As explained here, in the Motion Blur section:
https://docs.chaos.com/display/PHX4MAX/Liquid+Rendering
For each rendered frame, a new Phoenix mesh must be built from the voxel grid, and so it usually has a different number of vertices than the previous and the next frame. Because of this, individual vertices can not be traced back or forward in time between frames. Instead, motion blur of fluid meshes uses the velocity of vertices which is recorded by the simulation, and moves each vertex back and forward in time along the vertex velocity.
1) Generating the velocities in Phoenix
Make sure you have enabled 'Particle Velocity' channel in Output / Output particles / Liquid
Make sure you have enabled 'Grid Velocity' channel in Output / Output Grid Channels
re-Start the simulation if you changed it.
2) Enable the velocity channel for the mesh in PhoenixFD:
The velocity map is available only while the 'Export as' option is set to 'Mesh'
While rendering with Octane, you need to keep this option set to Mesh, even if the dialog mention 'It is highly recommended to Disable this after exporting!"...
To make sure it's enabled, and has valid data, select the PhoenixFDLiquid object, and open the Tools / Channel Info dialog.
Make sure there is a channel with ID '2:map', name 'velocity', and Num Verts non zero and matching the 'mesh' channel count:
Note that the velocity channel will appear the first time you set 'Export as' to Mesh, and have the correct vertex number.
Then, if you set 'Export as' to 'Disabled', the velocity channel will still exist, but will have 0 vertex.
Ref:
https://docs.chaos.com/display/PHX4MAX/ ... etoAlembic
In 3ds Max, while you have "Export as" set to "Mesh", the vertex velocity is in Map Channel 2 of the mesh, named "velocity".
3) In Octane Object Properties dialog:
Set Velocity channel to 2 for the PhoenixFDLiquid object. We will use the 'velocity' channel.
Make sure 'Vertex motion blur' is disabled.
(Reminder: Vertex motion blur is needed when a mesh is deformed, and keep a constant vertex count, like when using an animated Bend modifier)
Make sure 'Object motion blur' is enabled. That's the main control of motion blur per object.
Keep Motion blur steps to 2, as we can't generate useful intermediate positions from the mesh vertex and a velocity.
Note that Movable proxy is not used in the case of a PhoenixFD object. It is always set to movable proxy internally.
4) Adjust the blur effect.
Several settings affect the motion blur 'size':
-PhoenixFD: Rendering rollup: Motion blur Multiplier. Default is 1.0
-Octane: Render settings / Support / Common Scene Collection Settings / Generic Velocity scale. Default is 0.05
-Octane: Camera : Motion Blur: Shutter time, Motion blur duration, and all the motion blur settings.
Note that the Playback speed is also influencing the final velocity (3dsMax Time Configuration, Playback section)
You can temporarily set a very high shutter time to check that the direction of the velocity is as expected:
Particle motion blur:
The particles generated by Phoenix (like Foam, Mist) use a separate velocity and motion blur system.
The final blur effect for particles can be controlled by Render settings / Support / Phoenix FD / General Particle Velocity scale.
Here is a sample with both the liquid and the particles having motion blur:
See this previous post for particle settings:
Phoenix FD 3.0 particle usage with Octane plugin 4.33
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