View Full Version : Black spots in rendering?

12 December 2005, 06:39 PM
I've been using Maya mental ray to render a building design which i'm doing for my architecture classes; gi & fg settings finally work, but there's still a big problem left. On the glass surfaces big black spots appear, which sometimes even extend onto the concrete. It doesn't seem to be a refraction or reflection problem since i've tried to alter these settings in almost every possible way. The glass is made of a simple blinn material with the mib_shadow_transparency shader connected to it, to make sunlight fall through. The surface the material is connected to is a polygonal plane, so refractions inside the glass are impossible(or that's what i thought). In other words: increasing refraction-, reflection rays and trace depth in global render settings doesn't seem to work

I really don't know what to do anymore; hopefully any of you guys can help me out.


12 December 2005, 11:55 PM
i had this same problem yesterday. somebody told me that mental ray dosn't like area lights. are you using some? i havn't had a chance to switch them out, but i think the person was right. i'll get the link to the suggested method of fixing the prob...just a sec.

12 December 2005, 11:58 PM
.....its usualy caused by using a maya area light and rendering with mental ray. If this is what you are doing you should convert a pointlight to a mr area light(under the mr section of the lights att ed.).

12 December 2005, 09:51 AM
I'm actually not using an area light but Image Based Lighting and a directional light. In the beginning it seems to have worked fine, but now for some reason, or for some changed setting it has gone corrupted.

I'll try to delete the lights and create new nodes. Hiding them doesn't seem to help.

12 December 2005, 10:44 PM
can u share the file? It easier to check it, otherwise it remains guessing.

12 December 2005, 03:49 PM
It looks as if you have to crank up the Final gather resolution.

12 December 2005, 03:56 PM
try to set filter under the mental ray/final gather tab at render globals to 1 or 2 instead of zero (default)

12 December 2005, 01:47 AM
You might try a different HDR, as I experienced the same problem, but found it was linked to a corrupt HDR..

12 December 2005, 08:34 AM
Hey Guys,

I experienced the similar problem when I was working on the architectural visualization project last year. The scene was quite heavy and I used FG and GI along with some spot and point lights. No area lights. No IBL light. About 50% of objects in that scene contained reflections, but no refractions. I didn't have problem in the beginning, but I think it was because of rendering a single frame for shading purposes. After rendering the sequence using either batch or commandline, the small black spots - mostly square-shaped, not as big or bad as yours - were visible in some frames. I eventually have to fix it frame-by-frame at the end. I was using Maya 6.01.

The current project that I'm working on, is also heavy. I used IBL with .hdr image along with some coloured point lights and a directional light with `emit diffuse` attributes disabled for specular highlights. No shadows are casted from those lights, since IBL is enough in this case. No reflections and refractions. The small black spots are visible - around a object that is connected to ramp shader - in some frames. Sometimes the black spots are extend onto neighbouring simple lambert shaded objects. I'm using Maya 7.0.

The render settings for current project is:

Number of Samples
Min and Max samples: 1 and 2

Reflections: 0
Refractions: 0
Max Trace Depth: 4
Shadow Trace Depth: 0
Scanline: ON
Faces: BOTH

Final Gather
Final Gather Rays: 1000
Min and Max Radius: 0.5 and 5

Trace Depth: 4

It is obvious that reflective areas tend to have black spots and extend onto the neigbouring surfaces. I've got a feeling that they could be caused by ramp shaders?

I've attached a cropped image from my current project. Please check the red circle.


12 December 2005, 08:34 AM
holy arse, that's the worst artifacting i've seen lol.

i would just be grasping at straws but the best problem solving technique is a process of elimination; remove scene elements until it works - like what happens without HDR, just FG and then no FG? if it still happens its probably a shader issue, take out the shader nodes until it works etc.

good luck with it ;)

12 December 2005, 08:36 PM
Problem: Black spots appear in Mental Ray renderings.

Reason: Final Gather does not work with Multi/sub materials with Raytrace reflections that are "FACETED".

Solution: Turn "FACETED" OFF.

See 3DS MAX files attached. This apprears to be a similar problem in Maya.

Kenneth Loyd

01 January 2006, 10:11 AM
Thanks to everybody for your tips, still i haven't found the solution. Can anybody tell me if maya (7.0) also has a "faceted" function for i haven't been able to find it yet.I tried to reapply the material to the facade which gave an interesting result: black spots on the facade itself disappeared, but now the black spot appear behind the facade. When i render the picture without the facade (just floors and columns) no black spots appear at all!

I'm starting to get really annoyed by this problem! Does anybody have any idea how to solve it? Anything which might help me would be great!

Solved it! The problem apparently was the facades, after rebuilding them the black spots finally disappeared. I always thought they must have had something to do with the reflections or refractions on the glass surfaces but that just wasn't true. I don't know how the spots developed and i definitely don't know why they were gone after rebuilding the facade, but i'm glad with the results.

Thanks to everybody for trying to help, I guess there's just not a simple answer to why these spots appear and how to get rid of them!

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