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One of the more interesting aspects of working with Final Gathering and rendering with mental ray is that light objects, objects with shaders that have an incandescent value, can actually add light to a render without the help of any other direct lighting. So, to demonstrate this, I'm using this simple animation of a lava lamp. I have my lava lamp in the scene, and if I scrub on the timeline, you can see that these little guys are animated, moving around inside the lamp. So, I'm going to go to my Render Settings window. That's Window > Rendering Editors > Render Settings.
The first thing I'm going to do is go to the Common tab and turn off the default light. I'm turning this off, so there are no extra lights affecting the scene. So if I create a render now, I should see basically a dark room. There is nothing in here, because there is no lighting. So now, the next thing is go to the Indirect Lighting tab. I'm going to turn on Final Gathering. These are the default settings right here. I'm going to pump this up a little bit. So, let's set the Accuracy to 200, just to get something that looks all right to start with.
I'm also going to increase the Secondary Diffuse Bounces, so we get a little bit more light moving around in here. I'll set this to 1. Then what I need to do is apply a shader to the lava inside the lamp, to make it actually glow. So, I'm going to expand the lava group, so we can see our little bits of lava geometry in here. I'm going to select the first one and Shift+Select the last one, so that they are all selected. I'll just click on this Lambert Material icon, here in the Rendering shelf.
I'm going to just change a couple of settings here. The first thing I'm going to change is the color. I'm going to change this to a purplish color. Then I'm going to click on this and add a light purple to this value, and make it fairly bright, so it's a bright pink. So, I'm going to render that and see how that looks so far. It'll be probably be still fairly dark. We can't see that the lava is actually glowing. It's got this purple color to it. So we just need to boost a few settings to get some of that light going. So, I'm going to go into the Render settings again, and under Indirect Lighting, I'm going to click on the Primary Diffuse Scale swatch here.
So I'm just going to increase the scale. So, I have this set to HSV, which stands for Hue, Saturation and Value, so I'm just going to click on that. Under the Value slider, I'm just going to type in 4 here, just to pump up the value of the scale. So, let's do a render now and see how that looks. Now, we're starting to actually see objects in the scene. We can see that they're castings some light here on the background plane. The body of the lamp has a reflective shader here, so it's starting to pick up reflections of the lit environment.
I'm going to store that now. The next thing I'm going to do is take a look in here. I have a polygon plane that's hidden in the Outliner. I'm going to select it. This Plane1 is the background, so I'm going to select Plane2 and press Shift+H down to hide it. What we're going to see is that we have this plane here acting as kind of like a bounce card, like something a photographer might use. It's just positioned right here. This plane has a Lambert shader applied to it that is white with just a very dim gray applied to the Incandescence, just like I applied purple shading to the lava here.
So, this has got a little bit of a value in here. The other thing about the actual geometry is that if I select this geometry and go to the pPlaneShape2 tab here in the Attribute Editor, I've gone to the Render Stats, and I've turned off Casts Shadows, Receive Shadows, as well as Primary Visibility. So this plane is not actually going to show up in the render, even though we can see it here in the scene; it's not going to render at all. But the fact that it's casting light because of its incandescent shader is going to be applied in the seen. You'll also be able to see it in the reflection.
So, it's got Visible In Reflections that's left on, so you will see it in the reflection on the glass here, which will add a little bit of extra quality to the scene. So, I'm going to do another render here. You can already see that that bounce card is adding some white lighting here, even though we can't actually see it in the render. It's also being picked up in the reflection on the body of the lava lamp and then the glass. At this point, I can just mess with these settings a little bit more, maybe select one of the pieces of the lava. I'll go into the Hypershade.
I'm just going to graph the material on the lava, and here it is in Hypershade. Maybe I'll boost that Incandescent value a little bit and also add a bit of Saturation. I can actually put this above a value of 1. I can actually plug this 2, just by selecting that value and typing in 2, so we get something that's very bright. Let's see how that looks. Now it's really adding a lot more light to the scene.
So even though we don't have any actual lights in the scene, we are actually seeing the objects. This is because the shaders applied to the lava and the shaders applied to that invisible plane are both casting light into the scene. So, this is a technique that can be used for all kinds of effects. If you have something like a cylinder, a fluorescent light housing in a room, you could apply Incandescent shader and turn on Final Gathering. You'll see that that fluorescent bulb will actually add lights to the scene.
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