rest positions and other custom properties for MPM and particle simulations

[calibrix]

Hi BiFrost users,

Does anyone know how to create a rest position for particles and MPM simulations? I can add custom properties to the simulation, but can't seem to figure out how to populate them with data. I would like to have a rest position that I can use for UV mapping of a meshed MPM simulation. The aero UVW settings only work with aero simulations and the MPM source nodes don't output rest values. My only thought to get this working is caching a start frame with Id values and mapping the positions back onto the simulated object as a post process, but that seems really hacky.

Thanks,

Chris

[stamatk]

Hi @calibrix 

Currently, what you propose would be the only way to do that. The source has no concept of the solved positions of the particles so this information has to come from the solver.

I agree that it's hacky and we'll be sure to facilitate this workflow in the future. Perhaps a generalized version for what we do in Aero.  

Thanks for bringing it up!

Kosta

[calibrix · Accepted solution]

Hi Kosta,

I'm not sure about the "only" way to do this. I did come up with a pre-simulation solution that sorta works. I can use a set_geo_property to create a point_rest attribute and set those values to point_position. *see attached image* Then I can use a set_property node with the same name and those point_rest values will be set a per-particle values. I say this mostly works, because as you mentioned, the solver does the actual sourcing and the source node only sets up properties that tell the solver how to generate a source. In this case, it will extrapolate the point_rest values into the interior of the MPM particles. Not ideal, but good enough to map surface UVs. Ideally, we'd want full UVW information. I know you guys are working hard to add more features to the particle and MPM solvers. I think the ability to add and source custom data more accurately should be taken into account.

For reference, the way a software program like Houdini tackles this is to create the sourced geo before solve. The user can specifically apply any data set to the source and the solver just moves and updates the point_* data.

I also think it could be a good idea to update the BiFrost documentation on "add custom properties on emission" to further explain how to initialize per point / voxel values. I don't think that was adequately covered in the documentation and took a bit of guesswork to figure out.

Thanks,

Chris

[stamatk]

Hi Chris,

Glad to hear that workaround is adequate for you, but I figured if the input geometry didn't have a lot of vertices, the point_position data might not be effective as UV data on the sim particles. 

In any case, thanks for all the information and invaluable feedback. You've definitely stumbled upon a simulation aspect that is a bit less polished, and we intend to bring improvements and streamline how custom properties are added and populated. Your input here will be used to make sure we get it right 🙂

And of course, clearer documentation for all this will be part of that.

Thank you!

Kosta