Understanding the relationship each simulation pass has with its associated attachment constraints is key when it comes to fine tuning your simulation results. Since multiple attachment constraints co-exist and affect the same geometry in tandem, it is important to familiarize yourself with each type of constraint operating on a piece of geometry in each simulation context.

In the first stage of Muscles & Tissue simulation creation, you create the muscle pass for the simulation. The muscle pass creates the solid muscle geometry that moves and flexes with the animated bone geometry, and then drives the majority of the tissue’s deformations (point transforms) downstream.

The following section lists the steps for recreating the example muscles network above. You can adapt these steps to create your own basic muscles pass from scratch.

1. Import the static muscle surfaces, animated bone surfaces, and static t-pose bone surfaces.

   You need the static muscle surfaces, animated bone surfaces, and static t-pose bone surfaces for your character to be able to generate the solid muscles for the muscles pass.

   There are many ways to bring in or import geometry with SOPs. For example, you can use an Object Merge SOP node, a File SOP, a Stash SOP, an Alembic SOP, and so on.

   1. Import your muscle surface geometry. The sculpted muscle surfaces should be manifold non-intersecting polygonal geometry modeled in real-world scale (with 1 Houdini unit = 1 meter).

   2. Import your animated bone surfaces. The polygonal bone surfaces should be modeled in real-world scale (with 1 Houdini unit = 1 meter).

   3. Import your bone surfaces that are in their t-pose (rest position).

2. Name the muscles.

   Each muscles surface (self-contained series of connected primitives) that will represent a muscle in your Muscle & Tissue simulation must have a muscle_id attribute or name before it can be tetrahedralized.

   Create a Muscle ID SOP node, connect it to your static muscle surfaces import node, and then use the Muscle ID SOP to select each piece of your muscle surface geometry and assign them a Name.

3. (Optional) Remove any unneeded geometry groups.

   If you used geometry groups to select your muscle surfaces in step two, you no longer need those group attributes. You can remove them from your muscle geometry to make sure only necessary attributes are being passed to downstream nodes.

   Create a Group Delete SOP node, connect it to your Muscle ID SOP node, and then use the Group Delete SOP to select and remove the unneeded geometry groups from your muscles.

4. Set the t-pose for the bone surfaces.

   It is necessary to store the rest position or t-pose for your character’s bones and muscles as these values will be used to move your geometry into and out of the t-pose position throughout the Muscles & Tissue creation process.

   • Create a Set T-Pose SOP node and connect it to the static t-pose bone surfaces import node. The Set T-Pose tab menu item creates a Rest Position SOP node with preset store t-pose settings.

   • Use the Rest Position SOP to store the t-pose bone geometry’s current pose in the tpose point attribute.

5. (Optional) Triangulate the muscles externally.

   The skin surface must be triangulated before tetrahedralization. Traditionally this is done by using the Remesh Surface settings that are inside the Muscle Solidify SOP node.

   However, if you want more control over how the muscle polygons are triangulated, then you can use an external Remesh SOP node to triangulate the muscles geometry.

   Create a Remesh SOP node, connect it to the Muscle ID SOP node, and then use it to triangulate the muscles geometry before building their tetrahedra.

6. Triangulate and then tetrahedralize the muscle surfaces.

   1. Create a Muscle Solidify SOP node and connect it to the Muscle ID SOP node.

   2. Make sure that the input muscle surfaces are tessellated with clean equilateral triangles. The muscle surfaces must be triangulated before tetrahedralization. To do this, use the internal Remesh Surface settings on the Muscle Solidify SOP node.

   3. The Muscle Solidify SOP node automatically tetrahedralizes the input triangulated muscle surfaces. The solid muscles do not have an outside polygonal surface like the tissue pass or skin pass because the muscles' surfaces are created just-in-time within the Muscle Solver Vellum SOP node only when they are required by the Vellum sliding constraints.

   4. Adjust the Tissue Solidify SOP's parameter settings as you desire.

      • The Max Tet Size parameter sets the upper size limit for individual generated tetrahedra. The smaller the tetrahedra, the more detail the muscles will be able to convey to the tissue pass downstream. However, the bigger the tetrahedra, the faster the muscles' performance.

7. Groom muscle fiber direction.

   When a muscle flexes, it contracts in the direction of its fiber vectors. The fiber vectors define the orientation of the muscles' local axes. For your muscles to contract in the direction you want them too, they need to have well defined custom fiber direction vectors. You can paint these fiber direction vectors for your muscles with the Fiber Groom SOP node.

