Houdini 20.5 Nodes Geometry nodes

Skin Properties geometry node

Creates and modifies skin attributes in preparation for the skin solver.

On this page
Since 19.0

This node activates and assigns the physical properties and constraint attributes that govern the behavior of the skin’s simulation.

The Skin pass is comprised of two structures: the Skin Solid Layer (the interior tetrahedral mesh that gives the skin its thickness) and the Skin Surface Layer (the exterior triangulated polygonal shell). As a result, there are also two sets of constraints operating on the skin and connecting it to the underlying tissue.

Parameters

Group

Specifies the geometry group to which to apply the skin properties. If no groups are specified, then the skin properties are applied to all the geometry groups on the skin surface and skin solid geometries supplied on Input 1.

Group Type

Sets the component type for the geometry group that is specified in the Group field.

Guess from Group

Node guesses the geometry group component type.

Vertices

Group contains vertices.

Edges

Group contains edges.

Points

Group contains points.

Primitives

Group contains primitives.

Mode

Determines how the skin properties are applied to the skin.

Assign Values

Apply the skin properties to all areas of the skin.

Blend with Mask

Apply the skin properties to only the regions of the skin that have been painted with the attribute specified by Mask Attribute.

Mask Attribute

Identifies the mask attribute that is used by the Attribute Paint SOP node to paint this set of skin properties on the skin.

Invert Mask

Inverts how the skin properties are applied to the Mask Attribute regions of the skin.

Mask Blend

Determines what percentage of the skin properties are applied to the Mask Attribute region of the skin. A Mask Blend of 0.5 provides a blend of 50%. Also, you can animate this parameter.

Surface Layer

These parameters affect the physical properties of the skin surface’s polygons and how the skin surface attaches to the tissue. You can adjust the Attach Stiffness, Damping, and Rest Scale parameters for the skin surface to determine how tightly the outer surface layer will press on the underlying tissue.

Shape Stiffness

Override

When on, determines how strongly the skin surface is held to the tissue surface. This parameter is only available when the Absolute/Relative selector is set to Absolute.

Warning

For Muscle & Tissue set-ups and their .hip files that were created in Houdini v19.0.496 or earlier, please increase the Shape Stiffness/Relative Shape Stiffness parameter values by 10-100 times as the recently introduced Scale-Invariant ARAP method will apply stiffness values more accurately with respect to the area of triangles and the volume of tetrahedra.

Relative Shape Stiffness

Multiplier

See the Shape Stiffness parameter description. This parameter is only available when the Absolute/Relative selector is set to Relative.

Damping Ratio

Override

When on, determines how freely the skin surface is held to the tissue. A low damping like 0.01 will give no resistance and it will attach directly. A high damping like 0.75 makes it so that the skin surface has little energy to hold itself to the tissue surface and it will only give you a partial pull because it has a sluggish attraction force. This parameter is only available when the Absolute/Relative selector is set to Absolute.

Relative Damping Ratio

Multiplier

See the Damping Ratio parameter description. This parameter is only available when the Absolute/Relative selector is set to Relative.

Bend Stiffness

Override

When on, specifies the stiffness of the skin surface’s bend constraint. The bend constraint affects the skin’s surface polygons and limits how freely forces can change the relative angles between their adjacent triangles.

Relative Bend Stiffness

Multiplier

See the Bend Stiffness parameter description. This parameter is only available when the Absolute/Relative selector is set to Relative.

Bend Damping Ratio

Override

When on, determines how much energy is lost when applying the bend constraint to the skin surface.

Tip

You can use Bend Stiffness and Bend Damping Ratio to control the crinkliness of your skin or smooth out any pock-marked skin.

Relative Bend Damping Ratio

Multiplier

See the Bend Damping Ratio parameter description. This parameter is only available when the Absolute/Relative selector is set to Relative.

Mass Density

Override

When on, sets the mass per point for the skin surface’s polygons. Formulating mass as density like this keeps the behavior resolution of the skin surface independent. And the Surface Shape Stiffness and Damping Ratio parameter values also help to keep the skin’s behavior predicable over varying resolutions. This means that when you use Mass Density with your skin surface, changing the number of polygons the surface has will not affect its mass or how they appear in the simulation. This parameter is only available when the Absolute/Relative selector is set to Absolute.

Relative Mass Density

Multiplier

See the Mass Density parameter description. This parameter is only available when the Absolute/Relative selector is set to Relative.

Attach Stiffness

Override

When on, controls how tightly or loosely the skin surface attaches to the tissue surface. This constraint is affected by the Sliding parameters. This parameter is only available when the Absolute/Relative selector is set to Absolute.

Relative Attach Stiffness

Multiplier

See the Attach Stiffness parameter description. This parameter is only available when the Absolute/Relative selector is set to Relative.

