KineFX is similar to Crowds in Houdini as it is a joint-based system, unlike the object-level rigging tools which are bone-based.

There are several key differences between the bones found at the object-level and the points in a geometry-level SOP skeleton:

• Bones have length, whereas joints only take their local space translation into account in order to offset themselves from their parents.

• Bones have a fixed orientation, which means that a bone’s child will always be offset in -Z by the length of its parent.

• Bones do not support scale compensation, making squash and stretch animation pipelines very unwieldy, which impacts performance.

• Deformation of a capture (skin) is not driven by the bone itself, it is instead driven by the Capture Region SOP inside the bone object. The difference between the Capture Region SOP’s capture pose and the current pose drives the object-level skin’s deformation.

• Object-level capture poses are attached to their skins. This mean that the skin will break if you start deleting parts of the skeleton. However with SOP skeletons, their capture poses are defined by the Joint Deform SOP’s second input. This means that you can alter a SOP capture pose without having to re-capture (re-skin) its geometry. For example, you can make changes to a skeleton’s joint orientations or tweak a character’s pivots and not affect its skin.

You can create a skeleton from scratch by drawing the joints using the Skeleton SOP node.

1. In the network editor at the geometry level, have your character geometry available. Turn on the template flag for your character’s geometry node.

2. Create a Skeleton SOP node.

3. Select the Skeleton SOP and turn on its display flag. You will now be able to see your character’s geometry while drawing its skeleton.

   

   Tip

   You can change how your templated character geometry is displayed in the viewport. From the Display Options window (press D when hovering over the viewport), choose Set display options for ▸ Template Model Geometry. Then choose one of the options in the Draw drop-down menu. We recommend using either Smooth Shaded or Smooth Wire Shaded.

4. Select the Skeleton SOP, click in the viewport, and press Enter. You are now in the Skeleton interaction state. In this state, you have access to the Skeleton toolbar above the display, the menu, and the skeleton hotkeys.

5. On the Skeleton toolbar, choose one of the following options from the Joint Placement drop-down menu for placing joints on the geometry:

   View Based

   In this mode, joints are placed at the mid-point of the geometry. Consider a straight line that goes from your current view through the geometry. Joints are placed at the mid-point of the closest and furthest intersection points with the geometry surface.

   Recommendation

   When working in the View Based joint placement mode, switch to an orthographic view that splits your character down the middle (for example, the Right view).

   Normal Based

   In this mode, Houdini places joints at the mid-point of the geometry, using the normal of the surface. Consider a straight line that goes through the geometry perpendicular to the geometry surface. Joints are placed at the mid-point of the closest and furthest intersection points with the geometry surface.

   The white circle shows the location inside the geometry that the joint would be placed.

   Freehand

   If the construction plane is displayed, joints will be snapped onto the construction plane. Display the construction plane by clicking on the right toolbar.

   If the construction plane is not displayed, joints will be lined up on a plane perpendicular to your viewing angle when you placed the joints.

   Surface

   In this mode, joints are placed on the surface of the geometry.

6. Choose Mode ▸ Create on the Skeleton toolbar or press F. You can now draw your joint chain in the viewport.

   Tip

   F toggles the Mode between Create and Modify.

You can create a skeleton and easily align the joints by snapping them onto the construction plane.

1. In the network editor at the geometry level, have your character geometry available. Turn on the template flag for your character’s geometry node.

2. Create a Skeleton SOP node.

3. Select the Skeleton SOP and turn on its display flag. You will now be able to see your character’s geometry while drawing its skeleton.

   Tip

   You can change how your templated character geometry is displayed in the viewport. From the Display Options window (press D when hovering over the viewport), choose Set display options for ▸ Template Model Geometry. Then choose one of the options in the Draw drop-down menu. We recommend using either Smooth Shaded or Smooth Wire Shaded.

4. Select the Skeleton SOP, click in the viewport, and press Enter. You are now in the Skeleton interaction state. In this state, you have access to the Skeleton toolbar above the display, the menu, and the skeleton hotkeys.

5. Display the construction plane in the viewport by clicking on the right toolbar.

   

   If the construction plane handle is not displayed, click the construction plane icon and turn on Construction Plane Handle.

6. Set the orientation of the construction plane by clicking the construction plane handle and choosing Align normal with world [X|Y|Z] axis.

   For example, selecting the Align normal with world X axis option will orient the construction plane so that its normal points along the X-axis.

   

7. On the Skeleton toolbar, select Joint Placement ▸ Freehand.

8. Draw your joints. The joint chain will snap to the construction plane.

In addition to drawing your own skeleton joints, you can also turn any SOP polygonal line into a joint chain with the Skeleton SOP node or the Rig Doctor SOP node.

