Hit & Miss Tracking.


Hit & Miss Tracking.

Created by Chris_Ae
8 Months Ago
June 1, 2019 @ 0:09 pm

I do get baffled by the tracking thing. I don't get a good track when I expect to and vice versa. I have this HUD animation which follows the actor perfectly via a tracking marker placed on an earlobe. The actor is walking away from the camera and I fully expected the marker to lose its position - but I was pleasantly surprised (the actor ends up a long way from the camera).

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.

martin_mocha
8 Months Ago
June 3, 2019 @ 1:29 am

With Mocha, you have two main things to consider when tracking an object:
  1. How much texture there is
  2. How planar the motion is

With an arm, if there is not much texture detail in the skin or clothes but a lot of high-contrast detail behind the arm (i.e the grass you mentioned), then a more accurate roto is required to avoid the tracker finding the background more interesting. Sometimes in these situations you'll get a better track by placing the shape inside the object as opposed to around it, especially when the object has low-contrast detail compared to its surroundings.

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:

  • Faces/Heads track well (as long as you track to the natural planes of the head)
  • Arms/legs are usually more difficult, depending on how close you are tot he subject
  • Hands are usually okay
  • Fingers are a nightmare, unless up close
  • Torsos track well, but be aware they are bendy in the middle and may have two planes of motion
  • Eyes are pretty forgiving, even when the actor blinks

Chris_Ae
8 Months Ago
June 3, 2019 @ 9:23 am

Many thanks for that, Martin.

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.

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