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The nice thing about raytrace shadows is they have very few settings that you'll have to work with in order to increase the quality of the shadow. So, let's take a look at some of these settings right now. We have Light Radius, and this creates a softness to the raytrace shadows. So, in my scene here, I have a spotlight shining down on the tree, and here's our little hill geometry. So, I'm going to create a test render with this set to zero. You can see that, by default, the raytrace shadow creates a very crisp, clean shadow.
So, let's store that, and I'm going to minimize this, and I'm going to increase the Light Radius. Let's set it up to 0.05, and I'll create another test render. The shadows starting to get soft; you'll notice that it is kind of grainy though. So, how can we improve this? Well, I'm going to store this image, and I'm going to increase the number of Shadow Rays. I'm going to set this up to 12 and then create another render.
The thing that's interesting to note is that unlike depth map shadows, you get a more realistic shadow behavior out of raytrace shadows. You see how the branches that are closer to the ground cast a crisper shadow than the branches that are further from the ground. So these branches that are further up here and closer to light are getting blurrier than those that are closer to the ground, and that's essentially how shadows work in the real world. So you'll get a more realistic behavior out of raytrace shadows.
So I'm going to store this. The other setting is the Ray Depth Limit, and the way this works is this determines how many times in reflective surfaces you're going to see the shadow. So I have a mirror object here. I'm going to unhide this by pressing Shift+H. So this is just a plane, and it has a Blinn shader attached to it, and the Reflectivity of the shader is set to 1. So, I'm going to create a test render here. So you can see the tree and the ground plane do appear in the reflection; however, you don't see any shadow here.
In the reflection, the tree is just casting no shadow. So, if I increase this setting, if I select the spotLight and in the Raytrace Shadow Attributes, I set this up to 2, so this means that the raytrace shadow is going to be calculated for two surfaces, once for the initial surface here, on the ground plane, and again for this reflective surface. So, now we can see it calculated here in the reflection.
So, if I had another reflective surface that was reflecting what I see in this reflective surface, I would want to start to increase that value. You're only going to need to set Ray Depth Limit to a high value if you're starting to render something like a hall of mirrors or something like that. For the most part, you can keep this fairly low. So just remember, if you want to use raytrace shadows with a particular light, just go to the Raytrace Shadow Attributes in the Attribute Editor, turn on Use Ray Trace Shadows, adjust the Light Radius to add softness to the shadows.
This only needs to go up by small increments. To increase the quality of the blurred shadow, increase Shadow Rays, take out the graininess, and you should only need to increase Ray Depth Limit if the shadow is going to be reflected in other surfaces in the scene; otherwise, you can leave this setting alone.
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