   Create a Fiber Groom SOP node, connect it to the Muscle Solidify SOP node, and then use it to interactively groom the fiber direction vectors for your muscles so that they flow in the direction you want your muscles to contract. These custom fiber vectors are then stored in the materialW attribute, which is later used by the Muscle Flex SOP downstream.

8. Set muscle physical properties.

   The muscle physical properties determine the physical characteristics of the muscles like how receptive or resistant the muscles are to shape changes, which regions of the muscles are tendon, how stiff or floppy the muscles are, how much the muscles will bulge on contraction, and so on.

   Create a Muscle Properties SOP node, connect it to the Fiber Groom SOP node, and then configure the Muscle Properties SOP’s parameter settings as you desire.

   We recommend that you use the first Number of Assignments multiparm tab to set the general settings for all your muscles, and then use the subsequent multiparm tabs to set specific property configurations for individual muscles or groups of muscle.

   Important Note

   Please keep in mind that the Absolute and Relative versions of the parameters have different behaviors. The Absolute parameter values are applied as overrides, and the Relative parameter values are applied as multipliers of previously set values (from prior multiparm tabs) or default values.

   • Shape Stiffness determines how much a muscle wants to stay in its original shape (the tpose).

   • Damping Ratio determines how quickly a muscle’s shape can change.

   • Mass Density determines whether or not the number of tetrahedra in a muscle affects its overall mass or appearance in the simulation.

   • Fiber Strength determines how strongly a muscle is squeezed.

   • Fiber Damping controls how rapidly a muscle can contract.

   • Fiber Scale Range defines the min/max threshold of muscle tension for a muscle.

   • Fiber Volume Scale controls the volume scale force that is applied to a muscle tetrahedra when fully contracted.

   • Tendon Stiffness determines the scale factor that is applied to Shape Stiffness for the tendon regions of a muscle.

   • Tendon Mask Radius defines which parts of a muscle are the tendon regions.

   • Thickness Threshold defines the Tendon Stiffness threshold above which a muscle region is considered the belly of the muscle, and below which a muscle region is considered the tendon.

9. Preroll the bone surfaces.

   Create a Muscle Preroll SOP node, connect it to the upstream bone surfaces t-pose node, and then add a hold and/or a preroll of frames that transitions your character’s bones from their t-pose to the their world space position at the first frame of their shot animation. Optionally, you can also enter the node’s viewport state and use the transform handles to add a transformation offset to the character’s bones preroll.

   The transformation used to go from t-pose to start position is stored as preroll_ detail attributes on the output geometry. In the tissue pass and skin pass you will be able to reference this same transformation data using additional Muscle Preroll SOP nodes to move your character’s other components into the exact same position.

10. Set muscle constraint properties.

    The muscle constraint properties create and configure the constraints for the muscles. The muscle constraints attach the muscles to the bones and to each other, constrain the ends of the muscles, define the collision radii of the muscles, and determine how much velocity each muscle inherits from surrounding muscles.

    Create a Muscle Constraint Properties Vellum SOP node, connect its first input to the Muscle Properties SOP node and its second input to the Muscle Preroll SOP node, and then set all its parameters values as you desire.

    • The Muscle Ends parameters determines whether the muscles ends are constrained to pre-animated positions or nearby bones. Which target the muscles ends attach to is determined by the Muscle Ends parameter on the Muscle Vellum Solver SOP.

    • The Muscle to Muscle parameters control how muscles are constrained to each other.

    • The Muscle to Bone parameters control how muscles attach to the bone geometry.

    • The Velocity Blend parameters controls how much neighborhood velocity is imparted to the muscles.

    • The Collisions parameters define the collision radii of the muscles.

    Tip

    You can use the Visualize buttons in the Muscle Constraint Properties Vellum SOP node parameters to view visual representations of the different constraints' current settings in the viewport state.

11. (Optional) Mirror your muscles.

    If you only have half (just the left or right side, rather than both sides) of your character’s muscle geometry and your character is symmetrical, then you can use the Muscle Mirror SOP node to mirror or duplicate the muscles for the missing side.

    • Create a Muscle Mirror SOP node and connect it to the Muscle Constraint Properties Vellum SOP node.

    • In the Muscle Mirror SOP's parameters, specify the Naming prefixing and Mirror Plane position you want to use for the mirroring operation.

