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3D motion data can be used to drive the VectorBlur node directly, and there are two ways to do that. Let's take a look over here. What I have here is a checkerboard hooked to a card. If we switch to the 3D viewer and we have our camera--so this little 3D scene has motion on the card, which we're going to use to create motion blur.
Back to the 2D view. We hooked the viewer up to the scan line render node, and then if we check our channels, we now have forward motion channels. And it came from the camera and the card and the ScanlineRender node. If I open up the ScanlineRender node, you can see here on the Shader tab, this is where the ScanlineRender node has created motion vector channels: forward, U, and V. We'll close this and the 3D camera.
So at this point in the data stream we have motion UV data, so if I hook up viewer2 to the VectorBlur node, open up the VectorBlur node, we can set the channels for RGBA, set the UV channels for forward, and there we have it. You now have motion blur applied to the 3D render. Now why do we want to do this? In the ScanlineRender node there's an option for turning on motion blur, and you can do that and you'll get a very nice 3D motion blur. The problem is that it is very, very expensive.
So it's computationally cheaper to export your motion UV channels and then apply the VectorBlur afterwards. Another option, which we'll see in just a minute, is to actually import motion UV data from other applications where you can then use that in Nuke to apply motion blur to your objects. So the VectorBlur node is getting its motion UV data right here from Nuke's 3D render, and of course, we can dial that up or down, adjust it any way we want. So let's take a look at that third case where we've actually imported an EXR file and this EXR file contains the forward channel data right here.
We'll put that back in the RGBA layer and take a look at adjusting the VectorBlur node. I am going to close the old one. So this is our VectorBlur node. We'll set the channels to RGBA, and we'll set the UV channels to forward. Using the imported forward UV data, we now have the exact same motion blur that we had in the original 3D render, and even the alpha channel is motion blurred. I'll put that back to RGB.
Using the MotionBlur2D node to convert transformation data into forward UV data for the VectorBlur node is but one way to get the motion data; it can also be generated by Nuke 3D render or imported as one of the channels of an EXR file.
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