How to Attach a Surfboard (Snowboard or Skateboard) to the Feet The idea here is to build an IK Skeleton, attach IK handles so you can move and animate the limbs, then constrain the feet of the skeleton to a surfboard. Once the feet are constrained to the board then transformations made to the board will effect the skeletonŐs legs up to the waist. The result will mimic the kinematics of a figure surfing. Rotations and translations made to the board and the skeleton may then be keyframed for animation.
We will start with the board pre-modeled. I used a circle, modified the CVs to give the basic cross section of the board. Grouped it to itself for an extra pivot and animated the top node along a curve that provided the stringer or center of the board. Then, for the geometry of the board I set keyframes for scaling on the lower node and gave it an Anim Sweep. The scaling parameters where set from the tail to the nose. The modified circle is scaled small at the tail, wide at the center and very small at the nose to achieve a point. The stringer curve was modified to get the proper kick in the nose and tail before Anim Sweep.
Lets start first by building the Skeleton. Build in the Front window using the positive Z as the center axis for your skeleton. Open up the ASBD window for reference and to label nodes. Go to Windows - ASBD.
Go to Objects - Draw skeleton and you will begin creating the legs of your skeleton. Plot the first joint approximately at 0, 0, 8.
This joint will represent the center of gravity for your skeleton. Placing this node at the top of the hierarchy will stop limb translation around the waist and hips from affecting the rest of the skeleton. You will also be able to move the entire skeleton at this level.
In the ASBD window hold down the Ctrl and double click on this first joint node. Name it Lap.
Sometimes the modeling windows will interpret this action as a Magnet Snap if you are in Move mode. This will cause your joint to move. If you are in Pick - Object mode before you press Ctrl you will be able to name a node without a problem.
Plot the next joint at approximately .5, 0, 8. This joint will give you more freedom to move each leg independently. It will also allow you to create the right leg and not have to share the top node. This will be easier to depict motion that resembles hip sway. If you tried to transform an IK handle with the hip joint to the lap without this extra joint the whole skeleton would transform.
Label this node L_pelvis.
Plot the next joint at about .75, 0, 7.5.
Label this node L_hip.
Plot the knee in the Right Window at approximately 2.25, -.25, 3.75. You need to give the knee a slight bend. If you plot the knee joint in line with the hip and ankle joint you may get unpredictable results when you attach an IK handle. The IK handle constraint may lock the knee straight and transformations will be difficult.
The idea here is to build an IK Skeleton, attach IK handles so you can move and animate the limbs, then constrain the feet of the skeleton to a surfboard. Once the feet are constrained to the board then transformations made to the board will effect the skeletonŐs legs up to the waist. The result will mimic the kinematics of a figure surfing. Rotations and translations made to the board and the skeleton may then be keyframed for animation.
