Hello guys,
I am having a liquid setup as follows
- Liquid only (no splashes etc), outputting velocity and ID channels
- rendering liquid in mesh mode
- Using Max Physical Camera Setup (with camera motion blur enabled)
- rendering using Vray 5 (tried GPU and CPU)
The issue I get is that “motion blur” on the liquid is only rendered on every 4th frame.
Changing the “motion blur intensity” in the rendering tab does not change the issue.
Any ideas?
So as a workaround I thougth to turn off Phoenix Motion Blur and use Vray Velcoity pass and ad all motion blur in post:
But also Vray velocity pass is only rendering a “gray” image (no motion data).
Thanks,
Martin 
Hey,
Make sure that you have enabled both the Grid and the Particle velocity channels in the Output rollout.
Yes, the liquid Mesh is constructed from the Grid voxel data, so particle velocity is not enough for it - you need the Grid Velocity exported. Particle velocity is needed when you render a Particle Shader with motion blur.
Ah - bummer - I Have not had the grid velocity enabled. Is this a channel that I can “resimulate” or do I have to run the full simulation from scratch?
Thanks for the superquick answers, guys,
Martin
You will have to run the simulation again I’m afraid, you can’t add the velocity channel through resimulation 
Ah, I’m afraid you can’t resimulate it 
Instead, resimulation is often driven by the grid velocity channel when you need to make changes to the other channels, so it’s a bit more special than the rest of the channels..
I did resimulate and here´s what I get:
Motion blur renders fine, great!
Since I wanted the option to increase Motionblur in post I also did ad the Vray Velocity render pass, and here some other problem popped up:
Vray Velocity Render Pass does only render (only a grey channel). Is Vray Velocity pass not compatible with Phoenix liquid or is there someting I am missing out on?
Thanks for advice,
Martin
The Velocity render element should be working fine as long as you have the Velocity channels exported.
Note that in order to get correct results out ouf the Velocity Render Element, the motion blur in the V-Ray settings should be disabled.
Could it be that you have rendered using V-Ray GPU and with motion blur enabled? In this case he Velocity element will not be rendered as it will produce incorrect results.
Hello Georgi,
thanks for clarification. I do use Vray-GPU and in the render settings I do only have “Camera motion blur” enabled (regular “motion blur” on the same tab to the left is disabled).
So it should be working, right? But it does not. If I e.g. animate the camera I do get teh Veocity Render Element straight away though…
Martin
Is there something in the V-Ray log? The V-Ray log files can be usually found in the Temp folder on your PC.
C:\Users\<username>\AppData\Local\Temp
Could it be that you have motion blur turned on from the Camera settings? If the answer is no - Can you send over the scene with a single cache file so that we can check what is going on?
Ah I see. This is it then. The motion blur from the Physical camera is what makes the Velocity Render Element not renderable.
When using Physical camera its motion blur settings override the ones in the V-Ray Menu.
In order to get correct Velocity Render Element make sure there is no Motion Blur enabled neither in the V-Ray Menu, nor in the Physical camera.
In the latest V-Ray version there should be a similar warning in the log.
Hello Georgi,
thanks for final clarification - got it and it all works now!
Best,
Martin