    • The Muscle Mirror SOP automatically creates the muscles for the missing side of your character and makes sure that all the mirrored muscles receive the proper names (muscle_id attributes), fiber vectors (materialW attribute), and properties (physical and constraints).

12. (Optional) Paint tissue property masks.

    If you want to paint masks on your muscles so that only certain muscle properties and values apply to certain regions on your muscles, then create an Muscle Paint SOP node, connect it to the Muscle Constraint Properties Vellum SOP node, and then use it to paint those regions.

    You can paint the following preset muscle physical and constraint properties on your muscles:

    • Muscle Ends Mask lets you paint the End Regions for the Muscle Ends constraint on your muscles. This determines which area of your muscles the Muscle Ends constraint will affect.

    • Muscle to Bone Mask lets you paint the Stiffness for the Muscle to Bone constraint on your muscles. The brighter the heat map, the higher the stiffness. Stiffness determines how stretchy the Distance Threshold connection is.

    • Tendon Mask lets you paint the Tendon Mask Radius property on your muscles. Tendon Mask Radius defines which parts of a muscle are the tendon regions and how far from the muscle’s ends the tendon regions extend.

    • Muscle Glue lets you paint the Muscle Glue constraint values on your the muscles. The Muscle Glue constraint forms a rigid attachment between neighboring muscles so that they will never be able to separate.

    • Slide Rate lets you paint the amount of Slide Rate on your muscles. Slide Rate determines how much the Muscle to Bone constraint connections can drift along the surfaces of your bone.

13. Create muscle tension lines.

    The triggering of muscle activation and the increase and decrease of muscletension in your muscles is automated by the muscle tension lines. The muscletension attribute drives the flexing action of your muscles during their simulation.

    The Muscle Tension Lines SOP node lets you draw the muscle tension lines on any static t-pose geometry (this is most commonly your bone geometry). The name of each muscle tension line is stored in the autoflex_id attribute, the initial length of each line is stored in the restlength attribute, and the length of the line as it shortens and lengthens over the bone geometry’s animation is stored in the length attribute. The difference between the restlength and length values are what drives the increase and decrease of muscletension.

    • Create the Muscle Tension Lines SOP node, connect it to the bone surfaces preroll node, and then use it to create the muscle tension lines.

      • Draw each line on your static t-pose guide geometry (most commonly the bone geometry) making sure that the lines begin and end at the approximate origin and insertion points of each of the muscles.

      Tip

      You can use the Guides parameters to help you visualize your lines and their target guide geometry in the viewport state.

    • (Optional) Connect a geometry node or an import node that contains pre-drawn polylines to the second input on the Muscle Tension Lines SOP node to replace the internally generated muscle tension lines.

    Note

    You can use the muscle tension lines alone or in conjunction with keyframed muscletension values per muscle on the Muscle Flex SOP node.

14. Get preroll bone surfaces.

    You need the animated bone surfaces to drive the proxy animation in the Muscle Flex SOP node, so you need to retrieve the data stored in the bone preroll node upstream.

    Create an Object Merge SOP node and point it to the upstream bone preroll node.

15. Animate the muscle tension.

    • Create a Muscle Flex SOP node and connect its first input to the Muscle Constraint Properties Vellum SOP node, its second input to the bone preroll Object Merge SOP node, and its third input to the Muscle Tension Lines SOP node.

    • Set the Muscle Flex SOP node’s parameter values as you desire.

      • Use the Activation Link parameters to link your muscle geometry (muscle_id attribute) to the muscle tension lines (autoflex_id attribute) that drive their automatic flexion timing.

      • Use the Muscle Tension Lines parameters to adjust the activation ratio between muscle tension line length (restlength and length attributes) and what is considered a fully flexed state for the muscles (muscletension attribute). This sets the threshold for what is considered flexed or not.

      • Use the Muscle Flex parameters to animate the muscletension attribute which is then used by the muscle solver to help determine when muscles try to contract and relax during their shot animation. This animation can be created automatically by the muscle tension lines alone or in combination with keyframes on the Muscle Flex parameters. You can tell the solver when to activate the muscles and when not to activate them by keyframing those parameter values.

16. Simulate the muscles.

    Compute the tissue simulation using the Vellum solver.

    • Create a Muscle Vellum Solver SOP node, connect its first input to the Muscle Flex SOP node, connect its second input to the bone preroll Object Merge SOP node, and then set all the parameter settings you desire for the solve.

