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When using render passes, you're going to find that some materials work better than others, and some materials don't work at all. This is another thing that can drive you crazy when you're just first starting to use this feature. So, I'd like to demonstrate a few things. I'm going to switch to my renderCam camera here, and this is my scene, and I have my monster toy and simple lighting, and I just have master layer. I'm going to open up the Hypershade here, and then switch to the Work Area, and with the object selected, I'll select Graph Materials on Selected Objects, so we can see what's going on with this particular material.
It's a basic, standard Maya blinn, the same kind of blinn that you create when you click on this Blinn icon here in the Rendering Shelf, and I've added a texture map for the color to create the color of the monster's skin, and I've created a normal map to add bumping quality to the surface of the model to add some detail. And so this is just a standard blinn shader. So, what I like to do is create an ambient occlusion pass for the monster, and I'll just use the master layer, because that will work just fine to demonstrate this.
I'm going to right-click over masterlayer, and I'm going to Add New Render Pass > Ambient Occlusion. So now, I'm going to create a render from the renderCam camera. So here's our finished render, and it looks just fine. I'm going to go to the File menu and Load the AO Pass and here's the AO Pass. We see that ambient occlusion is working, it's on, but there's no detail in the ambient occlusion pass, none of the bump detail that you see like the stitching right here, the wrinkles on his forehead or the crow's feet, none of that is coming through in the model.
It's just ambient occlusion applied to the base geometry. So the problem is not the ambient occlusion. It's no setting in the ambient occlusion. The problem is the material I have chosen for my monster toy doesn't really work with render passes very well. So the solution to this is to use a different material, and I'll open up the Hypershade, and let's take a look at some of your material choices and then create tab of the Hypershade.
Under mental ray, if I select Materials, there are a few materials here that end with the word "passes." So these are car_paint_phenomena _passes, metallic_paint_passes, mia_material_passes, and there's also the fast_shader has a passes version. What this means is that these materials are specifically designed to work with mental ray render passes. As much as you can, you'll see that some of these materials work with render passes, some of them won't work and some of them will kind of work, like I demonstrated with the blinn shader.
If you know that you're going to use render passes in your scene, try and restrict yourself, as much as you can, to the materials that end in the word passes. If you can get away with it, use that. You'll find that in particular the mia_maternal_x_passes, you can simulate almost any material you can think of using this particular shader. It's an extremely versatile shader. So, everything from brick to metal to chrome to opalescent qualities - they can all be created with this material.
The only thing that's really difficult to create with this is car paint or subsurface scattering shaders, like skin, but you'll notice there's also passes versions for these shaders, as well. And in some cases, the materials that end in X, for instance, you can actually upgrade. So, if for instance, if I create a mia_material_x, select this, and go to the Attribute editor, you'll find that under Upgrade Shader, I can actually upgrade this to material_x_passes.
So try and stick to those materials as much as possible. The Maya documentation will actually list exactly which materials work with render passes and which don't, but for the most part, if we stick to those materials you are in good shape. So, I'll demonstrate. What I've done is I've created a mia_material_x_passes shader with the same color texture map and the same normal map, and I'm going to apply this to my monster toy. So I'll assign material to selection. I'm just right-clicking over the material and saying, Assign Material to Selection, and it's going to override the material and assign that, and now, I'll open up my Render view and do another render.
So now that's complete. I'm going to load the render pass and this time you can see my bump detail from the normal map is all there. It's a much better-looking pass, and the only thing I did was I used the mia_material_x_passes shader, as opposed to the standard Maya Blinn, and now I have the detail that I really need in my ambient occlusion pass. If you're in a situation where you need to use a shader that does not work with render passes, then you might have to develop a workflow that takes advantage of render layers and the render layer presets in order to get the most out of those shaders. But I think, by and large, as you gain experience using things like the mia_material_x_passes shader and the other mental ray shaders, you'll find that you can get pretty much almost anything you need out of these shaders, and they'll work much better with render passes.
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