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.
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