      • If your muscles use the Muscle Ends constraints, be sure to select how you want those constraints to behave with the Muscle Ends parameter on the Muscle Vellum Solver SOP node.

    • (Optional) If an animated muscletension attribute is not present on the muscle solver’s first input, you can connect flexing reference primitives to its third input to provide the animated muscletension attribute directly. In this case, muscles will look for matching muscle_id's coming in from the third input, and then use the muscletension attribute found on the first point.

17. Cache the muscles to disk.

    Create a File Cache SOP node, connect it to the Muscle Vellum Solver SOP node, and then use it to cache the results of the muscles simulation to disk.

18. Collect the muscles and bones.

    Collect the final data outputs from the muscles pass for use downstream in the tissue pass.

    • Create two Null SOP nodes.

    • Connect the first null to the File Cache SOP node’s output to copy (collect) the final muscle simulation cache.

    • Connect the second null to the bone preroll Object Merge SOP node to copy (collect) the prerolled animated bone geometry.

In the second stage of Muscles & Tissue simulation creation, you create the tissue pass for the simulation. The tissue pass creates the filler tetrahedra that surround the muscles and fill in the gaps between the muscles (core layer) and a thick tissue layer that functions as the base skin for your character. The tissue pass shows muscle definition and coarser skin wrinkling/creasing, whereas the downstream skin pass (detail skin) shows finer skin wrinkling/creasing and adds an additional level of detail to the outmost surface of your character’s Muscle & Tissue simulation. The core layer in the tissue pass also pulls in both the tissue layer and the skin pass with a sucking-type effect.

The tissue pass has two layers, and each layer is comprised of two separate geometry pieces. The composition of the tissue pass (from inner most to outmost) is the following:

• The core layer

  • A tetrahedral solid core inner mesh (core solid)

  • A triangulated polygonal core outer surface (core surface)

• The tissue layer

  • A tetrahedral solid tissue inner mesh (tissue solid)

  • A triangulated polygonal tissue outer surface (tissue surface).

Each of these layers have separate physical properties and constraints.

The following section lists the steps for recreating the example tissue network above. You can adapt these steps to create your own basic tissue pass from scratch.

1. Import the skin surface.

   Import your original high-resolution skin surface geometry. You need this surface to generate the geometry for the tissue pass.

   There are many ways to bring in or import geometry with SOPs. For example, you can use an Object Merge SOP node, a File SOP, a Stash SOP, an Alembic SOP, and so on.

2. Get the muscles simulation and animated bone surface outputs from the muscles pass.

   Retrieve the muscles simulation and animated bone surface geometry outputs from the muscles pass. The Tissue Solidify SOP node needs this geometry so that it can create the tissue’s tetrahedra.

   There are many ways to bring in or import geometry with SOPs. For example, you can use an Object Merge SOP node, a File SOP, a Stash SOP, an Alembic SOP, and so on.

3. Extract t-poses from the muscle solid and bone surfaces geometry.

   Your character’s muscle simulation and animated bone geometry needs to be in their rest positions (t-poses) before you can create the tissue for your character.

   Create an Extract T-Pose SOP node for both the muscle solid geometry output import node and the bone surface geometry output import node, and then connect them to those nodes. The Extract T-Pose tab menu item creates a Rest Position SOP node with preset t-pose extract settings.

   • Use one of the Rest Position SOP nodes to extract the tpose attribute from the muscles simulation.

   • Use the other Rest Position SOP nodes to extract the tpose attribute from the animated bone geometry.

4. Merge the muscle simulation and bone surface output t-poses.

   In order for the Tissue Solidify SOP node to able to use both the muscle simulation output’s t-pose and the animated bone surface output’s t-pose, you need to merge their point transform rest data.

   Create a Merge SOP node and then connect both the muscle simulation output’s t-pose node and the animated bone surfaces output’s t-pose node to the Merge SOP node.

5. (Optional) Triangulate the tissue surface externally.

   The skin surface must be triangulated before tetrahedralization. Traditionally this is done by using inside the Remesh Surface settings that are inside the Tissue Solidify SOP node. However, if you want more control over how the skin surface polygons are triangulated—especially if you are concerned about the scale of the triangles—then you can use an external Remesh SOP node and complimentary attribute nodes to triangulate the skin surface’s geometry.

