For starters, try tracking the inside corners of your tape markers. For example in the image you posted, try the bottom right corner of the tape at the bottom left corner of the green card. This will have better contrast and not get thrown off as easily as the corners of the green card your child is holding. There will be an offset, but you can use the Mimic Source option and the layer corner pin controls to expand the video to be slightly larger than the card. In the future, instead of placing the markers at the edge, I’d suggest placing them just inside the corner, so they are surrounded by the green on all sides. They can also be a little bit smaller.
If you still have trouble here’s the general worklofow I use to solve difficult tracking situations:
1. Start isolating: Instead of tracking everything at once, focus on one point at a time. Motion lets you enable/disable tracks while working on a shot. Uncheck failing tracks and let the software complete the "easy" ones first. Then go back and focus on the failing tracks.
2. For troublesome tracks, uncheck the completed tracks in the inspector to protect them and speed up analysis. Now focus on the failing track(s) one at a time. Select the track and play through and see if you can figure out why it might be failing (obscured point, heavy movement, etc.) From here you can try several things:
-Adjust the size of the track region.
-Increase the search area (good when things move quickly between frames).
-Reduced the fail tolerance. This will cause analysis to stop, but will allow you to more quickly adjust and reset.
With the tracker selected, watch the preview in the inspector, if it fails, stop analysis and reposition the tracker, then start analysis again.
3. Break the tracking up into sections. I point this out to people all the time, some shots will work better if you focus on segments rather than expecting analyze from the beginning and trying to go all the way through to the end. Sometimes you need to break the track into sections (using the in/out play range). As I mentioned previously, take care of the easy parts first, then move on to the more challenging sections.
4. Try tracking backwards. I've seen tracks that drift every time when tracked forward work flawlessly when tracking backwards. You can use the same techniques I've mentioned above. Some shots need forward/reverse and sectioned off tracks.
Finally,review the Motion Documentation on tracking, it’s filled with suggestions and workflows for what to do with failed tracks as well as tips for getting better results:
http://help.apple.com/motion/mac/5.1/#motn18191fa3
