There are many ways to add motion vectors to an image, usually, they come from the render you are working with. In this case, I’ll be showing you how to create motion vectors based on any Transform or Tracker.
In this simple setup, I have a BG image and an image of a bird. For examples sake, these will do just fine.
Once I have my bird merged over my BG. I add a transform and animate it over 12 frames from SL to SR.
Now here is where everyone likes to do their own thing. Some people manually animate a DirBlur or a VectorBlur, and I’ve even seen artists animate a normal Blur.
Before we just jump right into it and call it done, we need to think about what a Motion Vector is. Put simply, a motion vector or a motion vector pass, is an image filled with red and green values. Each pixel has a red value and a green value, red being U and green being V. U means “Unilateral”, think of it as X, and V means “Vertical”, think of that as Y.
So, you already have a transform or a tracker animated and driving your bird image. These values are the driving force of that image and its motion. It makes sense that we should use that information to create our motion vectors.
Here comes the MotionBlur2D node. Add it under the transform/tracker and boom.
You’ve just created motion vectors from your animated transform. (Note that if you have an exported camera and a depth pass, you can create motion vectors from that as well using MotionBlur3D)
Now to make use of these newly created motion vectors, we’ll be using a VectorBlur. Add it after the MotionBlur2D node. Set the channels to "RGBA" as we’re only wanting to effect those channels, and set the “uv channels” to “motion”.
This will add the correct amount of motion blur according to your animated transform. You can always use the “multiply” value to tone it down though if it seems too intense. That's all there is to it! I've also made a video on this topic because in my experience people learn very differently so I hope it helps!
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