Houdini 20.0 Nodes Geometry nodes

Muscle Tension Lines geometry node

Creates and attaches line segments to input bone surface geometry that then drive muscle flexion.

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Since 19.0

This node draws tension lines (primitives) onto your input bone geometry which is internally held at its t-pose (rest position). These lines are used as measuring sticks. The length changes of these lines throughout your character’s animation are used to drive its muscle flexion. Each tension line is linked to a specific muscle or series of muscles in the Muscle Flex SOP node.

Muscle tension lines and all visualization guides in the viewport state

At initialization, the length of each muscle tension line is measured and stored in a restlength primitive attribute. As your bone geometry is animated, the endpoints of the tension lines are automatically deformed along with the local transformation of the bone points in the vicinity of the endpoints. The lengths of the tensions lines are measured as their points are transformed and the results are stored in the length primitive attribute.

When the tension lines are at their rest length, they are at 100% of their length. When the lines shorten, they indicate that their linked muscles should be flexed. By default, when a tension line is at 75% of its rest length, its muscle is considered fully flexed. When the lines lengthen, they indicate that their muscles should be relaxed. You can adjust this activation threshold for the muscle tension lines from the Muscle Flex SOP node. For example, if you set the activation ratio for a muscle to 0.50, then when its tension line is at 50% its rest length, the muscle will be fully flexed. You can also control and animate the flex for individual muscles over your character’s animation with the Muscle Flex parameters on the Muscle Flex SOP node.

How-to

Tip

While bones are the most commonly used input geometry for the Muscle Tension Lines SOP, using bones as the guide geometry is not mandatory. You can draw the muscle tension lines relative to any input geometry. For example, like the input skin surface geometry.

To...Do this

Draw a muscle tension line on your character’s bone geometry in the node’s interactive viewport state

  1. In the viewport state, press A. You are now in tension line creation mode.

  2. LMB-click+hold on the bone surface location for the first tension line end point, LMB-drag the brush to the location on the bone surface for the last tension line end point, and then release the mouse button.

The new muscle tension line appears on your bone geometry and is named autoflex# by default. You can use the tension line’s Line ID parameter to change its name identifier. autoflex_id

Place a muscle tension line by placing end points on your character’s bone geometry

  1. In the Parameter Editor, select the Rest Geometry multiparm for the line you want to draw. Create a new multiparm if needed.

  2. Select the arrow icon next to the Point 0 parameter, and then in the viewport click the bone surface location where you want to place the first end point for your tension line.

  3. Back in the the Parameter Editor, select the arrow icon next to the Point 1 parameter and then in the Viewport click the bone surface location where you want to place the last end point for your tension line.

The new muscle tension line appears on your bone geometry. You can use the tension line’s Line ID parameter to change its name identifier.

Modify the placement of a muscle tension line’s end points

  1. In the viewport state, press S+hold to enter tension line select mode.

  2. Hover your mouse pointer over the line you want to select, LMB-click the line, and then release the S key.

  3. LMB-drag the muscle tension line’s translation handles to modify their placement. The endpoints only need to be near your bone surfaces. They do not need to be attached to them. However, moving the tension line end points will cause them to snap to the nearest surface.

Select a muscle tension line

Do one of the following:

  • In the viewport state, MMB-scroll to cycle your selection through all your current tension lines.

  • In the viewport state, press S+hold to enter tension line select mode, hover your mouse pointer over the line you want to select, LMB-click the line, and then release the S key.

Delete a muscle tension line

Do one of the following:

  • In the viewport state, select the muscle tension line and then press Delete.

  • In the Parameter Editor, remove the muscle tension line’s Rest Geometry multiparm.

State

Muscle Tension Lines state hotkeys

Hotkeys or Interaction

Action

Press S+Hold

Enter muscle tension line selection mode.

  • LMB-click to select a muscle tension line.

  • Once selected:

    • LMB-drag the end points from a muscle tension line to reposition them.

    • Press Delete to remove the line.

Example: Click-selecting muscle tension lines in the viewport state

Note

You cannot select Symmetry-generated mirror duplicate muscle tension lines in the viewport state.

MMB-wheel scroll

Cycle through muscle tension line current selections.

Example: MMB-scrolling through the current muscle tension line selections

Press A

Enter muscle tension line creation mode.

To add a new muscle tension line in the viewport state, click-drag from the location of the first end point for the tension line to the location of the last end point.

Example: Drawing a muscle tension line in muscle tension line creation mode
Muscle Tension Lines state right-click menu

RMB menu > New Muscle Tension Line

Enter muscle tension line creation mode.

RMB menu > Delete Current Line

Remove the currently selected tension line.

Attributes

autoflex_id

 integer

Sets and stores the descriptive label or name given to each muscle tension line.

length

 integer

Stores the active length of the muscle tension line over time.

restlength

 integer

Stores the length of the muscle tension line measured while its bone geometry is in its t-pose.

Parameters

Line Count

  • The number of muscle tension lines. This count does not include the symmetrical copies.

  • Adds or removes a Rest Geometry multiparm.

