houdini motion vector

How to render a motion vector pass with Houdini / Mantra to generate motion blur after that Nuke

INTRODUCTION

 

First of all I must thank Nacho Thomas, Pablo Hernández and Jose Antonio Navarrete for giving me advice and useful insight to figure this out, and hope this post will help others who (I´m sure) will get caught in the same problem: Getting Houdini / Mantra to render a motion vector pass that can be then used in Nuke properly to generate motion blur with the node “vector blur”.

 

I had to do quite a bit of research to make this thing work, as I couldn´t find much information about this subject, and personally I find this workflow very important and commonly used in VFX.

So going to the point I share with you all the set up that works fine for me to achieve this motion vector pass with Houdini / Mantra:

(you can get the project files HERE)

WORKFLOW

 

We have for example a Torous and spheres moving in our scene. Important to check “geometry velocity blur” in both objects.

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Both Torous and spheres need to have a v (velocity) attribute.

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In shop network we create a new “material shader builder” and inside we create our custom motion vector pass:

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You can see here the properties panel for each node in our custom motion vector:

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the final node “bind2” is storing the result information that will become our motion vector pass. You need to give it a name, in my case it is “myv”

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Going to our Out network, in our mantra node / rendering tab, we need to check “Allow motion blur” and unchek “Allow image motion blur” (as we don´t want to generate the motion blur in our render, just the motion vector pass)

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Next, in our image tab, we create an extra image plane and we type in the name we gave before to our custom motion vector pass (myv)

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You need to create a camera and make sure you have it selected in your mantra render node

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In your render view, right click and tick “view bar” so you will see your custom passes selector

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Now you can select your custom motion vector pass myv and see it working.

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In nuke, the set up is quite straight forward with the “vector blur” node. Here you can see the resut and the parameters:

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Please let me know if you find any error or a better way to do it. Take care!