Found this in vfxtalk.com
Use the “multiply” node. It is not in the menu, just go to others and >“update all plugins”, then go to the letter M and select “Multiply”, now select you VRayVelocity pass and deselect x and z and type -1.
Voila now you have applied a -1 to the Y. and Made the vectors go to the correct side.
Hope it helps
Sorry but that is not part of rules of the game.
Do not worry about refraction and try to post an image with motion blur using V-Ray Velocity pass.
> muoto :
Your motion blur is not correct because some objects don’t move and in your attached image all is motion blurred.
post an image of what is moving how then. troubleshooting without a target is not a game at all…rest assured it does work as we are using it daily in many of our productions.
this is why I asked for the Max file so I could re-render the Velocity pass - what you have shown looks strange to me. And I would never split channels out but embed everything into an EXR that I know works perfectly from Vray and in Nuke.
I have been using VectorBlur all morning here with this workflow to add MB to renders…
Looking at the image itself reveals that according to this velo channel everything IS moving. Do you have a moving camera etc etc.
As said troubleshooting doesnt make sence before knowing what should happen.
post a reel motion blur rendering from vray, so we can see what’s happening 
Ok guys, it’s just what I wanted to hear for Vlado :
The V-Ray Velocity Pass in Render Elements is not usable, it doesn’t work for any software.
That’s why I don’t give the max scene, to be sure you are not using another technique.
I have understood the contrivance of sv speaking of refraction.
For the moment, in the best cases you can have this :
with Reelsmart Motion Blur Vectors and V-Ray Render Elements Velocity pass
It may be noted that the motion blur is no more correct when objects are before another like in front of green plane but it’s work if there is nothing in the background, here where it’s gray.
It confirms that this Render Element pass should be amended so that we can add the speed of pixels in the blue channel, for having a valid Render Element Velocity pass like it’s explain at the end of this video :
http://www.cgarena.info/files/pigeon/PigeonImpossiblePodcast-019.flv
It IS usable. It may not work in any software. But it is working fine in many apps.
Using special tricks ? Such as getting the settings right in regards to the App used ? 
Posting the Scene might help to investigate on the issues you are experiencing. And keep in mind since it DOES work for many ppl simply changing it might break it for these ppl.
Regards,
Thorsten
On a quick note i get the impression you offsetted the speeds by tweaking the min/max parameters. Things that are supposed to be still have a value of 0.241
Subtracing that value from the velocity channel gets pretty close to what you posted. So i would guess it is a settings problem eventually.
Regards,
Thorsten
yes, I have to agree with Thorsten here.
The Velocity channel you posted shows values (so movement) on all the objects - load the velocity channel into a post app, read the values over the black clip thing - they show movement…
This is why I asked for the scene, because I wanted to be sure that the Velocity channel was being saved and stored correctly.
MultiChannel EXR through the VFB is a sure fire way to do this. I’m sure this is the way that Thorsten will be typically doing it too…
could you save an EXR with your two layers ?
and make sure that max aint that far out of range and make sure “clamp” is off.
Regards,
Thorsten
I’m going to have to give the multichannel .exr a try, i’ve been getting similar results (in fusion with the ReelSmartMB plugin) to what noel_d3 posted.
I usually add a velocity pass in the render elements, look at the “max velocity in last frame” and set my max velocity a bit higher, and while the result is sometimes passable during playback, if you inspect a single frame things look strange.
Well…if someone finds a way to have the vectorblur in nuke work, let me know…i thought i had a correct workflow, but it seems not…
thanks !
See the first page of the thread. Seems to work perfectly fine.
Regards,
Thorsten
Ok indeed, it worked with using the EXR multilayer from, vray directly.
Thanks!
Okay. Here’s feedback on the problematic files:
They are saved as linear. hence the weird 0.241. When read as linear instead of sRGB they show an offset of 0.5. Offsetting that to -0.5 pretty much does the job.
I assume the offset is caused by turning on clamping. I assume VRay does that automatically as negative values would be clamped otherwise and hence it is remapping to 0-1 i assume.
Regards,
Thorsten
nice one Thorsten. Good info. This is why I would only use EXR for CG graphics…!
Do you know as well how to get the relight of nuke work with the vraynormals?
the foundry web page has some downloadable scripts for the relight which were shown at the masterclass…
