¶AdnMuscle

AdnMuscle is a Houdini SOP for fast, robust, and easy-to-configure volumetric muscle simulation for digital assets. Thanks to the combination of internal (structural) and external (attachment and slide) constraints, this SOP can produce dynamics that allow the mesh to acquire realistic muscle-like behavior, including volume preservation, fiber activation to modulate rigidity, and attachment properties that allow the muscle to follow the global kinematics of the character through external objects.

The influence of these constraints on the simulated mesh can be freely modified by painting the available maps or by uniformly adjusting their influence using multipliers in the SOP parameters. In addition to maps and multipliers, there are many other parameters available to control the muscle’s dynamics and behavior, offering a wide range of configuration options.

¶How To Use

The AdnMuscle SOP is very simple to apply to geometry and set up in a Houdini scene. It works by connecting the geometry to be simulated to the single input of the SOP. Then, attachment objects are added in the Targets tab, and maps are painted to make the simulated geometry follow those objects. This introduces a significant portion of the solver’s dynamics. Additionally, there are several intuitive, easy-to-set parameters and other paintable maps that allow further tweaking of the simulation to achieve the desired results.

An AdnMuscle setup requires the following inputs:

• Targets (T): A list of objects with which the simulated muscle will interact. Targets can be nodes to define attachment points via their transformation matrix (e.g., rig joints, null nodes, etc.) or geometries to define those attachment points via closest point queries on their surface (e.g., bones or other muscles). Providing a list of targets is optional; however, targets are required to enable attachment and sliding behaviors on the muscle. One or multiple targets are supported. Refer to the Targets Section for more information.
• Muscle Geometry (M): The mesh to which the AdnMuscle SOP will be applied.

To create an AdnMuscle, follow these steps:

1. Go to the geometry context of the rig containing the muscle geometries and ensure that the muscle is a single geometry piece.
2. Press TAB and navigate to the submenu AdonisFX > Solvers to find the AdnMuscle SOP type.
3. Create it and connect the geometry to the input.
4. Go to the Targets tab in the AdnMuscle parameters, add a new entry to Targets to add a geometry target (e.g., the mummy).
5. Provide the object path of the target geometry in Target World Mesh.
6. The AdnMuscle is now ready to simulate using the default settings. Refer to the next section to customize its configuration.

¶Attributes

¶Solver Attributes

¶Time Attributes

¶Scale Attributes

¶Gravity

¶Activation Layers

¶Advanced Settings

Initialization Settings

Stiffness Settings

Override Constraint Stiffness

Mass Properties

Rest Shapes Attributes

Dynamic Properties

¶Deformer Attributes

¶Targets Attributes

¶Maps

¶Debug Attributes

¶Parameter Template

¶Paintable Weights

In order to provide more artistic control, some key parameters of the muscle solver are exposed as paintable maps in the SOP. The maps are point attributes that must be present in the geometry stream injected into the SOP. For that, the Houdini attribpaint node can be used.

¶Debugger

In order to better visualize deformer constraints and attributes in the Houdini viewport there is the option to enable the debugger, found in the switcher menu labeled Debug in the parameter interface. The visualization of the guide geometry is only activated when selecting the node to debug and cannot be enabled globally.

To enable the debugger the Debug checkbox must be marked. To select the specific feature to be visualized, choose it from the list provided in Features. The features that can be visualized with the debugger in the AdnMuscle deformer are:

• Attachments To Geometry: For each vertex with a geometry attachment constraint weight greater than 0.0, a line will be drawn from the mesh vertex to its respective geometry target closest point at rest.
• Attachments To Transform: For each vertex with a transform attachment constraint weight greater than 0.0, a line will be drawn from the mesh vertex to its respective transform target.
• Fiber Constraints: For each pair of vertices forming a constraint a line will be drawn. If the Triangulate Mesh option is disabled the debugged lines will align with the edges of the mesh polygons. If the Triangulate Mesh option is enabled the debugged lines will align with the edges of the underlying triangulation of the mesh.
• Muscle Fibers: For each vertex, a line will be drawn showing the direction of the muscle fibers. In addition, the colors of the fibers will be modulated given the Activation value by interpolating the Fibers Color and the Activation Color. This will allow debugging the activation of the muscle more precisely.
• Shape Preservation: For each vertex with a shape preservation weight greater than 0.0, a line will be drawn from each adjacent vertex to the opposite adjacent vertex.
• Slide On Geometry: If the Max Sliding Distance value is greater than 0.0, for each vertex with a slide on geometry weight greater than 0.0, a line will be drawn from the mesh vertex to the closest point on its respective geometry target.
• Slide On Segment: For each vertex with a slide on segment weight greater than 0.0, a line will be drawn from the mesh vertex to the closest point on its respective segment.

¶Advanced

¶Targets

Once the AdnMuscle SOP is created, it is possible to add and remove attachments from the muscle using the Targets tab. A target is evaluated as a mesh if its object path is added to Target World Mesh under the Targets multiparm (i.e., to set up attachment-to-geometry and slide-on-geometry constraints), and as a transformation matrix if its object path is added to Attachment Matrix under the Attachments To Transform multiparm (i.e., to set up attachment-to-transform constraints).

