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When you use area lights in a scene that you're rendering with mental ray, you should be aware that there are few settings that can help determine how the light cast by the area light appears in the scene, and it can also make a few aspects of area lights easier to work with. So, I'm going to create an area light by clicking on the areaLight icon here, on the Rendering shelf, and I'm going to use the Move tool to pull it up, scale it and rotate it so that it's facing the table.
So, I'm using the default settings here, so the Intensity is at 1. I'm going to create a test render, and if you see, the light is fairly blown out, but I'll save this. And in order to tone that down a bit, I'm going to turn and set the Decay Rate to Linear, and increase the Intensity, and create another test render, and this is what we get. So, it's falling off the further the light gets from the actual area light position.
But you notice it's still fairly blown out. Another aspect of working with area lights is if I scale this down, it will actually affect the intensity of the light. So all I did was scale it down, but you can see the intensity is somewhat diminished. So, let me scale this back up, and I'm going to activate raytrace shadows. You can see the shadow is very, very grainy.
Now, I can continue to tune these shadows using the Shadow Rays setting and the Ray Depth Limit, but what I want to demonstrate is what happens when I go to the mental ray section. So I'm in the Attribute Editor for the Area Light. I'm going to scroll down here and expand mental ray and look at what's under the Area Light settings. So these are mental ray's specific settings for area lights. Now, when I turn on Light Shape, what I'm really doing is I'm turning this into a mental ray area light, where when this is off, this is actually a Maya area light.
So it's a subtle distinction there, but I'm going to turn this on, and I'm not going to change any other settings. I'm just going to render it, with Use Light Shape on. I'm going to see the light is fairly dim, but without messing with any of the Shadow settings, you can see that number one, the appearance of the shadow, the shape of it is very different. Here it's like long and slender and here it's actually spread out. So to make this a little bit more obvious, let's increase the Intensity of the light, as I'll go back up here, and I want to double it, set it to 8, and create another render.
So you can see that there is a shadow there, but it's very soft, and it's very spread out, and you can see the shadows cast by the chair are extremely soft. The other aspect of working with mental ray area lights, in other words when this is on, is that if I scale this down and create a test render, it's not going to affect the intensity of the light at all. It is going to affect how the shadow shape appears though, and I find this a bit less confusing.
So, now I know that scaling the light will change the shape of the shadow, but it won't affect the intensity. I can continue to adjust the intensity using the Intensity slider and the Decay Rate, and those are the only two things I have to worry about, and I find that much less confusing than having to remember that the scale of the light as well as the Intensity and the Decay Rate, all factor into the intensity, when Use Light Shape is off. So, once again, let's just a quick comparison. I'm going to turn this off, render - this is what I get - save this image, turn this on, do another render, and I'm going to store this image and compare.
So that's with Use Light Shape off, and this is with Use Light Shape on, very different quality to the way that the light looks and the way that the shadows are cast. The decision to use one over the other just depends on what you're trying to achieve in the scene. However, when I'm rendering with mental ray, I tend to turn this on because I think it makes it much easier to use. So just as a quick recap, the Use Light Shape option, found in the Area Light section within the mental ray section of the Area Lights Attribute Editor, This Use Light Shape option is actually telling mental ray to use the mental ray options to control the area light.
I think you'll find it's easier to control how the light looks when you use this option. So Use Light Shape, and you can adjust accordingly from there, but that's probably the first thing that you want to turn on when you're working with area lights and rendering with mental ray.
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