Houdini Diary #6 - Chocolate River
- Sammy Lyu
- Apr 29
- 2 min read
Updated: 7 days ago
29/04/2025
So far this is my 2nd flip simulation that made its way to render, and I had the crazy idea of incorporating an RBD simulation into it, what was I thinking! The result was worth it though:
PROJECT FILE:
Key points from this week:
When starting work on a scene, BE AWARE OF SCENE SCALE! Is it a small 1 meter scene? Or is it a 100m wide scene? That it is going to affect the value that you need to input for each SOP.
To add to the next point, for myself, when working with RBD MATERIAL FRACTURE. Don't forget to check the "noise element size" to ensure that is not too small or large
UVs AND BOOLEANs
To reduce TILING SIZE of UVS use the UV LAYOUT SOP and go down to "TARGET RECTANGLES" section. You can then adjust the size of the actualy UV rectangles to make it either bigger or smaller
When using BOOLEAN to combine or subtract faces of objects, make sure you add UVs to the cutting surface or the inside of your edges WILL NOT HAVE UVs!
RBD BOUNCE AND ROTATIONAL STIFFNESS
The "rotational stiffness" in the physical section of the RBD bullet solver properties dictates how much an object rotates from the forces, impacts and collisions it receives. Important note, WITHOUT BOUNCE, the rotational stiffness will try to compensate for that by spinning more unnaturally. So it is always a good idea to HAVE A LITTE BOUNCE, not to leave it at zero.
Just to add to the above point, Bounce also helps when pieces are a little too small, because houdini needs to calculate the bigger pieces and smaller pieces in the same manner and without bounce it freaks out. Also side note, without bounce and WITH collision padding enabled, everything will just stick together and possibly clip.
VDB CONVERSIONS AND SOPS
When using VDB from polygons to create a VDB collider, you want to make sure that "FILL INTERIOR" is enabled so that the FLIPs or the PYROs or what you are simulating, does not go through the collider.
When you have a VDB conversion that does not fully pick up the shape of the polygons, you can use the VDB RESHAPE SDF sop to "fluff" up the VDB a little so it becomes a little more chonky and takes the shape of the original polygons better.
FLIP VOLUME SOURCING AND COLLISIONS
Make sure you always SOURCE "VEL" into the Flip solver, because by default the volume source does not bring in the velocity.
If you are using VDBs or VOLUMES as collision or proxy collision. Always make sure you turn on "Use volume veolcity for Collisions" and adjust the "volume offset" accordingly. This will prevent the FLIP particles from ignoring the collisions and possibly passing through it, which we don't want.
ANDD that concludes this week's key points ! Sorry if its quite a long one, its been quite an educational week for me. I leave you now with a little flipbook of my initial simulation:
