanks a lot in advance !

06 June 2007, 05:06 AM
any photons emitting from the light? you could be getting a occumlation of photons in those hot spots.... im not entirely sure but the dgs material can reflect 100% of photons.

Hope this helps!

06 June 2007, 08:39 AM
You are most likely running out of trace depth, and the dielectric material then "sees" the environment... which is the sky, in your case.

Up your trace depth, or set an explicit environment shader on the dielectric material that is black, or use mia_material instead, which does not refract the environment when running out of trace depth.


06 June 2007, 12:12 PM
MAV4d : Thanks for your help, but those aren't photons. For a very simple reason : the photons I emit are white, while the glow is blue...

MasterZap : You definitely got a point here. I couldn't figure out where this blue color was coming from, and actually, you got it : it's the sky's color !!
However, I tried up'ing the "max trace depth" in the MentalRay render globals (was already at 8, upped to 19), and nothing changed. I also maxed out the "reflection" and "refraction" counts in the render globals, but it didn't change anything...
Whatsoever, I had already tried the mia_materials. They do fix the "glow" problem, yet they take *forever* to render (as compared to dielectric, that is). I also tried using Maya's 8.5 "Glass plane" and "Thick Glass" presets, but it didn't work so well. Here's a sample render using mia_material :

By the way, do you guys have tips about when to use mia_material vs dgs_ or dielectric_ ?

Thank you all very much for your help

06 June 2007, 05:42 PM
mia_material is the more modern shader. Consider it a modern, optimized form of dgs_material (esp. the glossy parts), that also has a lot of flexibiity for glass types of materials in replacement for the dielectric_material.

BTW, I would have tried the black explicit environment shader that Zap mentioned, ie, environment shader for the shading group (mental ray tab) of the vase. In this scene, I don't see much need for the high levels of trace depth which could add to render time. 8 would be more than I would have used.

When the mia_material was behaving slower, were you using these high trace depth levels?

06 June 2007, 01:01 AM
Thanks for the help bgawboy,

I'm gonna try the explicit environment shader. However I quite don't get the point of this shader : when is it really used ?If this shader is used when I'm overflowing the trace depth, wouldn't making it black just end up darkening my scene ? And anyway, I think I'm just not getting past the max trace depth, but I can be wrong - as you said, 8 seems far enough, but Master Zap said it might help to raise it so I did.

As for the mia_material, thanks a lot for the informations. So, actually, it's probably a better idea to always use mia_material. Is it really ? Or is it better only if I need its advanced features ?

And for the (really noticeably) longer render times, no, everything was exactly the same. In both cases I did use 8 max trace depth. And this yelds to significantly longer render times with the mia_material.

Finally, just so you guys know : I did switch to mia_material, and things are getting a lot better : Any suggestion on how to improve the glass shader ? Or any suggestion on the picture in general ?

Thanks you all very very much for your valuable help !

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