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When you render animation, a lot of times you'll want to have motion blur applied to your scene. Now this just adds another layer of realism, and it creates the blur effect you have when you take a picture of an object that moves within the length of the exposure. So I have a simple scene here, and it's basically just a pool ball rolling into the scene. Now if I put it around frame 22, so it's somewhere near the center of the screen and I do a quick render, you'll see that it doesn't look like it's moving in this scene.
Now if we were to actually animate this and play it back, you'd see it kind of go from frame to frame, and it wouldn't have a sense of realism because it wouldn't have a blur to it, or a motion blur. We can fix that by adding in motion blur. We do this under the Render Properties panel. There is a rollout here called Sampled Motion Blur. Let's go ahead and open that up and click that on. Now if we just render with the default settings, we get nothing, because we need to have multiple motion samples.
We have two properties here: we have Motion Samples and Shutter Length. So if I turn Motion Samples up to 4, you can see how this effect works. Now what it's going to do is it's actually going to render the scene four times. And if you notice, each time it renders, that ball moves just a little bit. And then at the end, it takes those four renders and then combines them together into a single image and then creates the motion blur.
Now, the amount of blur that we have is dependent upon the Shutter Length, so this is how long the shutter is open. At a default level of 0.5, it's open for half of whatever my frame rate is. So if I'm at 24 frames a second, it's open for a 48th of a second. Now, I can certainly bring this down and this would lessen the effect, or I can bring it up. So let's bring this up to, say, 1, and let's take a look at it again.
Now again, it's going to render four times, and because I've got my shutter open wider, you can see that the ball is moving more in each render and I'm getting more motion blur. Now you can go above 1, and that will give you an enhanced motion blur, but you have to be careful because in that instance, the ball may actually move ahead of where it's supposed to be. But let's go ahead and give a larger value for our shutter speed, just to see how this works.
So I'm going to put in 3, and you'll see that it's actually going to render that ball moving quite a bit in each frame. Now when this happens, you'll see a little bit of a problem here, and the problem is that I don't have enough motion samples. I only have four samples, but the ball is moving quite a bit with each sample. And so what I'm going to have to do is up the number of samples to compensate for the fact that I'm opening the shutter for so long.
Now this will also apply to objects that are moving very quickly. So the faster an object moves through a scene, the more samples you'll need. So in this case, let's go ahead and add in, say, 10 samples, and the render is actually going to take quite a bit longer because we have to render that scene ten times. But you'll notice that in each render the ball itself is moving just a little bit less. That's because I'm dividing that three into ten parts.
So when I actually have more samples, you can see I get a much better and much smoother motion blur. So those are some of the basics of motion blur. And yes, it does add a little bit of render time, but it can a lot more realism.
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