If you want a muscle to have some details (for example, like showing grooves or indentations) when it contracts that it does not have when it is relaxed, then you can use this node to specify the target shape you want the muscle to try to achieve when it flexes.

1. In the Network Editor, import the static sculpted polygonal surface you want to use as the target shape for your muscle. This will be the shape your muscle will try to achieve anytime it flexes during it’s 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. Create a tpose point attribute from the original undeformed muscle surface.

   • Create a Set T-Pose SOP node. The Set T-Pose tab menu item creates a Rest Position SOP node with preset Rest ▸ Store Rest t-pose settings.

   • Use the Rest Position SOP to set the muscle’s original undeformed muscle surface as its rest surface.

     • Connect the output of the target shape’s import node to the first input (input 1) on this node, and then connect the output of your static muscle geometry import node to the second input (input 2) on this node.

       

     • In the Parameter Editor, type tpose in the Rest Attrib field.

3. Set up the relationship between the base muscle tetrahedral mesh and the modeled surface variation of the muscle that will allow the muscle to morph from one to the other.

   • Create an FEM Deform SOP node.

   • Connect the output from the Muscle Solidify SOP node to the first input (input 1) of the FEM Deform SOP node.

   • Connect the output from the target shape’s Rest Position SOP node (tpose) to the second input (input 2) of the FEM Deform SOP node.

     

   The surface points of the base muscle tetrahedral mesh are now constrained to the modeled muscle surface variation as it transforms to the target shape over the course of the muscle’s simulation. The muscle’s tetrahedral interior points are now also simulated using a quasistatic solver to maintain the muscle’s shape and volume.

4. Remove any time dependencies from the FEM Deform SOP's output.

   Since the FEM Deform SOP node produces a static model, you want to remove any time dependencies it may have before sending it downstream in the muscle network.

   • Create a Time Shift SOP node.

   • Connect the output from the FEM Deform SOP node to the input of the Time Shift SOP node.

     

   • Select the Time Shift SOP node.

   • In the Parameter Editor, keep the default Method (By Frame), remove the $F expression from the Frame parameter field, type 1 in the Frame field, and then press Enter.

5. Create a targetshape point attribute from the modeled variation of the muscle.

   • Create a Set T-Pose SOP node. The Set T-Pose tab menu item creates a Rest Position SOP node with preset Rest ▸ Store Rest t-pose settings.

   • Use the Rest Position SOP to set the modeled variation of the muscle as its target surface.

     • Connect the Muscle Solidify SOP node’s output to the first input (input 1) on this node, and then connect the Time Shift SOP node’s output to the second input (input 2) on this node.

       

     • In the Parameter Editor, type targetshape in the Rest Attrib field.

6. Specify a custom shape activation attribute for the muscle.

   • Select the Muscle Solver Vellum SOP node in your muscles network.

   • In the Parameter Editor, scroll down to the Activation Attributes section in the Simulation tab.

   • In the Shape parameter field, delete the muscletension attribute, type the name of the custom shape activation attribute (for this example’s procedure, that would be muscleshape) you want to use, and then press Enter. You can use this attribute to drive and animate the muscle’s morph between its original undeformed muscle surface shape (relaxed state) and the shape of its modeled muscle variation (flexed state).

     

7. Connect the muscle to the custom shape activation attribute.

   • Just below your the main Muscle Flex SOP node in your muscle network that sets up the muscletension for you muscles, insert a second Muscle Flex SOP node.

     

   • Select this new Muscle Flex SOP node and then open the Parameter Editor.

   • In the Attribute field at the top of the listed node parameters, type the name of the custom shape activation attribute you specified in the Activation Attributes ▸ Shape field on the Muscle Solver Vellum SOP node in step 6, and then press Enter. For example, muscleshape.

   • Click Detect New.

     All incoming muscles are automatically detected and one Muscle ID parameter (flex control) per muscle_id appears in the Muscle Flex tab.

   You can now animate these parameters to determine when the muscle tries to achieve the shape of the target surface and how much of the target surface is reflected in the muscle’s shape as it contracts.

   

8. Animate the custom shape activation attribute over the course of the muscle’s simulation.

   On the shape activation’s Muscle Flex SOP node, select the Muscle Flex tab, and then keyframe the Muscle ID parameters over time as needed. A value of 0 is 100% the tpose shape (rest surface) for the muscle, and a value of 1 is 100% the targetshape shape (target surface).

   This will morph the muscle between its defined tpose shape and targetshape shape over the course of its simulation.