   Create and use the following nodes:

   • Create an Attribute Paint SOP node, connect it to the skin surface geometry’s import node, type the name of the target mesh size attribute you want to use in the Attribute Name parameter field, and then use the node to paint the target mesh size attribute values on the tissue surface to affect the scale of the triangles.

   • Create an Attribute Remap SOP node, connect it to the Attribute Paint SOP, set its Original Name parameter value to the same attribute you defined in the Attribute Paint SOP, and then use it to map the target mesh size painted values to the desired minimum and maximum values you'd like to use in the Remesh SOP node.

   • Create a Remesh SOP node, connect it to the Attribute Remap SOP node, and the use it to triangulate the skin surface geometry before building the tetrahedra.

   • Create an Attribute Delete SOP node, connect it to the Remesh SOP node, and use it to delete the target mesh size attribute you created with the Attribute Paint SOP as it is no longer needed in the Muscle & Tissue workflow.

6. Triangulate the tissue surface internally and then tetrahedralize it.

   1. Create a Tissue Solidify SOP node, connect the Merge SOP node to its Input 2, and then connect the imported bone surface geometry’s Extract T-Pose SOP node to its Input 4.

   2. Make sure that the input skin surface is tessellated with clean equilateral triangles. The skin surface must be triangulated before tetrahedralization. To do this, use the internal Remesh Surface settings on the Tissue Solidify SOP node.

   3. The Tissue Solidify SOP node automatically constructs the skin pass from the triangulated input skin surface geometry. The tissue pass is made up two layers, which in turn are composed of two geometry pieces each: a core layer with a tetrahedral solid core mesh (core solid) and a triangulated polygonal core surface (core surface), and a tissue layer with a tetrahedral solid tissue mesh (tissue solid) and a triangulated polygonal tissue surface (tissue surface).

   4. Adjust the Tissue Solidify SOP's parameter settings as you desire.

      • The Surface Offset parameter specifies the distance to inset this surface prior to tetrahedralization. This offset distance is intended to give some allowance for the skin pass downstream.

      • The Tissue Relative Thickness parameter controls how deep to build the tissue solid relative to the local depth of the space enclosed by the skin surface.

      • The Core Falloff parameter creates a weight mask attribute that represents the distance from the core surface layer to the geometry provided on the Tissue Solidify SOP node’s Input 4. This corefalloff attribute is also used by the downstream Tissue Vellum Solver SOP node when stiffening the attachment constraint between the core solid and the muscles and bones.

      Notes

      • In most cases, the Surface Offset value should match the value of the Skin Thickness parameter on the Skin Solidify SOP node downstream. However, this is not a requirement.

      • (Optional) You can supply external core surface geometry with the Input 3 on the Tissue Solidify SOP node if generating it procedurally isn’t sufficient. If you supply your own core surface, then it will override the Tissue Relative Thickness parameter’s value.

      • (Optional) You can supply custom poly lines for the core’s falloff with the Input 4 on the Tissue Solidify SOP node. These lines determine from where the sucking in force emanates to pull the tissue in towards the muscles and bones. This force’s strength is determined by the Rest Scale parameter on the Tissue Properties SOP node.

7. (Optional) Cache the tissue geometry to disk.

   We recommend that you save your tissue geometry to disk instead of cooking it each time you touch your tissue pass network.

   Create a File Cache SOP node, connect it to the Tissue Solidify SOP node, and then use it to cache the results of the tissue’s tetrahedralization.

8. (Optional) Paint tissue property masks.

   If you want to paint masks on your skin so that only certain skin properties and values apply to certain regions on your skin, then create an Attribute Paint SOP node, connect it to the Tissue Solidify SOP node, and then use it to paint those regions. You can then use the Mask Attributes parameter on individual Tissue Properties SOP nodes to apply specific skin property settings to the mask regions you painted.

9. Set tissue properties.

   Create a base Tissue Properties SOP node that sets the general settings for all of your tissue, and then create individual Tissue Properties SOP nodes that each set tissue settings that contribute to a specific look or characteristic you want in the tissue. For example, you could create a Tissue Properties SOP to define the settings for tight tissue, and then create a separate Tissue Properties SOP to define the settings for saggy tissue.

   Since the tissue pass is made up of two geometries, the tissue surface and the tissue layer, there are also two sets of constraints that operate on the outer and inner tissue and connect it to the underlying core layer.

