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I looked at the doc: https://docs.blender.org/manual/en/latest/render/layers/passes.html

It says that the Vector pass outputs: "Motion vectors for the Vector Blur node. The four components consist of 2D vectors giving the motion towards the next and previous frame position in pixel space."

However, it looks like the XYZ component of the speed vector. This is also what I get when using a file output node with .exr format (I get the XYZ in the RGB and no A channel)

My question - is it possible to get the output as stated in the docs? that is, two 2D vector fields, one from current frame to previous and the second from current to next frame?

This can be good since in general the motion transformation might not be invertible (e.g. several pixels in one frame become one in the other frame).

Many thanks!

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2 Answers 2

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In order to "see" the Alpha channel populated, you need to turn on the Use Alpha check box of the composite or viewer output.

See this for further details:

Incorrect Vector Pass output (No Alpha & Zero Values)

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    $\begingroup$ Thanks! that did the trick in a much simpler fashion! $\endgroup$
    – matanj
    Commented Apr 25, 2020 at 16:04
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If it helps anyone, I ended up using the "Nonlinear animation" panel to duplicate and reverse my animation, and then after rendering I can use the 2nd half of the vector field as the motion field from current-to-next frames of the original animation.

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