This video will show you how to use the motion vector ROP. Then I will give a brief overview of the material setup in UE4. And then I'll go into some of the details regarding how we calculate a useful vector in texture space.
You can download the latest motion vector ROP from the development branch of our Game Development Toolset on GitHub.
For more info about motion vectors and creating a UE4 material check out Klemen Lozar's awesome tutorial - Frame Blending with Motion Vectors