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.
​
Both Torous and spheres need to have a v (velocity) attribute.
In shop network we create a new “material shader builder” and inside we create our custom motion vector pass:
You can see here the properties panel for each node in our custom motion vector:
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”
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)
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)
You need to create a camera and make sure you have it selected in your mantra render node
In your render view, right click and tick “view bar” so you will see your custom passes selector
Now you can select your custom motion vector pass myv and see it working.
In nuke, the set up is quite straight forward with the “vector blur” node. Here you can see the resut and the parameters:
Please let me know if you find any error or a better way to do it. Take care!