Post-Processing: Including SPH soil particles on Blender

[Rebeca Guimarães]

Hello all!

I am currently working on VIPER rover terramechanics SPH simulations using the the ChTireTestRig and have been struggling to generate images and animations that include the wheel, soil and complete rig mechanism.

I was previously using ParaView, but `ChTireTestRig` doesn't have a built-in function to save VTK files of the mechanism itself. Because of this, my animations only showed a floating, single wheel gliding through the soil. Let's just say that for the purpose of presenting results to people who aren't familiar with terramechanics single-wheel tests, it looks a bit mind-boggling! Haha.  

So, I decided to try Blender, since I've seen beautiful images and animations on SBEL's website and knew, from SBEL's NASA Workshop slides, that they were generated there. However, now I have the opposite problem: I can only export the solids, not the soil.  

Here is what I have tried so far to bridge the gap:

• Exporting soil from ParaView to Blender: I tried saving the soil as an animated scene in ParaView, but it only exports in the .vtk.js extension, which Blender doesn't support.

• Using SciBlender: I managed to export the soil in other formats, and while it generates files for all timesteps, Blender only reads them as individual static scenes.

• StopMotionOBJ: I tried using this add-on to handle the time sequence, but I couldn't get it to show in the export/import menu, despite installing it.

• Exporting the mechanism from Blender to ParaView: I attempted to bring the rig into ParaView, but I couldn't export it as an animated scene.

Currently, the only workaround I have is generating static images in ParaView by exporting the mechanism from Blender as a .glb file at the exact timestep I need. Unfortunately, this means I cannot generate animations.

  

I apologize for the long text, but I thought it was important to share what hasn't worked. Could anyone provide some guidance or a recommended workflow on how to render animations that include the soil, the wheel, and the rig mechanism all together?

Thank you very much!

--Rebeca

[Khai S]

Hello Rebeca!

I recently did something similar for a PyChrono mini-example. Here is the way I went about exporting the terrain and rendering the sim in Blender (whole workflow is in Python): 

1. During my sim loop, I call sys_sph.SaveParticleData(sph_dir) where sys_sph is the sim's ChFsiFluidSystemSPH instance. This dumps per-frame fluid*.csv files with columns  x, y, z, v_x, v_y, v_z, |U|, acc, rho, pressure to the location specified by sph_dir. 

2. For rendering this in blender, I have a stdlib-only script that runs headless. It first discovers all of the fluid*.csv files, sorts them by frame number, and registers a frame_change_pre handler so each Blender frame reloads the matching CSV and updates the particle positions in the scene. Geometry Nodes (a built-in Blender feature) then draws a small sphere at each of those points, which is a faster technique than placing individual sphere objects. 

Caveat: You do have to set the particle radius manually. I pick it to roughly match my SPH initial spacing so the soil renders as a continuous material instead of scattered dots. Domain size also plays a role to some extent. 

Hopefully this helps! Please let me know if you have any additional questions :) 

~Khai 

[Huzaifa Mustafa Unjhawala]

Hello Rebeca,

To generate the images you see in this paper for instance - https://arxiv.org/pdf/2507.05643,

1) If I want to render the particles as particles, I export them as CSV, if I want to render as a surface, I use the splashsurf export plugin on Chrono Main
2) Then I had ai generate a blender script for me (see attached). This script relies on using the ChBlender plugin for the Chrono bodies and the above csv/splashsurf surface. 

Feel free to use the script. It will probably be rough on the edges (hard coded paths etc.) but should be modifiable for your purpose.

Best
Huzaifa

[Rebeca Guimarães]

Hello there, Khai and Huzaifa!

Thank you for claryfing, this is amazing since I already save the CSV with the particles data for all my simulations, because of Paraview, so I won’t have to rerun them. 

I must confess I’m new to Blender, so when I got to the post-processing phase, I hit a wall. 

Huzaifa, thank you very much for sharing your scripts, they’ll definitely save me when it comes to post processing my thesis simulations.

I’ll be sure to update this post once with the animation once I’m done :)

Best regards,

Rebeca