Attach Compression Stiffness

Override

When on, affects the compression stiffness of the surface attachment. High compression stiffness will resist any shortening of the rest length distance. Low compression stiffness will allow the skin surface to press into the tissue surface with less resistance. This parameter is only available when the Absolute/Relative selector is set to Absolute.

Relative Attach Compression Stiffness

Multiplier

See the Attach Compression Stiffness parameter description. This parameter is only available when the Absolute/Relative selector is set to Relative.

Attach Damping

Override

When on, determines how freely the skin surface attaches to the tissue surface. With a low damping like 0.01, the skin surface will give no resistance (it will attach directly). With a high damping like 0.75, the skin surface will not have the energy to attach, and it will only provide a partial pull because of its sluggish attraction force. This parameter is only available when the Absolute/Relative selector is set to Absolute.

Relative Attach Damping

Multiplier

See the Attach Damping parameter description. This parameter is only available when the Absolute/Relative selector is set to Relative.

Attach Rest Scale

Override

When on, specifies how far the skin surface can move. This value scales the distance between the skin surface and the tissue surface. A value of 1.0 maintains their distance, but a value of 0.5 would put the rest scale halfway across the gap between the skin surface and the tissue surface. By default, this value is so small so that the skin magnetizes to the tissue. This parameter is only available when the Absolute/Relative selector is set to Absolute.

Relative Attach Rest Scale

Multiplier

See the Attach Rest Scale parameter description. This parameter is only available when the Absolute/Relative selector is set to Relative.

Collision Scale

Override

When on, specifies a scale multiplier for the air bubble or air bag between the tissue and skin. Acts like a bumper guard. This is similar to the Collision Radius Scale on the Skin Solver Vellum SOP. This parameter is only available when the Absolute/Relative selector is set to Absolute.

Relative Collision Scale

Multiplier

See the Collision Scale parameter description. This parameter is only available when the Absolute/Relative selector is set to Relative.

Solid Layer

These parameters control the physical properties and constraints on the skin solid’s tetrahedrons. You can adjust the Attach Stiffness, Damping, and Rest Scale parameters for the skin solid to determine the degree to which the solid mesh sticks to or detaches from the underlying tissue.

Shape Stiffness

Override

When on, determines how much the skin solid wants to preserve its shape. With a high Shape Stiffness, the skin solid will not want to change its shape and it will remain quite rigid. This parameter is only available when the Absolute/Relative selector is set to Absolute.

Warning

For Muscle & Tissue set-ups and their .hip files that were created in Houdini v19.0.496 or earlier, please increase the Shape Stiffness/Relative Shape Stiffness parameter values by 10-100 times as the recently introduced Scale-Invariant ARAP method will apply stiffness values more accurately with respect to the area of triangles and the volume of tetrahedra.

Relative Shape Stiffness

Multiplier

See the Shape Stiffness parameter description. This parameter is only available when the Absolute/Relative selector is set to Relative.

Volume Stiffness

Override

When on, determines how much the skin solid wants to preserve its volume. This parameter is only available when the Absolute/Relative selector is set to Absolute.

Relative Volume Stiffness

Multiplier

See the Volume Stiffness parameter description. This parameter is only available when the Absolute/Relative selector is set to Relative.

Damping Ratio

Override

When on, determines how quickly the skin solid is allowed to change its shape. This parameter is only available when the Absolute/Relative selector is set to Absolute.

Relative Damping Ratio

Multiplier

See the Damping Ratio parameter description. This parameter is only available when the Absolute/Relative selector is set to Relative.

Mass Density

Override

When on, sets the mass per volume for the tissue solid’s tetrahedrons and controls how mass is computed per point. Formulating mass as density like this keeps the behavior resolution of the skin solid independent. And the Shape Stiffness and Damping Ratio parameter values also help to keep the muscles' behavior predicable over varying resolutions. This means that when you use Mass Density with your skin solid, changing the number of tetrahedra the skin solid has will not affect their mass or how they appear in the simulation. This parameter is only available when the Absolute/Relative selector is set to Absolute.

Relative Mass Density

Multiplier

See the Mass Density parameter description. This parameter is only available when the Absolute/Relative selector is set to Relative.

Attach Stiffness

Override

When on, this parameter controls the attachment of the innermost boundary of points on the skin solid’s tetrahedra to the tissue surface. This attachment constraint does not slide, but you can weaken it as much as desired to detach the skin from the tissue surface completely or make sure it remains adherent. This constraint does not support Sliding. This parameter is only available when the Absolute/Relative selector is set to Absolute.

Relative Attach Stiffness

Multiplier

See the Attach Stiffness parameter description. This parameter is only available when the Absolute/Relative selector is set to Relative.

Attach Damping

Override

When on, determines how quickly the skin solid will snap to the tissue surface. This parameter is only available when the Absolute/Relative selector is set to Absolute.

Relative Attach Damping

Multiplier

See the Attach Damping parameter description. This parameter is only available when the Absolute/Relative selector is set to Relative.

