metaballs

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Hello,


for what do they use metaballs in houdini? In the “particle slide tutorial with umbrella” they tell something about metaballs, but there is no explanation.


Thx
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Well, the nice thing about metaball is that it's not a solid surface. It's more of a “force”, if you think of it that way. One common uses for metaball is, well, for forces. You can use it as an affector (provided that there are correct attributes supplied to it) in the POP network. Or you can use it as a “magnet” and such.

HTH,
Alex
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http://www.siggraph.org/education/materials/HyperGraph/modeling/metaballs/metaballs_mward.html [siggraph.org]

Link on some metaball info.



My personal take on Metaballs is this…..they are Sirens. When working on a shot their lovely song sings out to you and you happily use them….once a deadline looms their evil claws come out and attempt to destroy you.
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Hello,


thanks for the great info and examples.
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Metaballs are used as the backbone to i3d and the creation of volumetrics. The surface that two metaballs create is just a way of visualizing where a particular weight or amount of influence is met when one, two or more metaballs interface with each other. You can change where the surface is evaluated as true by changing the weight of the metaball, the kernel that the metaball uses to define the weighting, the size, shape using the XY and Z Exponents.

The weight or influence a metaball has increases the closer you get to the center as defined by the Kernel Function to a maximum of 1.

It is this weighting property that makes them very useful.

I use metaballs as influence objects for particle systems, softbodys (spring SOP).
I use metaballs to generate i3d files.
I use metaballs to model organic shapes with lots of holes. I also use the iso SOP to do this but is a bit more involved as you need to write mathematical functions to describe the limit surface.
You can use metaballs to generate surfaces for raindrops, sheeting and running water, blood, spashing water and much more. That is where you need lots of small metaballs and as Jim mentioned above, to crank up the quality, you need even more smaller metaballs to avoid the “blobby” look. Sort of looks like bird poop. Not too nice.

Blobbies are quite handy for some things and in typical Houdini fashion, can be used almost anywhere.


There are ways to optimize the use and display of metaballs in the viewport and in networks.

First you can reduce the LOD in the Viewdisplay Properties dialog. Crank it rignt down. Yes you will loose the tiny ones but the base of the effect can be seen.

You can set up the viepwort to display hulls only. This allows you to see the spherical hulls of the blobbies without seeing the actual final limit surface generated by the blobbies (metaballs). To do this, just turn on the display of hulls then in the Optimization folder of the Display Options, turn on “Display Hulls Only”.

You can use a Convert Meta SOP to polygonize the metaballs for display purposes. Many times this is much faster than having OpenGL calculate the limit surface with much more accuracy. For animated shots, you can cache out the .bgeo's and read them back in for much quicker playback.

You can run a switch SOP or two chains of SOPs and substitute spheres in place of Metaballs. Do the display-render flag technique to display spheres and render metaballs. There is a long-standing RFE (Request For Enhancement or SUG in alias-speak) to have a display option to visualize metaballs as spheres that hasn't made it in yet.

The most problematic property with metaballs is you just can't add more without significantly changing the look. If you add more blobbies, you need to reduce their scale then tweak the look again. Because of this, I break all my rules of starting simple and working complex. I go for broke and add what I think is the final density of blobbies, substitute in spheres and tweak tweak tweak.

There has been much progress in the render speed of metaballs in PRMan11 and Mantra as of version H7. Render times have been reduced significantly making them viable for surface generation again.

Hope that gives you a bit of what blobbies can do. They just don't get used to build surfaces!
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Thanks Jeff, for the great examples!


bernard
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