   • The tissue surface and tissue solid parameters are separated into their own tabs. On both tabs you can find material property parameters that affect Shape Stiffness, Volume Stiffness, Damping Ratio, and Mass Density.

   • The attachment parameters in the Tissue Surface Layer tab control how tightly or loosely the tissue surface will attach to the muscles and bones within your character. This constraint is also affected by the Sliding parameters.

   • In a similar fashion, the attachment parameters in the Tissue Solid Layer tab control the attachment of the innermost boundary points of the tissue solid tetrahedra to the core surface. This attachment constraint does not slide, but it can be weakened as much as desired to detach the tissue from the core completely or have it remain adherent.

   • When the attachment Rest Scale parameter’s value is set to less than 1.0, it will pull the tissue in towards the muscles and bones.

   • Sliding Rate determines how freely the attachment can slide.

   • Distance Limit creates a distance limit constraint that determines how far a point on the tissue surface can slide relative to its initial attachment point in the t-pose.

   • Limit Stiffness specifies the stiffness of the spring that imposes the distance limit.

   • Damping Ratio controls how sluggish vs. how springy the Distance Limit constraint is.

   • You can affect the Rest Scale for each of the tissue layer’s 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 state) and shrink or expand accordingly.

   See the Tissue Properties SOP node doc for detailed descriptions of each parameter.

10. (Optional) Make areas of your tissue rigid.

    If you need parts of your tissue to attach as rigidly as possible to the tissue’s surface layer and ignore sliding, then create a Group Create SOP node, connect it to the Tissue Solidify SOP node, and then use it to define a rigid skin point group in your tissue pass network.

    If you define a rigid skin point group, then you also need to also enter the Group Name you defined on the Group Create SOP in the Advanced tab ▸ Rigid Group parameter field on the Tissue Solver Vellum SOP node downstream.

    Tip

    Periodically you may want to blast away portions of your character’s tissue to simulate a subsection for testing purposes. In situations like this, the points on the periphery of the deletion may become unconstrained and free-fall or slide undesirably (for example, imagine a sleeve of tissue sliding right off a limb). In these situations, we recommend that you use a point group like this to temporarily anchor the severed tissue regions and prevent the rest of the tissue from being affected adversely.

11. Preroll and hold the tissue.

    Move the tissue pass to its start position in its character’s shot animation. The tissue needs to be in the correct world space position and pose before computing the tissue simulation.

    Create a Muscle Preroll SOP node, connect it to the last Tissue Properties SOP in your chain of properties nodes, and then use it to move your tissue pass to its start position and hold its position. This node should match the transforms from the upstream bone preroll from your muscles set-up.

    Even though the tissue pass is static and not changing over time, the Muscle Preroll SOP node takes advantage of its ability to match the transforms that the muscles and bones have undergone while transforming from the t-pose rest position to the shot start frame position.

12. Get the muscle solid and bone surface outputs from the muscles pass.

    Retrieve the muscle solid geometry and bone surface geometry outputs from the muscles pass. This geometry is animated and has the preroll baked in. The Tissue Solver Vellum SOP node needs this geometry so that it can simulate the tissue.

    There are many ways to bring in or import geometry with SOPs. For example, you can use an Object Merge SOP node, a File SOP, a Stash SOP, an Alembic SOP, and so on.

13. Merge all muscle and bone output geometry.

    In order for the Tissue Solver Vellum SOP node to able to use both the muscle solid output’s t-pose and the bone surface output’s t-pose, you need to merge their geometry.

    Create a Merge SOP and then connect both the muscle solid output’s import node and the bone surfaces output’s import node to the merge node.

14. Simulate the tissue.

    Compute the tissue simulation using the Vellum solver.

    Create a Tissue Solver Vellum SOP node, connect the upstream tissue preroll node to its Input 1 and the merge node to its Input 2, and then set all the parameter settings you desire for the solve.

15. Clean up the tissue simulation.

    Create a Blast SOP node and connect it to the Tissue Solver Vellum SOP node.

    And then create a Clean SOP node and connect it to the Blast SOP node.

    Use the Blast SOP and Clean SOP nodes to remove the solid (tetrahedral) component of the tissue (since you no longer need it) and any attributes you no longer need. However, be sure to keep the tpose attribute as it is used in the later stages of the Muscles & Tissue simulation process.

16. Cache your tissue simulation to disk.

    Create a File Cache SOP node, connect it to the Clean SOP node, and then use it to cache the results of the tissue simulation to disk.

