Currently I have grain being continually emitted from a piece of animated Alembic geometry (a rectangle moving slowly along Z).

No matter what I do, I keep getting a significant amount of stepping artifacts as the grain falls to the ground - see screenshot for example. I've cranked up the constraint iterations (280) and minimum substeps (90) - but even still, stepping prevails.

Any idea as to what might be causing this?

Thanks!
Luke

Jitter the birth time, and/or interpolate the source, and you should get away from needing extra sub steps.

There are settings for those under the POP Source Birth tab.

Hmm. Doesn't seem to be making much of a change - as it seems to be occurring a few frames after emission rather than before.

And as I type, I realize that this is most likely occurring because my collision geometry is an alembic file, without in-between meshes - and as the mesh moves in it's frame increments, Houdini isn't creating the in-between meshes to collide with - thus creating the steps.

Instead, I should import the collision mesh as a single frame file, then import the transform data separately & apply it to the stationary mesh - which would then allow Houdini to have data for the time between frames.

Convert Alembic primitives to psoup/poly surfaces then append a Time Blend SOP to get linear in-between deformation of your geometry colliders?

After trying out that idea (a very promising one) it seems the problem is more complicated - as the stepping begins after the grain passes through a very large hole in an obstacle.

It appears to be leaning more in the durection of a bug rather than user error (though I would love to be proved wrong!).

I've attached the scene file & the animated geometry meshes for further investigation.

(As a side note - why does the grain get pushed far out to the side instead of falling directly down? There shouldn't be any initial velocity.)

Figured it out! My collision box was being created with a VDB subtract setup (large container box removing smaller container to create a cavity).

It turns out that the edge of my smaller box was touching that of the larger - and while the subtract process looked complete, with a clean hole created - there was just the slightest amount of overlap which was causing collision issues.

After enlarging the smaller box, I got a much cleaner cut, and things looked a lot better.

It seems there was more than one cause - as I'm still experiencing a stepping type issue due to poor interpolation of my collision geometry, as seen in the attached screenshot & scene file.

The sim in the screenshot is low-res, but you can see the distinct stair-stepping at the bottom edge of the particle flow.

Things I've done that should have fixed it:

- Collision geometry is a single piece of geo that is translated inside Houdini via keyframes rather than an Alembic Cache or geometry sequence.

- I added the time-blend node as recommended by jeff.

- I checked the “Re-evaluate SOPs to Interpolate Geometry” box in my static object node in my DOP network.

- Increased simulation substeps to 30 (The grain-stepping effect seems to generate one step per frame, so increasing the simulation substeps doesn't appear to have any effect. )

Despite all of that, stepping still occurs; this one really has me stumped. Any help would be appreciated!

Thanks,
Luke