I am relatively new at Houdini. Recently, I Alembic imported an animated rig into my scene, used an object merge sop in a geo node, and then turned the rig into a collision object for FLIP particles. Later after turning on “Use Deforming Geometry”, the sim time became too high. I tried to use a resample node to try to somehow lower it with not that much success.

What is the best thing to use to turn the animated rig geo into a lower res proxy?

I think you're getting the long sim times cause FLIP handles collision through SDF. When you enable deforming geo, you're basically creating a new SDF for each frame which can be quite expensive depending on the size and resolution of your SDF. I mean, it's less about the resolution of your model than it is about the resolution of your collision volume (SDF)
You can try to isolate, if possible, the areas of the rig that you want to collide with by deleting the remaining primitives, or you can decrease the resolution of the SDF by going to the Collisions>Volume tab on your RBD object and toning down the uniform divisions.

Of course, I'm assuming a workflow and the details of your setup might differ a bit…

Cheers

You can cache the sdf in sops, and reference a sop file node in dops so it doesn't have to calculate it every frame (after you've cached it once and loaded it back in via a file node of course). You can do it by setting the proxy volume on a static object in dops. I have an example file in here:
http://www.sidefx.com/index.php?option=com_forum&Itemid=172&page=viewtopic&t=27495&highlight= [sidefx.com]

Thanks for the files, Ian! I spent the time studying and applying your technique to my scene and it definitely has made things faster. Also, the behavior of the FLIP is much more accurate when colliding. Some things I noticed when I am using the technique:

1.) As seen in the attachment, when the particles meet with the collision volume, there is a sudden explosion of smaller particles flying about. Not sure what causes that.

2.) When I turn up the isoOffset max axis, I noticed the particles collide with a box-shaped volume instead of the character geo volume. How can I make the collision volume conform more to the object? Should I use fog volume? Put “density” in the name in the IsoOffset sop?

3.) The proxy definitely made the sim faster on lower setting. However, even when I rop cached the character geo and the isoOffset SDF geo, the sim/cache time gets pretty high when I tried to raise the detail more (soon I will have to). I’m not sure if I set up the SDF volume right, so I included my scene to show how my networks are set up.

Update: Well, I used a fog volume to check out what my collision volume looked like and I found this. I have no idea how to fix that :shock:

In the mode dropbox, try ray intersect, it usually gives good results.
Also, in the construction tab disable laser scan. In objects with lots of holes and concavity, laser scan tends to be problematic.
Lastly, increasing the resolution isn't always a solution, I've had cases where a slightly lower res gave me better results than the higher one. Start at 100 and increase at 25 to 50% increments at a time until you're satisfied with the detail amount.

Cheers

You should have the isooffset to sdf volume, with ray intersect like what rafels said. You don't want volume sample set on the isooffset node, you want it on the static object in dops. You will probably see a white box when you are viewing the sdf because the sdf contains positive values on the outside of the object, and negative values on the inside of the object. You can ‘preview’ it by viewing it first as an isosurface or like you did as a fog volume to make sure it's not giving you whacky lines like you are seeing in your image, just make sure you set it to sdf before you cache it out because that's what dops will expect.


Edit: opened your file – looks right to me, minus the laser scan, which may help with the lines. Also, the resolution in dops (uniform divisions) won't do anything, so having it set to 350 won't help. If you need a higher resolution collision object, do it in sops.

Some other tips: to help with collsiions – if your collision object isn't changing point counts, put a trail sop on it, set to calculate velocities, then on the flip solver set Volume Motion/Collisions/Velocity type to point. You can also try changing the Particle Motion / Collision Detection to Move Outside Collision. Also make the bounding volume limits only as big as you need the sim to be, otherwise it's possible to waste memory, and things can slow down a lot.

I took both of your advice and things make much more sense now, though there are still some problems persisting with the collision interaction involving the FLIP particles.

On first contact with the collision geometry, a number of small particles suddenly explode and fly off in all directions. I was told that it may be the little details on the character geometry like the moustache, those pinches between the arms and shoulder, etc.

I am also wondering why the FLIP particles unnaturally wrap around the character geo as it is moving forward instead of bouncing off. It’s almost as if the FLIP fluids try to meet again after being separated.

I was told that also the FLIP solver may have a role in constantly creating an SDF (more so than the static solver) which makes the sim slower. I'm not sure exactly what they meant though.