Cycles 'vector' pass meaning of W, X, Y, Z I have a scene with moving objects and I want to extract the optical flow (apparent motion as seen by the camera). I am using Cycles. I include the 'Vector' pass and save the result as 'OpenEXR MultiLayer'. When I open the resulting file in Python I get the following channels:

RenderLayer.Vector.W
RenderLayer.Vector.X
RenderLayer.Vector.Y
RenderLayer.Vector.Z

What is the convention here? Could you provide a source for the information?
Guided by @micaelh in the comments, I ended up doing some experiments moving objects in a controlled way, the results are below.

Vector.X - X displacement from previous frame to current frame
Vector.Y - Y displacement from previous frame to current frame
Vector.Z - X displacement from current frame to next frame
Vector.W - Y displacement from current frame to next frame

Values are in pixels. X displacement is positive to the left, Y displacement is positive downwards.