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Bifrost Liquid - Difference between a guided simulation and a motion field??

Message 1 of 12
918 Views, 11 Replies

Bifrost Liquid - Difference between a guided simulation and a motion field??

I'm trying to do some RND to make my own beach scene and have seen a few different ways on youtube to do it (there are some similarities though)


Problem is that the videos range from 2016 to 2019 and the 2016 videos cover things that the newer ones dont.

so this has caused some confusion on some parts on my end...

One thing im trying to figure out is what is the difference between using a BOSS spectral wave as a guidded simulation and using it as  a motion field?


What would be the correct way to do this?

Message 2 of 12
in reply to: snake3y3s

For a beach scene, most likely a motion field or a collision geometry acting as a wave maker will give superior results to a guided simulation.


A guided simulation is best suited for deep open water type of scenarios. A guided simulation essentially creates a moving collision geometry underneath the surface and on top of which the liquid will be moving freely while inheriting the motion of the collision geometry underneath.
The benefit of this is that you can get crashing waves and feasible wakes from boats etc while still maintaining a resemblance to the input BOSS surface.

A motion field will act as an acceleration and constrain the velocities such that the liquid is not able to move as freely as with a guided method, but on the other hand it doesn't require a certain depth in order for it to affect the liquid.

Hope this helps.

Michael Nielsen

Principal Engineer
Message 3 of 12
in reply to: michael_nielsen

So if I have a beach set up (contained volume with an inclined plane) could I use a BOSS spectral wave to drive the particles? which would be best suited for this approach? (guided or motion field)

on the other end... should i even be using BOSS for this, or should i just go ahead and use a plane with a texture deformer with a noise or wave texture and have that drive the particles... using this approach, again, what would work best (guided or motion field)


Basically, i have watched too many different videos from different people (official autodesk and other) from different years between 2016 and now that has had improvements made to Bifrost and Arnold added in between that has now confused the crap outa me on which the best approach is to do what. Its not exactly as straight forward as most would make it out to be... there are a few pitfalls here and there.

Message 4 of 12
in reply to: snake3y3s

The best is probably to take outset in the tutorial you've found that most closely resembles the effect you're trying to achieve and then follow up on this thread if you run into a specific issue.


Here's a bit of general advice that may help in settling on an approach to pursue:

You should be able to use any mesh geometry including one driven by BOSS as input (see "follow a mesh" here:
Again, for a beach scene I don't think a guided approach will work so well because as the liquid gets thinner near the shore the guiding method won't be able to affect the liquid there.
A motion field or waves generated by collision geometry acting as a wave maker would most likely perform better.
Let me elaborate a bit on the wave maker collision geometry approach: basically, at the deepest end of your basin, create a box geometry that either pushes the liquid back and forth or rotates around its center axis or a combination thereof.
This can be used to generate waves rolling onto the shore.

Whether you should use BOSS, a texture deformer with noise or just collision geometry acting as a wave maker depends on the traits of your shot.

If your shot does not need any specific art direction or timing, wave maker collision geo is recommended as it will generate waves that move freely and lets the physics generate all the detail for you.
If your shot requires accurate timing of when the waves must hit the shore, their amplitude and spectrum, then BOSS or a texture deformer combined with a motion field is the way to go.

BOSS will give you physically plausible wave motion, so when you use that to control the waves you will get something that looks physical for free.
Using a texture deformer with noise can be used for highly art directed shots and shots where the liquid does not necessarily move in the physically correct way, but requires more skill to look real.

Michael Nielsen

Principal Engineer
Message 5 of 12
in reply to: michael_nielsen

Thank you for that information... very valuable.


One of the earlier videos I watched, they used an Accelerator... i'm assuming that this is a legacy method and has been replaced by the motion field. Is there any difference between them?


Also, when creating waves using BOSS, is there a way (only using BOSS as a motion field) to make the waves roll larger, or would it be advised to make another motion field container or something (with no geometry selected) and animate that going into the more subtle waves that are already made with BOSS.


Thank you for your answers and explanation so far, it is a great help



Message 6 of 12
in reply to: snake3y3s

Hi @snake3y3s 


The motion fields did indeed replace the accelerators and are much more versatile and controllable.  Accelerators are no longer exposed in Bifrost Liquid.


There are a couple ways to approach making larger waves using boss as a motion field, but I think the second option you mention would probably be best.  If you already have one motion field driven by the boss geo, you can then make a second motion field, with boundaries enabled, and move it around to amplify the velocities in certain areas and get bigger displacement.


Hope this helps!

Konstantinos Stamatelos

Bifrost FX Product Owner and Designer

Message 7 of 12
in reply to: stamatk



Message 8 of 12
in reply to: stamatk

Thank you so much for that reply Konstantinos


That is some Valuable information.


I have a different issue now with maya scale and bifrost...

Bifrost assumes 1 unit equals 1m

maya's units are 1 unit equals 1cm


so I have modeled everything to 1cm scale, so how would I do a water sim (beach waves) now without killing the system?


Would I have to scale down all my objects by a factor of 0.01 then do simulations?? but then this would break the real world scale for the lighting calculations.


Or should I scale down all my objects by 0.01, then do the sim, export the mesh and sims and then re import the sim back into the original scale scene? But then how would i do/render the foam if I want to render the foam particles? I dont want to mesh those.

Message 9 of 12
in reply to: snake3y3s



In Maya almost everything expects one default unit to be one meter despite being labeled "cm".

Also the Intensity values for lights when using Arnold can easily become incredible high when using "cm" as cm for larger scenes and not using a radius or area for the light.


Usually scaling a model is far easier than scaling the values for a simulation,

but you can also scale the simulation by adapting the values for some Attributes.

If you already have a complex lighting setup where you don't want to adapt intensity settings then you should do this, however if light intensity is based on "light size", then a simple scale should work.

Message 10 of 12
in reply to: mspeer

in my case its more of a issue that I have a character in the scene aswell, that has SSS on it, and scaling the character down with the "set" would affect the SSS.

should i then just adjust the sss values on the shaders?

Message 11 of 12
in reply to: snake3y3s



Yes, Arnold SSS Scale needs to be adjusted if you change object scale ("0.01"), but there is also a global Scene Scale that you could use to correct this ("100") in Render Settings -> System -> Render Settings.

Message 12 of 12
in reply to: mspeer

Ah HA! now that is putting me on the right path.

So group everything, scale down to .01, do the simulation, change render scale to use custom (set to 100) or set to meter. render


thank you... will check that out

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