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Exact explanation of Inflate needed badly


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Hi all,

can anyone describe me the theory of metaball capture and muscle deformation?

In my present understanding a muscle is a series of metaballs which are oriented along a curve called "center line". The radius of the metaballs contol the influence of the metaball on a given point. A point can belong to more than one "influence group" and if my tests are accurate the a point can belong to more than one metaball inside an influence group. Since metaballs can be rotated and scaled it's clear that they define a transformation space, so Houdini only needs to blend the transformations of the metaballs that influences a point to get it's deformed position.

Until this point it's clear and the behaviour is also predictable. What I don't understand is that how inflation and sliding works. What frustrates me the most is that I'm not even able to create a test scene where toggling the "enable slide" parameter makes any difference at all (of course with a coefficient of 1). Also enabling inflation, only makes a very little difference, and tweaking the parameters of the inflation tab only allows me to achieve erroneous results or this minimal one.

I'm struggling with this for days. Please, if you have any results or insight or you're just sure that these features are as useless as it seems to me, than share your knowledge.

thanks in adwance


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No idea how this really works, but it does something slidingly. The key point here is to add a rest anchor, whatever that is. Adding the path for the muscle deformers is also unusual (.../muscle.../muscle).

Sorry but maybe it helps a little.

The character tools look great but I wish they were better explained.


Edited by Macha
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Apparently I was missing the rest anchor. Some further research led to the conclusion that inflation and sliding is understandable and indeed are useful features. :)

So it seems like inflation works in the following way:

- The Capture Metaball node records the normals of the deformed geometry at the capture frame and stores it in the "muscleInflateNormal" point attribute.

- Inflation is the process of pushing the points along this stored normals until their distance from the muscle's surface is less than the "Inflate Tolerance" parameter.

- When sliding is turned on, Houdini does two deformations. The first is a muscle deformation, which I described in my previous post. This deformation pulls the points with the metaballs. The second is created by transforming the geometry with the transform of the Rest Anchor and inflating the surface to the muscle. This deformation does not pull the points with the metaballs at all.

- The sliding coefficient controls a blend between the two deformations.

This is more or less speculation so be cautious and do your own research. If you find results that contradicts my "theory", please report.



ps.: one source of possible confusion is the documentation of the Rest Anchor parameter. First of all it says "geometry" instead of "transform". The Rest Anchor has nothing to do with geometry, in fact it can only refer to OBJs and not SOPs. The last sentence is also clearly erroneous. The corrected version would sound something like this: "Sliding is done by blending the regular deformation result with the result obtained by transforming with the rest anchor and inflating by the muscle."

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