Houdini 20.5 Nodes Geometry nodes

Tissue Properties geometry node

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

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Since 19.0

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

Each tissue parameter injects a corresponding point attribute into the node’s output stream that the downstream Tissue Solver Vellum SOP node will then use to affect the constraint or material property it is tied to. In other words, this node only creates constraint attributes and material attributes.

You can assign each tissue parameter as either an Absolute or a Relative value. When a parameter is Absolute, its value is used explicitly (as an override). When a parameter is Relative, its value is used as a multiplier of whatever its current Absolute value (incoming or default value) is. If you do not specify a user-set value for a parameter, then the default value for that parameter is used.

In cases where a parameter is turned off, the incoming value for the associated attribute will be passed through the node. If the attribute does not yet exist, then its default value will be used by the tissue solver.

The parameters for the Tissue Surface Layer (exterior triangulated polygonal surface) and Tissue Solid Layer (interior tetrahedral mesh) geometry components are separated into their own respective tabs. However, in both tabs you can find parameters that affect Shape Stiffness, Volume Stiffness, Damping Ratio, and Mass Density.

Setting up tissue properties

When setting up tissue properties, think of the entire set of properties for the tissue as layers of a sort. You can make the first Tissue Properties SOP node in your tissue network the base properties layer for the tissue (the properties node that initializes and applies all the tissue properties for the entirety of the tissue), and then make all the subsequent Tissue Properties SOP nodes in the network the specialized properties layers (property nodes whose settings produce different tissue effects like saggy skin or tight skin).

You can also use the Attrib Paint SOP node to paint masks and blend layers of properties for your tissue. Using masks and chaining together a series of Tissue Properties SOP nodes can effectively composite tissue characteristics.

There are two ways that you can use the Tissue Properties SOP node:

You can also use the Mask Blend parameter to dial the strength of the composite up or down without having to adjust your painted masks.

Set-up Tips

The best ways to achieve more muscle definition are:

  • Reduce Core Rest Scale to less than 1. Core Rest Scale imparts a pull onto the tissue from the inside. A small reduction is normally sufficient, and a value of 0.9 is aggressive in most cases.

  • Reduce the Tissue Surface Layer tab > Attach Rest Scale parameter to less than 1. This brings the tissue surface closer to the muscles and bones. A value of 0.0 will make the Tissue Surface Layer coincident with the muscle and bones within, but that is quite extreme. A value of 0.5 is usually enough to make the muscles more evident under the tissue surface.

Parameters

Group

Specifies the geometry group to which to apply the tissue properties. If no groups are specified, then the tissue properties are applied to all the geometry groups on the tissue 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 tissue properties are applied to the tissue.

Assign Values

Apply the tissue properties to all areas of the tissue.

Blend with Mask

Apply the tissue properties to only the regions on the tissue 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 tissue properties on the tissue.

Invert Mask

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

Mask Blend

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

Tissue Surface Layer

These parameters specify the physical properties and attachment constraint settings for the Tissue’s exterior polygonal surface (Tissue Surface Layer).

Shape Stiffness

Override

When on, determines how much the tissue surface wants to preserve its shape. With a high Shape Stiffness, the tissue surface 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.

Damping Ratio

Override

When on, determines how quickly the tissue surface 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.

Bend Stiffness

Override

When on, specifies the stiffness of the tissue surface’s bend constraint. The bend constraint affects the tissue’s surface polygons and limits how freely forces can change the relative angles between their adjacent triangles. This parameter is only available when the Absolute/Relative selector is set to Absolute.

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 tissue surface.

Tip

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

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 tissue surface’s polygons. Formulating mass as density like this keeps the behavior resolution of the tissue surface independent. And the Surface Shape Stiffness and Damping Ratio parameter values also help to keep the tissue surface’s behavior predicable over varying resolutions. This means that when you use Mass Density with your tissue 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 tissue surface attaches to the muscles and bones within your character. 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 tissue surface to press into the muscles and bones 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 tissue surface attaches to the muscles and bones. With a low damping value like 0.01, the tissue surface will give no resistance (it will attach directly). With a high damping value like 0.75, the tissue 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, sets the rest scale for the tissue surface’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 tissue in towards the muscles and bones. 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, sets the collision multiplier for the tissue surface. 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.

Tissue Solid Layer

These parameters specify the physical properties and attachment constraint settings for the Tissue’s interior tetrahedral geometry (Tissue Solid Layer).

Shape Stiffness

Override

When on, determines how much the tissue solid wants to preserve its shape. With a high Shape Stiffness, the tissue 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 tissue 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 tissue 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 tissue solid independent. And the Shape Stiffness and Damping Ratio parameter values also help to keep the tissue solid’s behavior predicable over varying resolutions. This means that when you use Mass Density with your tissue solid, changing the number of tetrahedra the tissue 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 tissue solid’s tetrahedra to the outer surface of the core geometry (Core Surface Layer). This attachment constraint does not slide, but you can weaken it as much as desired to detach the tissue from the core 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 tissue solid will snap to the core 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.

Sliding

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

Sliding Rate

Override

When on, determines how freely the attachments can slide. A value of 1.0 allows tissue to slide freely. Values closer to 0 produce less and less movement until the tissue becomes a fixed attachment. 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, creates a distance limit constraint that determines how far a point on the tissue surface can slide relative to its initial attachment point at its t-pose rest position state. 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 stiffness of the spring that imposes the distance limit. 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, 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

You can affect the Rest Scale for each of the Tissue Layers' components (triangles and tetrahedra) with these parameters. During the tissue simulation, the tissue’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 and Core 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 tissue to pull inward and tighten against the internal muscle and bone geometry. By balancing the interplay between core, tissue solid, and tissue 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.

Important Note

The Tissue Surface Layer’s triangles are connected to the Tissue Solid Layer’s tetrahedrons, which are in turn connected to the Core Surface Layer. So any changes in scale on any of the layers will have a push-pull two-way effect on the neighboring components. However, the Core Surface Layer is bound to the muscles and bones in a rigid-as-possible manner, so the pull force on the tissue components is effectively one-way.

Tissue Rest Scale

Override

When on, specifies the scale (relative to the t-pose state) that the Tissue Solid Layer 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 Tissue Rest Scale

Multiplier

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

Core Rest Scale

Override

When on, specifies the scale (relative to the t-pose state) that the Core Layers 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 Core Rest Scale

Multiplier

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

Surface Rest Scale

Override

When on, specifies the scale (relative to the t-pose state) that the Tissue Surface Layer 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.

Inputs

Input 1

Tissue and Core geometry layers.

Input 2

When the Mode > Blend with Mask parameter option is selected, then this input allows you to merge 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

Tissue and Core geometry layers with constraint and material attributes.

See also

Geometry nodes