How to render/store velocity vectors in EXR?

[catchyid]

Hi,

I have a moving geometry that stores velocity vector on each point (keyframed animation + trail node). I want to do motion blur in post, so I need to store velocity vector as EXR layer, however I don't see any VEX variable for that (i.e. mantra->images->extra image planes have no parameter for velocity, I just find point, depth, color but no reference to velocity)???  

Also, if I want to Houdini to store this velocity vector for all objects in the scene, then what's the right way to do that? Should I go in each object and append trail node? What if some objects are not moving...

Thanks,

Edited January 4, 2017 by catchyid

[catchyid]

Hi,

https://www.sidefx.com/forum/topic/34339/?page=1#post-158971

In addition to my original question, an user on the above post says it's better to use Mantra built-in motion blur than using velocity vectors + post? Is this correct? I am not expert but I "think" that exporting velocity vectors and using them in post for motion blur will be way faster than computing extra samples in render time to create motion blur? Anyways, I still cannot export velocity vectors

 

[dimovfx]

modify the shader to take v@v and export it to v@vel, then in mantra add extra image reading v@vel and you'd have 3D vectors.

if you want 2D vectors the link you posted should work, but I think  you have to enable mblur in mantra and disable image mblur.

regarding the speed... you can try it yourself. mantra's mblur is quite fast.

 

 

edit: here is an example scene, hope it helps

vectorsForPostMBlur.hip

Edited January 4, 2017 by sasho78

[catchyid]

Thanks Alexander...I am looking at your file and I am not sure exactly how the vel2d and vel3d get calculated! There is no shader assigned to your mesh yet in the rendering node you export vel2d and vel3d and it does work!!! The only thing I see is trail node which computes velocity. Could you please elaborate more on how this "magic" is done  

[dimovfx]

sure, you just have to dive into the squab geometry, there is a shop network and finally there is the shader in question SquabSkin. Inside you can see the nodes I added in red.

little inception game there is

 

edit: oh, and you can assign shaders with material SOP, as you can see in this case.

Edited January 4, 2017 by sasho78

[catchyid]

I see Alexander Thank you, really really appreciate it   

[Atom]

Thanks for explaining the workflow Alexander. I had to try it out for myself. Other packages just offer velocity as preset pass but in Houdini you have to use a shader. I did not expect that.