Once you have finished simulating the tissue for your character, you have the option to create a skin pass for your character. The skin pass allows you to create an addition tissue layer for your character that adds an additional level of detail to its outmost surface that you can’t achieve with the tissue pass alone. For example, by varying both the skin’s properties and solver settings, you can create skin effects like fine wrinkling and crepeing or delicate folds and creases.

The skin pass is comprised of two separate pieces of geometry:

• A triangulated polygonal skin outer surface (skin surface)

• A tetrahedral solid skin inner mesh (skin solid).

By building the skin pass with this added thickness, buckling, folding, and dynamic effects can behave more like a fatty epidermal layer and less like a layer of cloth.

The following section lists the steps for recreating the example skin network above. You can adapt these steps to create your own basic skin pass from scratch.

1. Import skin surface geometry.

   Import your original high-resolution skin surface geometry. You need this geometry to generate the skin pass.

   There are many ways to bring in or import geometry with SOPs. For example, you can use an Object Merge SOP node, a File SOP, a Stash SOP, an Alembic SOP, and so on.

2. (Optional) Remesh the skin surface externally.

   The skin surface must be triangulated before tetrahedralization. Traditionally this is done by using inside the Remesh Surface settings that are inside the Skin Solidify SOP node.

   However, if you want more control over how the skin surface polygons are triangulated, then create an external Remesh SOP node, connect it to the skin surface import name, and use it to triangulate the skin surface’s geometry.

3. Triangulate the skin surface and then tetrahedralize it.

   1. Create a Skin Solidify SOP node and connect it to the high-resolution skin surface geometry’s import node.

   2. Make sure that the input skin surface is tessellated with clean equilateral triangles. The skin surface must be triangulated before tetrahedralization. If it is not triangulated, then use the internal Remesh Surface settings on the Skin Solidify SOP.

   3. The Skin Solidify SOP node automatically constructs the skin pass from the triangulated input skin surface geometry. The skin pass is made up of two separate geometry pieces: a polygonal outer surface (skin surface) and a tetrahedral solid inner mesh (skin solid).

   4. Adjust the Skin Solidify SOP's parameter settings as you desire.

      • The Skin Thickness parameter specifies how deep the skin solid layer should be.

      • The Regular Layers parameter determines how many layers of tetrahedrons are packed into the skin’s overall thickness.

      Note

      In most cases, the Skin Thickness value should match the value of the Surface Offset parameter on the Tissue Solidify SOP node upstream. However, this is not a requirement.

4. (Optional) Make areas of your skin rigid.

   If you need parts of your skin to attach as rigidly as possible to the tissue’s surface layer and ignore sliding, then create a Group Create SOP node, connect it to the Skin Solidify SOP node, and then use it to define a rigid skin point group in your skin network.

   If you define a rigid skin point group, then you also need to also enter the Group Name you defined on the Group Create SOP in the Advanced tab ▸ Rigid Group parameter field on the Skin Solver Vellum SOP node downstream.

   Tip

   Periodically you may want to blast away portions of your character’s skin to simulate a subsection for testing purposes. In situations like this, the points on the periphery of the deletion may become unconstrained and free-fall or slide undesirably (for example, imagine a sleeve of tissue sliding right off a limb). In these situations, we recommend that you use a point group like this to temporarily anchor the severed tissue regions and prevent the rest of the skin from being affected adversely.

5. (Optional) Paint skin property masks.

   If you want to paint masks on your skin so that only certain skin properties and values apply to certain regions on your skin, then create a Attribute Paint SOP node, connect it to Skin Solidify SOP node, and then use it to paint those regions.

   You can then use the Mask Attributes parameter on individual Skin Properties SOP nodes to apply specific skin property settings to the mask regions you painted.

   Tip

   You can paint a thickness multiplier attribute to vary the skin’s thickness. To do this, turn on the Thickness Multiplier Attribute parameter on the Skin Solidify SOP node upstream and then specify the name of the attribute you want to paint with. By default, this attribute is skinthickness. Then on the Attribute Paint SOP node, create a Attributes tab ▸ Attributes multiparm and type the attribute’s name in its the Attribute Name parameter field.

6. Set the skin properties.

   Create a base Skin Properties SOP node that sets the general settings for all of your skin, and then create individual Skin Properties SOP nodes that each set skin settings that contribute to a specific look or characteristic you want in the skin. For example, you could create a Skin Properties SOP to define the settings for sliding skin, and then create a separate Skin Properties SOP to define the settings for loose skin.

