PDA

View Full Version : Reflection Attribute, Mental Ray, General Complaints


dweinin
03-08-2006, 05:43 PM
The questions I have pertain to the reflection attributes of the shaders while rendering in Metal Ray using Maya 6.0 any help or suggestion are appreciated.

I use Image Based Lighting through mental ray. Basically Maya creates a sphere, maps an HDR Image to it and that in turns cast light and will light up my scene... but shouldn't it only cast light if the final gather is turned on to "see" it? I've noticed that if I do not turn final gather on nothing renders... except reflections. Any shader (blinn, phong) that has some reflectivity to it... that part of it will render. But there is no light, direct or indirect and everything else is completely black. So why is it rendering anything?

Second. If I take a phong shader and map a checkered map to the reflectivity attribute the result of that (when mental ray renders) is that where the checkered map was white reflections will occur and where the checkered map is black no reflections will occur. This is what I'm trying to do but instead of using a checkered map I'm using something like cloud or fractal. However the result of this when Mental Ray renders isn't the same. We're there is white, I get reflections and where there is black I still get reflections, however it seems to me that there should be no reflections.

Are these things bugs? Am I doing something wrong?

If anyone has answers I'd appreciate hearing them.

Thanks.

Dweinin-

jeremybirn
03-08-2006, 06:24 PM
I use Image Based Lighting through mental ray. Basically Maya creates a sphere, maps an HDR Image to it and that in turns cast light and will light up my scene... but shouldn't it only cast light if the final gather is turned on to "see" it? I've noticed that if I do not turn final gather on nothing renders... except reflections. Any shader (blinn, phong) that has some reflectivity to it... that part of it will render. But there is no light, direct or indirect and everything else is completely black. So why is it rendering anything?

I don't understand what the problem is; is there something you want to have happen that isn't happening?

Second. If I take a phong shader and map a checkered map to the reflectivity attribute the result of that (when mental ray renders) is that where the checkered map was white reflections will occur and where the checkered map is black no reflections will occur. This is what I'm trying to do but instead of using a checkered map I'm using something like cloud or fractal. However the result of this when Mental Ray renders isn't the same. We're there is white, I get reflections and where there is black I still get reflections, however it seems to me that there should be no reflections.

Try using a lower number, maybe even negative, for color offset on those textures, it sounds like your black just isn't black enough.

-jeremy

dweinin
03-16-2006, 04:33 PM
Hi Jeremy-

Thanks for replying I hope you get this. I created that post the day I had some friends visit for vacation. I'm just getting back into the daily grind. Sorry for the delay.

In trying to completely understand MR, Indirect lighting, Image Based lighting, Reflections and all that jazz I've bumped into a little something that I can't figure out.

Let me respond to your first questions. Yes. There is definitely something happening that I feel should not happen. I have a sphere in my scene and I am using the IBL to get a nice even "over-all" light onto my sphere and good reflections (See ChromeBall image). I'm also using a Spot light casting shadows. When I render with FG things are good. However...when I turn FG off and my spotlight to 0. I still get heavy reflections. Which shouldn't be there because I've taken the FG off and all the light out of the rendering equation. Is mental ray still computing the IBL? The simple solution is to delete my IBL. But I was thinking that since FG is off the IBL would not have an effect especially since it is not casting any light. (Image2)


Problem # 2: I'm trying to have great control over my reflections. I work in an industry where materials I use in real life are important to represent properly before hand in the concept renderings. For example a piece of metal or chrome will reflect it's environment. However there is often sort of a cloudy reflection or smudges, fingerprints all of those kinds of things go into a piece of metal looking realistic.
So I was wanting to control where my reflections are strong and where they will be week.
I used the example of the checkered pattern because it has definite "areas" (Top Of Image3) of reflection but I when use something different then a checkered map I get weird results.

Set Up:
I am back to using IBL and a spotlight to light up my scene. I have a spehere with a crater map assigned to the reflectiviy channel. (actually I set it up in the color channel first do all the adjusting so I can see it easily then break that connection and map it to the reflectiviy) When I render I get strong reflections. So I took all my color channels in the Crater map and turned them black so in the "theory in my head" I should get no reflections. However the sphere still reflects. (see image 3) bottom

dweinin
03-16-2006, 04:41 PM
Also. I was wondering why I get such crappy reflections when I map the same shader to the nieghboring cube? The reflections just look so much different. They are fuzzy and don't seem to reflect the same distances(?) as the sphere. For example, the sphere reflects a large looking environment the cube reflects... well you can't really tell. See example.

Thanks.
Drew.

CGTalk Moderation
03-16-2006, 04:41 PM
This thread has been automatically closed as it remained inactive for 12 months. If you wish to continue the discussion, please create a new thread in the appropriate forum.