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I want to use the Speed aka Vector pass in the compositor for some motion blur. Currently, it works based on rendering multiple frames. This is time consuming in Cycles. So I am wondering if there are any ways to fake it.

Since the output is a vector, it should just be an RGB or XYZ. Can this be faked with a material? Could you do something like make separate mesh and render it on a different layer with a shadeless material? Or rig up a widget that uses drivers and the position of an object to control a material? Has anyone tried this sort of thing before?

I don't really have enough info on how the speed pass works in order to know what the best method is. Can anyone shed some light on how it works and what would be necessary?

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  • $\begingroup$ Thats a cool idea but you get a vector pass from Cycles already? $\endgroup$
    – 3pointedit
    Jan 17, 2018 at 10:25
  • $\begingroup$ Only if you render multiple frames and have animation. $\endgroup$
    – Ascalon
    Jan 17, 2018 at 10:42

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Vector Pass: 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. [ref: https://docs.blender.org/manual/en/dev/render/cycles/settings/scene/render_layers/passes.html ]

The VectorPass stores these 2 vectors using an RGBA only the node editor renders the socket as being a vector. The R+G channels are used to store the motion towards the previous frame. and the B+A channels are used to store the motion towards the next frame.

Your goal of influencing this in a material is possible by using a Transparency Shader and Emision Shader and mix those together.

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