1. In the network editor, create a SOP node that will generate a polygonal line, for example, a Line SOP node.

2. Select the Line SOP, click in the viewport, and press Enter. The line geometry will be displayed in the viewport.

   In the parameter editor, adjust the properties of the geometry. For example, increase the Length and Points parameters for the Line SOP. Each point in the geometry will become a joint in the skeleton. To see the points in the geometry, turn on Display Points on the right toolbar of the viewport.

   

3. In the network editor, create a Rig Doctor SOP node or a Skeleton SOP node.

4. Connect the Line SOP to the Rig Doctor SOP or Skeleton SOP.

   These nodes initialize the point transforms for the geometry as well as give them valid names. In KineFX, a valid hierarchy requires valid point names.

   

   You can now see the new KineFX skeleton in the viewport.

   

   Tip

   If you want to visualize your joint chain outside the Rig Doctor SOP or Skeleton SOP, you can use the Visualize Rig SOP node.

For simple joints chains, you can automatically orient joints using the Orient Joints SOP node.

1. Create an Orient Joints SOP node.

2. Connect the output of the Skeleton node to the input of the Orient Joints node.

3. In the parameter editor, specify a Reference Vector for the orient operation.

   This sets the direction of the joint axes that points toward the child joint. For example, if you want to orient the joints in your joint chain so that their X-axes look at their child joints, then specify a Reference Vector of 1 in the X-axis field.

   You can also set the direction of the Up vector by turning on the Up Vector parameter.

   The joints in your joint chain will automatically orient to the Reference Vector (and Up Vector) you specify.

   

Orientation picking allows you to orient joints to their parent or child joint. You can use orientation picking with the Skeleton SOP node when creating or modifying your skeleton, or with the Rig Pose SOP node when cleaning up your retargets.

1. In the network editor, select the Skeleton SOP or Rig Pose SOP.

2. Click in the viewport and press Enter to enter the Skeleton SOP or Rig Pose SOP’s interaction state.

3. If using the Skeleton SOP, turn on Child Compensate on the Skeleton toolbar.

   If using the Rig Pose SOP, turn on the World Space parameter.

4. click in the viewport, and turn on Display Joint Axes and Show Handle from the pop-up menu.

5. Select the joint whose orientation you want to change.

6. Press ; to enter orientation picking mode. The manipulator handle will change to the orientation picking handle.

   

7. Set the main axis (primary axis) that the joint will rotate around. We want the Z-axis to remain as is (since it is pointing down the arm from the elbow to the wrist), and have the other axes orient themselves around the Z-axis. So we will set the Z-axis to be the primary axis.

   ⌃ Ctrl click the blue box shape at the end of the Z-axis handle. The box will change to a pyramid shape. The Z-axis is now set as the primary axis.

   

8. Set the axis that you want to line up with a particular edge (the link between joints). This is the secondary axis. When you set the secondary axis, the joint orientation will rotate around the Z-axis (primary axis) to line up the secondary axis on the same plane as the selected edge.

   In this example, we want to line up the X-axis with the shoulder joint:

   • Click the red box at the end of the x-axis handle. The box will turn yellow to indicated that it is selected.

   • Click the edge that connects the shoulder joint to the elbow joint. Imagine a plane that includes the shoulder-elbow edge and the elbow-wrist edge. The x-axis will rotate about the Z-axis (primary axis) and align itself with this plane.

   This technique will give nice orientations for elbows and knees.

9. Press ; to exit orientation picking mode.

You can orient joints by specifying the primary and secondary axes from the viewport state parameters.

1. In the network editor, select the Skeleton SOP node.

2. Click in the viewport and press Enter to enter the Skeleton interaction state.

3. click in the viewport, and turn on Display Joint Axes from the pop-up menu.

4. Hold S and drag to select the joints you want to orient.

5. Hover over the viewport and press P to bring up the Parameters window.

6. In the Viewer State Dialog tab, change the Primary Axis and Secondary Axis:

   Primary Axis

   Axis that points toward the child.

   Secondary Axis

   Axis that points in the up direction.

7. click in the viewport and choose Orient Selected Joints from the pop-up menu. The axes in the joint chain will be updated.

You can adjust the joint rotation axes for a joint chain using the construction plane. In the below example, we will be orienting the axes of an elbow in a biped arm.

1. In the network editor, select the Skeleton SOP node.

2. Click in the viewport and press Enter to enter the Skeleton interaction state.

3. Zoom in on the joints you want to orient.

4. Select the joints in the joint chain surrounding the joint you want to orient. For example, if you want to orient the elbow, select the shoulder, elbow, and wrist joints.

   

5. Display the construction plane in the viewport by clicking on the right toolbar.

6. click in the viewport and choose Orient Construction Plane to Selection from the pop-up menu. The construction plane will orient itself to the selected joints.

7. Select the joint(s) you want to orient. In this example, we will select the elbow joint.

   

8. click in the viewport and choose Snap Selection to Construction Plane from the pop-up menu. The joint orientations will now be updated. This technique will give you nice orientations for elbows and knees.