    1 - Deletes the currently selected multiparm tab.
    2 - Inserts a new multiparm tab to the right of the currently selected multiparm tab.
    3 - Currently selected mulitparm tab.
    4 - Adds or removes multiparm tabs from the end of the list. Increasing the number adds tabs and decreasing the number deletes tabs.
    5 - Adds a new multiparm tab to the end of the list.
    6 - Removes the multiparm tab from the end of the list.
    7 - Deletes all the current multiparm tabs.

Muscle Tension Lines

Rest Geometry

Line ID

Specifies a descriptive label for the muscle tension line. This parameter’s value is stored as the autoflex_id primitive attribute.

The default Line ID for new muscle tension lines is autoflex#. For easier matching when setting up the link between the muscle tension line and the muscle(s) it will be driving, give the tension line a Line ID that is similar to the muscle_ids of its target muscle(s).

Point 0

Specifies the local space position of the first end point (end point 0) for the muscle tension line. You can use the arrow icon next to this parameter to interactively place this end point in the viewport state.

Point 1

Specifies the local space position of the last end point (end point 1) for the muscle tension line. You can use the arrow icon next to this parameter to interactively place this end point in the viewport state.

Inputs

Bone Geometry

T-Pose

Specifies the pose the bone geometry is in when you are setting up its muscle tension lines.

Use Initialization Frame

Use the bone geometry’s pose at the specified Frame.

From Attribute

Use the bone geometry pose that is stored in the attribute specified by Attribute Name.

Frame

Specifies the initialization frame number. By default, the initialization frame is set to the first frame of your playback bar’s range ($FSTART).

This parameter is only available when T-Pose is set to Use Initialization Frame.

Attribute Name

Specifies the name of the attribute that contains the saved t-pose point positions for the bone geometry. By default, Attribute Name is tpose.

This parameter is only available when T-Pose is set to From Attribute.

Additional Lines

Default Line ID

Specifies the name prefix for all user-supplied secondary input tension line geometry. For example, if Default Line ID is the extraflex default value, then all external tension lines are automatically assigned the sequential autoflex_ids of extraflex0, extraflex1, extraflex2, and so on.

This parameter is only available when there is external poly line geometry attached to Input 2 of the Muscle Tension Lines SOP node.

Note

You cannot give each external tension line a fully unique name.

Symmetry

These parameters automatically generate the muscle tension lines and their Line IDs for the second half of your symmetrical bone geometry once you have created the muscle tension lines for the first half.

Automatic symmetry is only applicable to whole intact symmetrical models. If your input bone geometry is only one half of a symmetrical model, then you can use the Mirror Muscles SOP downstream in your Muscles & Tissue network to make sure that all your muscle tension lines are created and assigned the appropriate Line ID values.

Enable Symmetry

When on, the muscle tension lines on the other side of your model are automatically created and given Line IDs using the rules defined by the From Prefix and To Prefix parameter settings.

Note

Since the mirror duplicates of the muscle tension lines are created procedurally, so you won’t be able modify their endpoints in the viewport state.

From Prefix

Specifies the name prefix that is used for your Line ID values. For example, this parameter value would be L_ if your Line ID values are L_gastro, or L_extensor, and so on.

To Prefix

Specifies the name prefix that will replace the From Prefix in all of the automatically generated Line ID values for the mirrored duplicate tension lines. For example, if the parameter value for From Prefix is L_, then your To Prefix parameter value should be R_ for R_gastro, or R_extensor, and so on.

Symmetry Handle

When on, the symmetry plane and its handle/manipulator is shown in the viewport state. You can use this handle to interactively adjust the Origin, Distance, and Direction of the symmetry plane.

Origin

XYZ position of the symmetry plane relative to the origin of the bones.

Distance

Distance offset of the symmetry plane from the Origin.

Direction

Across which plane (X, Y, or Z) to mirror the tension lines. Also determines the normal direction of the symmetry plane.

Guides

Display Bone Geometry

When on, you can view the guide bone geometry in the viewport state. You can use this geometry as a placement guide when drawing your muscle tension lines.

Bone Geometry Color

Specifies the RGB color and transparency (alpha) for the guide bone geometry.

Line ID

When on, the line IDs for the current selected muscle tension line appears next to its line geometry in the viewport state.

Line Length

When on, the length for the current selected muscle tension line appears next to its line geometry in the viewport state.

Line Color Random Seed

Changes the random colors that are assigned to your muscle tension lines in the viewport state.

Symmetry Color Intensity

Adjusts the intensity of the line colors that are applied to the mirrored copies of the muscle tension lines.

This parameter is only available when Enable Symmetry is turned on.

Inputs

Input 1

Guide geometry for placing the muscle tension lines. Most commonly this is bone geometry.

Input 2

User-supplied poly lines that replace the auto-generated muscle tension lines. Each external line is automatically assigned an autoflex_id attribute.

Outputs

Output 1

Line IDs (autoflex_id) for each muscle tension line and their rest (restlength) and variable length (length) values.

See also

Geometry nodes