KarlRichter

You also may want to check out this little video by the very talented Dave Stewart:

Also, make sure the "cache simulation" box is unchecked on your dop node when your running the file cache to save ram!

Yeah, I noticed that as well. I think (guess?) it is an effect of how the DOPS black box processes sims. I messed around with the order of inputs, but either the cloth or the RBD would be a frame out of sync. It seemed to work better solving the RBD stuff after the cloth, so I left it at that. But in the end, if you grab the output of both objects at once from the DOP node, everything seems to line up with no interpenetrations. (if you check the object merge tab of the DOP node, the vellum object and bullet object are what's being exported).

What version of Houdini are you using? It might be related to this issue in the change log: Houdini 18.5.460 OpenVDB support has been reduced to 7.1.0 pending investigation of a CSG Interesction failure detected in 7.2.1 version. 7.2.1 was introduced in Houdini 18.5.452

you can use the "attribute interpolate" SOP, or the "primitive attribute" vop to do what you want.

Nice, Glad it worked out for you! Slow and working is better then not working at all!

Yeah, with the time warp node set it to interpolate between frames, and in the volume tab, set the blend mode to "by voxel position". then turn off "integer frame values" and scrub the time line to make sure the VDB is interpolating ok. Use that as the input so the "re-evaluate" thing in the static solver works properly. If the VDB changes too much between frames, this might not work, but you can give it a try! Also, really make sure the vdb is activated on the inside, that might help with the arms. Use the "vdb activate sdf" and set the halfwidth to 10 or whatever fills inside. Use the "vdbvisualizetree" to make sure.

Yeah, that's tough. Other things you might try are to increase the sub steps in the vellum solver and and then turn on "re-evaluate sops to interpolate geometry" in the static solver. If this is a VDB sequence, you might try using the time warp sop to interpolate the VDB on the subframes. Also make sure that the VDB is fully active on the "inside". You also might try making your VDB a little bigger with the VDB reshape. In the end, vellum just doesn't collide with a volume that well! In that past, i have even tried making my own collision "solver" to resolve this issue. I got t to work mostly, but it was super slow.

It is possible to use a VDB as a vellum collision object. You must setup your sim in a dop network, then add a static object collider with the collision type set to volume. Add the SDF VDB to the proxy volume slot, and set the mode to volume sample. While this does work, its not as accurate as using a geo collision object, and will have issues with fast moving colliders as there is no velocity data from the VDB. See attached for a simple example vellum_x_vdb.hip

Not long ago I was tasked with a simulation of bags of hard candy falling and splitting open on the floor, with the candy of various shapes and sizes spilling out. This turned out to be way harder then I thought. There is really no built in way for vellum and bullet dynamics to interact with each other in Houdini. After searching the internet, I did find one tutorial on kind of faking the interaction, but it seemed overly complicated to me. So after a bit of R&D I came up with this setup. Its very simple and fast, and solves both the vellum and bullet sims in one network at the same time: It basically works by importing the RBD geo into the vellum collision solver, and the vellum geo into the bullet solver. The mass of the RBD has to be faked in somewhat, so it interacts with the cloth in a more realistic way, but that's it! See hip for example: vellum_x_bullet.hip

Maybe try to clamp the volume rather than absolute value. I know volumes can render strangely with negative values.

"extract transform" SOP is your friend...

The main advantage to this method is that you can use all the hair tools to setup and animate the wires to drive your deformations. Note that this does not work with instances, as each copy is actually being deformed, not just translated and scaled, by the wire.

See attached for an example .hip of a simple method to preserve UV seams in a flip sim: UV_to_FLIP.hipnc