Motion Vector Pass for Volumes?


(Siddhantrane) #1

Anybody know how to get a motion vector pass for smoke and fire out of Cycles?
I need to get an MV pass for compositing the motion blur in post, but Cycles renders the vector of the entire domain box, not the volumetrics.


(zanzio) #2

I don’t think motion blur for volumes is supported in cycles atm.


(Siddhantrane) #3

Aw man. Thanks for your answer… I guess I’ll just use some other render engine for volumetrics… Any recommendations that are able to render volumetric vectors?


(zanzio) #4

Good question. Most of my experience is with cycles. I’m going to guess that renderman can deliver a motion vector pass because I found this research paper from 2016 that talks about a new data structure they’ve developed for rendering volumetric motion blur.


(Siddhantrane) #5

Thanks! I’ll look into that.


(sozap) #6

indeed blender use vertex position to render the motion pass, so it’s not possible for volume.
You can use RealSmart motion blur to make a vector pass or add motion blur to the smoke in post process.


(Siddhantrane) #7

I’m aware of rsmb but I am trying to keep my pipeline as free/open source as possible so it’s not an option… Even if I’d consider rsmb, I’ll have to get AfterEffects or Nuke… I’m gonna settle with the motion blur effect in Hitfilm as that seems to suffice for one smoke layer.


(sozap) #8

rsmb is working with natron, that is open source , but if you’re already good with Hitfilm …


(Siddhantrane) #9

My primary node based comping software is Natron (lol). That is where I’m doing everything but there isn’t a good (free) vector blur solution available for Natron…
The best one in Natron (as of now) is the Poor Man’s Vector Blur node, but its not very effective as it doesn’t create good results on stuff having an alpha channel and it doesn’t take depth pass into account…
I know one of the PyPlug developer who’s working on a better vector blur node for natron…
I know that RSMB is available for Natron, but I don’t have money to buy it and the project I’m working on is going to go out commercially so I can’t get away with a trial…
So the effects in Hitfilm express are my best bet for now…


(Siddhantrane) #10

So far I’ve got decent results with the vector blur node in the blender compositor, given that I have a vector pass… The big crap i stepped in is that the project involves pyros and fluids… blender can render vector passes for fluids and particles but not for volumes (yet)… Hope they implement this soon…


(lolwel21) #11

I’m not sure if Natron has this, but if you can find a free program that can calculate an Optical Flow image, then you can use that as an approximation for the motion vectors of your volumetric speed pass.

To post-process true motion blur onto volumes you’d need deep compositing, but if you’re okay with an approximation this 2d approach should be fine.
Another note: If you do this, you definitely want to render your volumetrics in a separate pass.


(Siddhantrane) #12

I am rendering the smoke and fire on new layers so I can comp them separately as every other layer has a vector pass so I can comp them with motion blur in Fusion or Natron.
Only the volumetrics were a problem but I guess that the optical flow motion blur effect in Hitfilm will work good enough for me…
Thanks!