However, when I had the same actor standing on the spot and bending his arm at the elbow because he's looking at his missing lower arm that's just been blown off - Mocha Ae had trouble following that simple and slow movement - despite some high contrast going on - a basically white arm against a sea of grass. While playing with the spline tool though, I noticed the more accurate setting up of the track the better (of course). So, I drew around the arm more accurately after the first track failed - but the clincher was adding a ton of handles. I swamped the shot with them and the result was a very accurate track.
Arms, being organic and fairly non-planar, will also not track well if they rotate along their axis a fair way, simply because more information will be revealed or obscured as the arm turns.
An ear on the other hand, is fairly planar and has a lot of ridges and detail to track, so will often do better.
For human bodies we generally find:
Placing the shape inside the object - very interesting! I shall certainly try that.
I love it that Mocha is integrated into After Effects - I'm sure many Ae users like myself really appreciate that. I'm not hugely experienced at this, but Mocha Ae worked a treat in a previous project where Ae struggled. I was trying to track a cartoon UFO and the animation was .PNG - therefore transparent - but despite no busy background to confuse the tracking, Ae markers broke free and became wayward. Mocha Ae though, worked perfectly in that instance.
Thanks again for your detailed response.