First runs of new MagPoly tool on the femur:

First user performs several clicks on perceived contour of the femur. Computer calculates the edge pattern next to the click and selects a "snapping" location setting an anchor point.
Two important points:

a) Even if snapping did not work properly - operator proceeds with the next points (there will be a cleanup stage later).
b) Work is performed on a full Zoomout, so User does not have to Zoom In and back and scroll!
c) In between the anchor points, a cardinal spline with application-specific tension is used.
d) 10-voxel snapping radius was used.

Each anchor point corresponds to a single click

In the followup stage. User returns and drags some anchors to improve their positions. In this example about 6 anchors were corrected.

After dragging 6 anchors.