How to get motion blur (motion vector) data from editor?

Hi all,
I am trying to get the motion vector from the engine. from the editor, I can see the data with arrows is the motion vector which I need(by setting the mode of view as in figure), but How can I export and save them?

I did follow the operation as this question, but it seems GBuffer returns some weird value.
https://forums.unrealengine.com/development-discussion/rendering/64183-pixel-tracking-methods-optical-flow-method

I also tried the new feature “Previous Frame Switch” in UE4.15, It looks like what I need, but I have no idea how to use it correctly, no documentation at all.

Any ideas for that? so appreciate!
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