   Since the skin pass is made up of two layers, the polygonal surface outer layer and the tetrahedral mesh inner layer, there are also two sets of constraints that operate on the skin and connect it to the underlying tissue.

   • The Surface Layer tab has all the parameters that affect how the skin surface attaches to the tissue as well as the physical properties of its triangles. To determine how tightly the skin surface will press onto the underlying tissue’s surface layer, adjust the Attach Stiffness, Damping, and Rest Scale parameter settings.

   • The Solid Layer tab has similar parameters that apply to the tetrahedra in the skin solid. To determine the degree to which the skin solid sticks to or detaches from the underlying tissue surface, adjust the Attach Stiffness, Damping, and Rest Scale parameters.

   • You can also adjust the the parameters in the Sliding and Shrinkage tabs to adjust those behaviors respectively.

   See the Skin Properties SOP’s node doc for detailed descriptions of each parameter.

7. Get the tissue simulation cache.

   Retrieve the tissue simulation cache’s data. The Skin Solver Vellum SOP needs this geometry to compute the skin simulation.

   There are many ways to bring in or import geometry with SOPs. For example, you can use an Object Merge SOP node, a File SOP, a Stash SOP, an Alembic SOP, and so on.

   If your cached tissue geometry does not have a built-in preroll, then also create one with a Muscle Preroll SOP node so that the tissue is in its start position before the skin simulation is computed.

8. Preroll and hold the skin.

   Move the skin pass to its start position in its character’s shot animation. The skin needs to be in the correct world space position and pose before computing the skin simulation.

   Create a Muscle Preroll SOP node, connect it to the last Skin Properties SOP in your chain of properties nodes, and then use it to move your skin pass to its start position and hold its position. This node should match the transforms from the upstream bone preroll from your muscles pass.

9. Simulate the skin.

   Compute the skin simulation using the Vellum solver.

   Create a Skin Solver Vellum SOP node, connect the upstream skin preroll node and the tissue cache import node to it, and then set all its parameter settings you desire for the solve.

   When simulating the skin pass, it is important to understand the following constraint relationships:

   • The skin surface is constrained to the tissue surface. Collisions and sliding are enabled for this constraint relationship.

   • The skin solid’s interior points are constrained to the tissue surface. Collisions are disabled and sliding is enabled for this constraint relationship. However, a distance-limiting spring attachment restricts sliding from exceeding the Distance Limit set on the Skin Properties SOP node. This means that the skin surface’s sliding is restricted indirectly by the stiffness of the skin solid.

10. Clean up your skin.

    Create a Blast SOP node and connect it to the Skin Solver Vellum SOP node.

    And then create a Clean SOP node and connect it to the Skin Solver Vellum SOP node.

    Use the Blast SOP and Clean SOP nodes to remove the solid (tetrahedral) component of the skin (since you no longer need it) and any attributes you no longer need. However, be sure to keep the tpose attribute as it is used in the final stage of the Muscles & Tissue set-up process.

11. Cache your skin simulation to disk.

    Create a File Cache SOP node, connect it to the Clean SOP node, and then use it to cache the results of the skin simulation to disk.

Once you have completed simulating your character’s Muscles & Tissue simulation, you can then transfer all the point transformations from the tissue (if you did not use a skin pass in your set-up) or skin simulation to the high-resolution mesh you will be using in your shot.

To transfer the simulation generated point transforms to your high-resolution mesh:

1. Import your original high-resolution skin surface geometry (the original mesh you used to create the tissue and skin passes) and then also fetch the simulation cache from your skin simulation.

   There are many ways to bring in or import geometry with SOPs. For example, you can use an Object Merge SOP node, a File SOP, a Stash SOP, an Alembic SOP, and so on.

2. Use an Extract T-Pose SOP node to extract the t-pose for your character’s skin from the node you used to fetch the skin simulation cache. This will give you the rest position (tpose attribute) that you can then use for the point deform operation.

3. Transfer the point transforms from your skin simulation cache to your character’s original high-resolution skin surface geometry with the Point Deform SOP node.

   If your original surface had UVs, textures, and so on, they are preserved as the Point Deform SOP only uses the simulation output to deform the original mesh.

   Tip

   You can adjust the Point Deform SOP’s parameters to vary the final look of your character’s skin by introducing more or less averaging (blurring) into the point transforms.