Metaball surface node

Metaballs provide a method for creating an organic-looking surface. The metaballs are blobs (“fields”) of “density” floating in space. Houdini skins a surface around areas where the density is greater than some threshold.

When two metaballs overlap, Houdini adds their field effects together. Because the density of the force field can be increased by the proximity of other metaball force fields, metaballs have the unique property that they change their shape to adapt and fuse with surrounding metaballs. This makes them very effective for modeling organic surfaces.

You can specify negative Weights to create “pusher” metaballs. This lets you create holes in the surface by effectively subtracting density.

The field is controlled by a kernel function and a weight. The kernel function controls the gradient of the density value of a metaball field from 1 (at the center) to 0 (at its outside edge). The weight scales the density to shift the location of the surface closer or further away from the center. You can currently specify the shape of the field as an ellipsoid or a super-quadric.

Instead of the regular ellipsoid field shape, you can increase or decrease the XY and Z exponent parameters to create “star-ish” (exponent > 1) or “square-ish” (exponent < 1).

A metaball is known as a super-quadratic if either exponent is not equal to one.

Each metaball has a “sphere of influence”. When you merge two metaballs and they extend into each other’s sphere of influence, they react in a way similar to drops of water: the surface tension works to form a smooth bridge between them. This is useful for making organic “blobby” shapes which meld into each other.

Metaballs can be thought of as spherical force fields whose surface is an implicit function defined at any point where the density of the force field equals a certain threshold. Because the density of the force field can be increased by the proximity of other metaball force fields, metaballs have the unique property that they change their shape to adapt and fuse with surrounding metaballs. This makes them very effective for modeling organic surfaces. For example, below we have a metaball. The surface of the metaball exists whenever the density of the metaball’s field reaches a certain threshold:

When two or more metaball force fields are combined, as in the illustration below, the resulting density of the force fields is added, and the surface extends to include that area where the force fields intersect and create density values with a value of one. For more information on Metaballs, see the Geometry Types section.

You can see a metaball’s sphere of influence by turning on Display primitive hulls in the Display options toolbar.

This node is ideal for creating organic models, liquid effects and other blobby surfaces.

Houdini also uses metaballs in different situations to define “fields of influence”, such as for the Force node which pushes or pulls Springs, and for capturing complex character geometry with the Capture Metaball node.

Metaball expressions are user-specified attributes that determine how metaballs are to be merged. These expressions can be built using a combination of quoted group names, unquoted primitive numbers, and the operations min, max, and sum. An example of a valid expression is

sum(max("group1", "group2"), max("group3", "group4"), 0, 1)

where group1, group2, group3 and group4 are the names of groups, and 0 and 1 are the primitive numbers of metaballs. The min, max, and sum operations can take an arbitrary number of arguments and support nested expressions such as sum(max(...), ...). Arguments to min, max, or sum are separated by commas. Providing a quoted group name is equivalent to simply adding each metaball primitive from that group to the current argument list. For example, if group1 contains metaballs with primitive numbers 0, 1 and 2, then the expression

max("group1")

is equivalent to

max(0, 1, 2)

Duplicate argument names are also supported by this expression. For example, if a metaball with primitive number 0 is defined, then specifying the expression sum(0, 0) is equivalent to merging two distinct metaballs at the same location as metaball 0.

The following images show two metaballs merged using the sum, max and min operations, respectively.

This expression can be set by creating a detail/string attribute with the desired expression on any geometry containing metaballs. This functionality is also partially exposed via the MetaGroups SOP. This SOP allows users to easily generate expressions of the form

max(sum(...), sum(...), ..., sum(...))

by establishing a set of Metaball groupings. Each of these groupings consists of a list of group names, separated by spaces. For example, if two metaball groupings are defined by the SOP, one as 'group1 group2', and the other as 'group3 group4', then the SOP generates a Metaball expression equivalent to

max(sum("group1", "group2"), sum("group3", "group4"))