Attach Rest Scale

Override

When on, sets the rest scale for the skin solid’s attachment constraint. This is the scale the attachment constraint wants to maintain or go back to. When the Attachment Rest Scale's value is set to less than 1.0, the attach constraint pulls the skin solid in towards the tissue surface. This parameter is only available when the Absolute/Relative selector is set to Absolute.

Relative Attach Rest Scale

Multiplier

See the Attach Rest Scale parameter description. This parameter is only available when the Absolute/Relative selector is set to Relative.

Sliding

These parameters determine the sliding behavior for all the Skin Layers' components.

Sliding Rate

Override

When on, controls how rapidly the skin geometry will find a new target on the tissue surface. A low value makes it so that attachment is reluctant to allow the skin to move further than the Distance Limit. A higher value makes it so that when the skin’s tetrahedral mesh moves, it changes what it is attaching to fairly easily. This parameter is related to friction. This parameter is only available when the Absolute/Relative selector is set to Absolute.

Relative Sliding Rate

Multiplier

See the Sliding Rate parameter description. This parameter is only available when the Absolute/Relative selector is set to Relative.

Distance Limit

Override

When on, specifies an additional constraint that prevents a point on the skin from sliding farther away than the rest distance. When a skin point hits the distance limit, it will spring back (not just stop). This allows regions of skin to slide while limiting other regions. Buckling and wrinkling effects will occur in skin regions that oppose each other. This parameter is only available when the Absolute/Relative selector is set to Absolute.

Relative Distance Limit

Multiplier

See the Distance Limit parameter description. This parameter is only available when the Absolute/Relative selector is set to Relative.

Limit Stiffness

Override

When on, specifies the strength of the Distance Limit constraint and determines the elasticity of that connection. This parameter is only available when the Absolute/Relative selector is set to Absolute.

Relative Limit Stiffness

Multiplier

See the Limit Stiffness parameter description. This parameter is only available when the Absolute/Relative selector is set to Relative.

Damping Ratio

Override

When on, this parameter controls how sluggish vs. how springy the Distance Limit constraint is. This parameter is only available when the Absolute/Relative selector is set to Absolute.

Relative Damping Ratio

Multiplier

See the Damping Ratio parameter description. This parameter is only available when the Absolute/Relative selector is set to Relative.

Shrinkage

These parameters define the rest scale for the skin. A value of 1 is the default rest state. Smaller values will cause the skin to shrink as it simulates, and larger values will cause the skin to expand.

You can affect the Rest Scale for each of the Skin Layers' components (triangles and tetrahedra) with these parameters. During the skin simulation, the skin’s components will attempt to achieve this scale (relative to the t-pose rest position state) and shrink or expand accordingly.

In their rest positions (tpose attribute), the Tissue Layers, Core Layer, and Skin Layers record rest volumes and rest shapes for each primitive in their geometries. Over the course of a simulation, the rest configuration can be modified relative to its initial state.

You can use the scale values for the rest shape and volume to adjust this rest configuration and create a shrink-wrapping effect that causes the tissue and skin to pull inward and tighten against the internal muscle and bone geometry. By balancing the interplay between core, tissue solid, tissue surface, skin solid, and skin surface rest scales, you can achieve a variety of different tissue looks for your Muscles & Tissue simulation. For example, shrinking tissue tetrahedrons while expanding surface triangles can contribute to additional buckling over a tissue’s surface.

Tip

Shrinkage allows you to produce spandex-type effects and fine wrinkling (when the skin solid layer is tighter than the skin surface).

Surface Rest Scale

Override

When on, specifies the scale (relative to the t-pose state) that the skin surface will try to achieve during the simulation (shrinking and expanding accordingly). This parameter is only available when the Absolute/Relative selector is set to Absolute.

Relative Surface Rest Scale

Multiplier

See the Surface Rest Scale parameter description. This parameter is only available when the Absolute/Relative selector is set to Relative.

Solid Rest Scale

Override

When on, specifies the scale (relative to the t-pose state) that the skin solid will try to achieve during the simulation (shrinking and expanding accordingly). This parameter is only available when the Absolute/Relative selector is set to Absolute. This parameter is only available when the Absolute/Relative selector is set to Absolute.

Relative Solid Rest Scale

Multiplier

See the Solid Rest Scale parameter description. This parameter is only available when the Absolute/Relative selector is set to Relative.

Inputs

Input 1

Skin surface geometry.

Input 2

When Mode > Blend with Mask is selected, then this input allows you to input two skin property nodes into one to create a stream blend.

A Mask Blend value of 0 = 100% of Input 1's value, and a Mask Blend value of 1 = 100% of Input 2's value.

However, if you also specify a Mask Attribute, then the connected Attrib Paint SOP node will drive the blend.

Outputs

Output 1

Skin surface (polygonal surface) and skin solid (tetrahedral mesh) with constraint and material attributes.

See also

Geometry nodes