ryew

No worries, have fun with vellum! Lots of fun examples to play with out there

Using the VEXpression in advectByVolume to multiply by an existing attribute does work (confirmed by enabling and disabling VEXpression to see difference), however hair is sitll hair - if you push an end of it, the rest will bend and twist accordingly unless you've adjusted the hair attributes otherwise - modifying the hair bend and stiffness attributes for the behavior rather than just the forces will likely help more in getting what you're after. odf_hairadvect.hiplc

The scene is missing the wall geometry and your DOP Network, so you should include them to make it easier for others to help you. If you intend to just use particles, unfortunately copying to particles will not be able to make the balloons collide with each other - check out Vellum in order to do this, there is even a Balloon preset you can try and lots of tutorials out there

Perhaps try using VDB Convert From Polygons on the pighead and set your VDB Combine to "SDF Intersection" instead. Having said that, if you are looking for the effect of small balloons, this is something Vellum should handle easily and convincingly, search for the "pressure" threads on here or check the SideFX Masterclasses on Vellum for examples (and I'm sure google will show plenty more videos)

Unless I'm misunderstanding, this seems to essentially be a shape/object packing problem - there have been threads on that here with a number of example files, so if that is indeed what you're after, I'm sure you'll find a solution previously posted Plus an entagma tutorial you might find useful: https://entagma.com/houdini-17-quicktip-packing-geometry-using-the-uv-layout-sop/

Dropping down a new Heightfield Erode node in H18.5.499 appears to work okay, but the one in your scene fails even if I set all the settings the exact same. Houdini version issue, perhaps?

Try creating a Point group of the outermost edge using a Group Create SOP set to use Include by Edges-> "Unshared Edges", then add noise with a Mountain / VOP / expression etc. to that point group. Before or after subdivision should work

Connect a Null SOP to the Constraints outputs of your nodes and work your way through the network to visualize the constraints you are creating - it appears your center of mass is offset for fractured pieces being fed in, so your RBD Constraints From Rules nodes are connecting to a centroid out in space. You'll want to go back and ensure the centroids of your fractured geometry are being correctly defined, otherwise the motion when the pieces are being simulated are most likely going to have the wrong motion.

You will likely get help faster if you post a simple example scene for others to investigate/tweak for you

I would recommend posting a reduced/simplified example file that demonstrates the problem in order for people to better help you. It is understandable if you might be reluctant to share assets, etc. but try deleting the majority of your geometry down to a level you are comfortable with sharing or create alternate, simplified geometry that still exhibits the same problem for people to investigate.

Unless I'm misunderstanding, but it sounds like you'd like to do a type of layered "texture bombing" - if that's the case, the tutorial by @konstantin magnus may be helpful (and this is a topic that has come up repeatedly so a google search should give you a good amount of other reference if needed).

Ah, yep, Tomas' tip about the Edge Cusp SOP is a better option - you still have to PolyFill the open faces, but you don't have to deal with fusing disconnected points. Facet has been in the toolbox so long I always default to it, but I'll have to start using Edge Cusp more

You can expand your bounds with a VDB Activate SOP in the "Expand" tab, but I believe the actual issue is not clipping but rather that your velocity data stops at the bounds your input frame is frozen on so your loop is multiplying by 0 in those areas - you might need to pipe in velocities from later frames or fake some by creating a custom velocity field

If you have edges you just need to separate, you can select those points and use the Facet SOP with "Unique Points" enabled to split them apart. Then fuse the points along the new separate border edges (to reconnect the adjacent edges to each other) and use polyfill to close the open faces where it's been split.

Can you use the "patchname" attribute to assign each object a unique name you can use to delete that entire object when it is outside the frustrum instead of just a handful of points?