View Full Version : Photon pass without color?

01 January 2006, 01:20 PM
I'm doing writing up a tutorial on render passes, lighting, shading, and compositing them back together. For my photon pass I have photons being emit from a light source. The light source's illumination is turned off and there is no final gather. So basically the only light occurring in the scene is from the photons. I have the color enabled on my shaders so the photons pick up the colors for bleeding, however I'm not completely sure if I actually want the color of the shader to show up in the render.

I'm using a color pass completely seperate without any source of lighting, so if I include the shader's color information in the photon pass, it will add color information on top of color information that I already have. And I don't want that. Is there any way to get just a render of color bleeding information?

Let's say a photon hits a green box and bounces onto a white wall. The photon mark on the wall will inherit the green from the box, but now I also have the box showing up as green because the photon hit it and lit it up. I don't want the box's green color to show up because I already have that information in a seperate pass. I just want to pull the change in hue information in the photon.

I dunno if I'm making any sense or not. Now that I'm reading over what I just wrote, I'm even confusing myself. Thanks for any help or suggestions.

edit: Oh, and I'm using Maya 7.0 and Mental Ray

01 January 2006, 02:34 PM
In the shader, under the Mental Ray tab, there's a section called "Photon Attributes." It sounds as if you have the "Derive From Maya" box checked there, making a surface's influence on photons match its visible appearance. If you want them to be different, you can un-check that box, assign colors or maps for photon color bleeding, and leave the shader white.

That'll give you what you're asking for. But on the issue of whether you want the color of the shader to show up in the GI pass, most of the time the answer is yes. You generally do want surface colors and textures to appear in your GI pass and each of your lighting passes.


01 January 2006, 03:55 PM
Thanks for the quick reply Jeremy. I always wondered what those Mental Ray attributes did in the shader. I was just curious about this because I wanted to do some tests for a few different ways for dividing up the pass information rather than just the traditional diffuse, spec, and shadow. It'll probably turn out that I will need a lot of the texture information in the lighting passes like you said, but it can't hurt to check out how the results will differ. Thanks a lot for the help.

03 March 2006, 08:00 AM
Hi All........Was just curious if anyone has rendered out a final gather pass for use in compositing? I'm just starting out trying to composite and would like soft final gather type shadows against my stand in geometry. I'm using maya by the way and have tried different shaders like use background ( which does'nt seem to like final gather or ray tracing ) with no luck. Any help at all would be greatly appreciated.



03 March 2006, 11:20 AM
You'll want to generate an occlusion pass. There are a few ways to do it. If you have Maya 7.0 you can use the occlusion render layer preset. Basically it applies a surface shader to all your objects. It has the MR occlusion shader attached to it's color attribute, so when it renders you get grayscale image of shading. Using this way allows you to tweak the softness and value ranges of the pass inside the occlusion shader, which is pretty nice. However, it can tend to take awhile to render this way.

The other way I usually do it is by creating my own "make shift" occlusion pass. I'll give all the objects in my scene a white lambert. If I have any custom diffuse or bump maps I then apply them to the white lambert. Create an IBL node and change the type from "image file" to "texture." Put the texture color to white. Turn on final gather and hit render. Using this way allows you to include the bump and diffuse information in the pass for a bit more accuracy. Also it renders quicker than the other way, but takes a bit of time to minimize final gather flickering.

03 March 2006, 02:44 PM
Hi Jared, thanks for the quick reply - most appreciated and very useful.

I was also wondering if there is a way to be a bit more selective with how the shadows are cast? What I mean is....say you've got an image of a building from which you've created stand-in geometry with all the nooks and crannies and windows etc. Say you also want a cgi object to cast shadows on the building. When you do a final gather you get light occlusion in all those areas in which the shadows already are in the original photograph thereby sort of doubling the intensity of the shadows. What you really want is the shadow from the cgi object to affect the building without getting any self-shadowing by the building if that makes sense? Is there any way of doing that?

Regards and again thanks


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