• Add geometry targets:
  1. Increment the number of entries in the Targets multiparm. This adds a new entry to the list.
  2. Enter the object path of the geometry to add. The path can be absolute or relative to the SOP.
  3. Make sure to recook the AdnMuscle at preroll start time for this change to take effect.
• Remove geometry targets:
  1. Locate the target to remove in the Targets multiparm.
  2. Remove it using the X button for that item.
  3. Alternatively, to remove all targets, click the Clear button of the Targets multiparm.
  4. Make sure to recook the AdnMuscle at preroll start time for this change to take effect.
• Add attachment matrices:
  1. Increment the number of entries in the Attachments To Transform multiparm. This adds a new entry to the list.
  2. Enter the object path of the node to add. The path can be absolute or relative to the SOP.
  3. Make sure to recook the AdnMuscle at preroll start time for this change to take effect.
• Remove attachment matrices:
  1. Locate the object to remove in the Attachments To Transform multiparm.
  2. Remove it using the X button for that item.
  3. Alternatively, to remove all attachments, click the Clear button of the Attachments To Transform multiparm.
  4. Make sure to recook the AdnMuscle at preroll start time for this change to take effect.

Transformation nodes such as joints or locators are used to create attachments to their world transformation matrices. Meshes, on the other hand, are used to create attachment-to-geometry and slide-on-geometry constraints. Refer to A Simple Setup for more information on painting influence maps for these constraints.

¶Slide On Segment Constraint

In addition to the previously described constraints, muscles can optionally use a constraint that defines a segment along which the muscle can slide. This constraint is configured in the Slide On Segment Constraints section of the Targets tab.

• Add segment:
  1. Increment the number of entries in the Slide On Segment Constraints multiparm. This adds a new entry to the list.
  2. Enter the object path of the node to use as the root of the segment. The path can be absolute or relative to the SOP.
  3. Enter the object path of the node to use as the tip of the segment. The path can be absolute or relative to the SOP.

• Remove segment:
  1. Locate the segment to remove in the Slide On Segment Constraints multiparm.
  2. Remove it using the X button for that item.
  3. Alternatively, to remove all segments, click the Clear button of the Slide On Segment Constraints multiparm.

¶Rest Shape

The muscle solver supports an art-directed shape that drives the fiber, shape, and volume constraints. This shape is typically a sculpted version of the muscle (and must be topologically identical) representing the muscle in its fully activated state. Artists can add or remove this custom shape from the dedicated Rest Shape Attributes section in the Advanced tab.

• Add rest shape:
  1. Enter the object path of the geometry to use as the rest shape in the Rest Shape Mesh parameter.
  2. Set the Rest Shape Weight to a value greater than 0.0 for the feature to take effect.

• Remove rest shape:
  1. Clear the object path in the Rest Shape Mesh parameter.
  2. Alternatively, set the Rest Shape Weight to 0.0 to ignore the rest shape.

Additionally, if the muscle activation input is connected to the Rest Shape Weight, the influence of the rest shape on the final result will vary coherently with the activation level during the simulation.

¶Fibers Multiplier

Painting the fibers multiplier map allows to concentrate the activation and volume of a muscle in certain areas which would allow for more artistic control over the final shape of the muscle after contraction (activation). Not painting the fibers multiplier map will cause the muscle to contract uniformly over its whole volume without concentrating the activations in the belly of the muscle. Painting to 0.0 the tendinous areas and painting to 1.0 the belly of the muscle will allow (after combing fibers and activating the muscle) to activate only the areas that had been painted with a value of 1.0.

¶Activation Layers

The activation layers allow to drive the muscle activation by multiple sensors combined together without the need of using an AdnActivation node. In practice, having an AdnActivation node with multiple input sensors connected to the activation attribute of an AdnMuscle is equivalent to connecting those sensors directly to the activation list of that muscle.

The activation layers contribute to the final activation of the muscle solver, where the first layer will always be the activation scalar attribute. Then, every value in the activation list array plug is applied on top taking into consideration the operator and the bypass flag. The resulting value is the global activation that the solver will use for the simulation, and it is written onto the read-only output solver activation attribute.

The operators available are:

1. Over (Override): Overrides the accumulated solver activation value with Value. If the current accumulated activation value is 1.0 and Value is 2.0 then the new accumulated value will be 2.0.
2. Add (Add): Adds the accumulated solver activation value with Value. If the current accumulated activation value is 1.0 and Value is 2.0 then the new accumulated value will be 3.0.
3. Sub (Subtract): Subtracts the accumulated solver activation value with Value. If the current accumulated activation value is 1.0 and Value is 2.0 then the new accumulated value will be -1.0.
4. Mult (Multiply): Multiplies the accumulated solver activation value with Value. If the current accumulated activation value is 1.0 and Value is 2.0 then the new accumulated value will be 2.0.
5. Div (Divide): Divides the accumulated solver activation value with Value. If the current accumulated activation value is 1.0 and Value is 2.0 then the new accumulated value will be 0.5.

¶Connections

Connections in AdonisFX for Houdini should be handled in two ways:

• Detail expression: detail("/obj/geo1/L_adnLocatorRotation_armFlexionShape", "adnActivationRotation", 0) where the first component should contain an API compliant naming convention and the second the detail attribute name that some of the AdonisFX SOP nodes output. This should be used when the requirement is for the connected geometry to cook before retrieving the detail attribute. This could be used for example to drive a parameter of the node using the activation value output from a sensor/locator.
• Channel expression: ch("../AdnMuscle1/envelope") where the first component should contain an API compliant naming convention and the second the referenced channel to the parameter name. This could be used to for example connect a float attribute to drive a parameter on the node.