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The Ambient Occlusion texture in mental ray has an option for creating Reflection Occlusion. Reflection Occlusion is very similar to Ambient Occlusion in that the parts of the surface that are in the cracks and the crevices where fewer photons of lights can reach tend to be less reflective. They're not reflecting as much of the environment. So in this rendering I have two teapots. Both of them have just a standard Maya Blinn applied with a very high reflective value of 1. So they're almost like mirrors or chrome and the only difference is that this Blinn texture has an Ambient Occlusion node connected to its Reflectivity channel and in the settings for the Ambient Occlusion node, I have Reflection Occlusion activated.
So I am going to show you how to set this up really quickly. So I'm going to select the teapot on the left and open up its blinn texture. This is a blinn1 texture, and I'm going to go down to the Reflectivity option here. I'm going to go down to the Specular Shading section and for Reflectivity, I'm going to click on the texture box right next to Reflectivity. That opens up the Create Render Node box, and under mental ray I'm going to click on Textures and choose Ambient Occlusion.
This pulls up the Connection Editor because Maya at this point is not exactly sure how you want to connect the Ambient Occlusion node to the Reflectivity channel. I'm going to point out a couple of things. We want to connect the outValue of the Ambient Occlusion node to the Reflectivity channel here in the Specular Shading. So if I look in the Connection Editor under mib_amb_occlusion, if I click on outValue notice on the right side it highlights certain options and grays out others. Meaning that the grayed out versions can't be connected to outValue.
So if I look down here, I'll see Reflectivity is grayed out. So what does this mean? Well the problem is that outValue is a vector. It has three values. Red, green, and blue. So that's three options, but Reflectivity is a single channel. So you can't connect a vector to a single channel easily. Maya just doesn't understand how you want to make the connection. So what you can do is you can use a few utility nodes within the Hypershade to make this connection to convert the vector into a single channel.
But if you're a little bit lazy like myself you'll take an easier way out, which is just to choose any one of these values. So for instance outValueR for the red channel, and then just hook up that to the reflectivity, and that will work just fine because the outValue, the light color of the Ambient Occlusion node, is going to be White. All right! So if I click on the Arrow button to the right of Reflectivity and you can see here are the options for Ambient Occlusion. The Bright value is going to be white; the Dark value is going to be black.
And with computer graphics, a white color is the combination of red, green, and blue. So it's a red value of 1 plus a green value of 1 and a blue value of 1. Same with black. black as outValue of red is 0, outValue of green is 0 and out value of blue is 0. So this should work just fine because really we just need one of these channels. This is just going to be a range of 1 or 0 for the Reflectivity, and that'll hook up just fine. So that's generally the way I do it. Once I have that set, I'm going to increase the Samples just to take out some of the graininess, and I'm going to increase the Max Distance, because remember the Max Distance of 0 means that mental ray is going to search for an infinite amount of space until it encounters any other objects in the scene and those objects are going to cause the shadowing or cause the reflection occlusion on the surface.
But I don't want this teapot to necessarily determine its occlusion based on this teapot; I just want the areas and the cracks and the crevices. So I want to increase this above 0, so it's no longer infinite, but a finite value. Something like 4 works pretty well for this scene. The last thing I need to do is turn on Reflective. That changes the type of occlusion from Ambient Occlusion shadowing to Reflective Occlusion. If I turn that on and create a render, the two teapots should now look the same. There you go, now they look fairly similar, and Reflection Occlusion is useful because it can make your objects look a little bit more realistic.
If you have an overly reflective object, a lot of the details are going to be lost in the reflection. So this is a way to bring back the detail in the surface.
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