User Guide > Characters > Rigging   

Capturing a Simple Arm

In this lesson you will create a simple arm setup that lets you explore the basic elements of binding geometry. In Houdini, the bones contain Capture regions that provide a 3D region for capturing geometry.

To follow through this lesson you can use the following start file. This file contains an arm surface. You will build the bones and the bind the arm.

Load Start File

1. Draw the Arm Bones

In the Viewer pane, press tab > Character > Bones. Before drawing, go to the Operation Control bar and set the chain Chain Name to arm and Kinematics to Inverse Kinematics.

Left click at the top of the arm, at the elbow and in front of the wrist area. It is important to put a bit of a kink in the chain to help give the IK solver some direction. RMB-click to accept. If you draw too many bones, press the delete key to step back.

In the Network pane, you can see that the chain is created with a root node, two bones and a separate end effector. You also get a KIN_Chops network manager that places your CHOP network where it can be easily bundled with the character if you want to create a digital asset.

2. Get Ready to Capture the Geometry

Press tab > Generic > Transform. Press the t key to focus on the transform handle then click on the End Effector and RMB-click to accept. Move the end to straighten the chain to align with the arm geometry.

Press tab > Character > Edit Capture regions. Select both of the arm bones and RMB-click to accept. Adjust the capture regions using the handles so that they appear to nicely encompass the starting geometry. The capture regions are represented by a number of handles that let you interactively define the volume in which the capturing will occur. By manipulating the handles, the regions can be set up for the particular geometry you are working with. You can shift-click to manipulate all the handles on a region at the same time.

3. Capture the Geometry

Press tab > Character > Capture Geometry. In the Operation Control bar you can name your Channel group prefix to be more descriptive. Since this name will be used for any animation channels created for this arm, it is good to edit it before capturing.

Leave the Capture frame set to 0. This is where Houdini will calculate the pose and bind the geometry to the skeleton. Leave Type set to Regions this way even if you later add detail to your arm model, the new geometry will be bound as long as it continues to lie within the capture regions.

LMB-click on the arm geometry then RMB-click to accept. Next, LMB-click on the root of the arm chain then RMB-click to accept. You’ll see the geometry colored to match that of the bones to which it has been captured. If the initial capture isn’t totally correct, you’ll see your geometry colored white. If that’s the case, you will have to re-edit the capture regions.

4. View the Geometry (SOP) Network

Select the arm geometry and go into the SOP network, you can see that a Capture Operator and a Defrom Operator have been added to the existing chain. These nodes are using the bone movements to deform the geometry.

5. Pose the Arm

Go back up to the object level. Press tab > Character > Pose. Click on the forearm bone then RMB-click to accept. Move the resulting handle to pose the arm. You can now see the arm deforming with the bones.


The Capture Frame

The geometry was captured with keyframes set on the capture pose at frame 0. If you go to frame 0, you can move the end effector and therefore adjust the bone positions at the capture frame. This will change your capture solution to use this new pose.