Standard metaballs merge into each other like blobs of slime.

Can I use metaballs that push into each other like balloons?

I can change the weight into the negative but then the balls dissappear.

Not quite sure, but here's my feeble attempt because I thought the problem was interesting.

Oh crickey! That's a lot more involving than I thought! I wanted to add them into Lsystems and thus get nice flowerbuds and cactus geometry but holy moley it's much trickier than I expected!

Strange, I'd have thought the way metaballs work would make it almost trivial, like a checkbox or pull down menu…

Well, it's considerably simpler if you don't care about the expansion as in real ballons. In that case, all you need is the meta expressions attribute, see the helpcard on the Metaball SOP.

Ok, that makes kind of sense but, err, I can't figure out where to type the expression in..

I make 2 metaballs. Create a group for each? Merge them? Where do I enter the expression?

ops:

By the way, somewehat related: I get a strange rendering artifact when applying metaballs to an LSystem. It looks like flat planes missing alpha maps. Is that a bug or a feature? See attached image.

Soothsayer
By the way, somewehat related: I get a strange rendering artifact when applying metaballs to an LSystem. It looks like flat planes missing alpha maps. Is that a bug or a feature? See attached image.

The artifact is most likely due to the fact that you probably have
open cuves (poly-lines) left in your geometry. These are rendered as
curves in mantra (light-weight geometry).

If you look at the info button of the render SOP, you'll probably see
some polygons (which are what the lines are).

Just a guess.

Soothsayer
I make 2 metaballs. Create a group for each? Merge them? Where do I enter the expression?

If you want all the metaballs to only use the max function, then you can just create a single group for all of them. However, then you have you create the “metaExpressions” attribute manually using the AttribCreate SOP instead of using the MetaGroups SOP. I've attached a simpler example.

Some links to explain what are geometry attributes:

http://www.sidefx.com/docs/houdini9.5/model/attributes [sidefx.com]
http://odforce.net/wiki/index.php/GeometryAttributes [odforce.net]

Thanks guys! Solved!

I didn't realize LSys renders polys when in skeleton mode! I changed it to tube and made the thickness 0.

Edward, thanks for scene. In my previous attempt I constructed the metaball part a little differently from you. I simply added a metaball as a leaf and made a new rule F=JFJ. The copy SOP is more flexible I think!

Phew, Houdini makes me feel a little stupid. I don't know how everybody else learns this app but thanks for the numerous helpful advice here. I've been doing 3d for many years now but I'd be lost with just the help docs and my pea-brain.

Actually, using the leaf input is probably more efficient than using the Copy SOP. Just copy the AttribCreate SOP over and you're done.