Finding Erratic Trackers

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‌Finding Erratic Trackers ‌

Here we're talking about an actual tool, "Find Erratic Trackers" on the Track menu, that does just what it says. It examines trackers over the range of the shot, looking for trackers that don't move consistently with the rest. Unlike the Tracker Clean- up tool, it does this before the shot is solved.

The erratic trackers tool can find things like moving trucks in aerial shots and trackers on phantom locations, for example where two sticks or wires cross, forming an X, directionally-dependent voids such as holes in foliage, isolated blowing leaves, flying bugs, lens flares, etc. It does not find short-term issues, for example where a tracker hops onto a different fence post and then hops back a few frames later, or short-lived trackers which don't exist long enough to do something untoward.

The tool operates by examining many pairs of frames which have many valid trackers in common. It picks some of the common trackers to form a kernel, which is then used to predict where the other trackers should appear in one frame, based on the other. If the prediction is off by too much, it is flagged. The tool tests many different kernels on the same pair of frames, so that most kernels do not contain bad trackers.

Any tracker that is flagged as bad on too many kernels is permanently declared to be erratic. When the tool completes (usually quickly), all the erratic trackers are selected for your review and possible deletion.

The Find Erratic Trackers tool has fair number of controls, which are described individually in detail in the Find Erratic Trackers reference section. (As is common in SynthEyes, the controls also offer explanatory tooltips.)

The tool produces an informative textual output with information that can be used to adjust the settings. That's also discussed in the reference section.

This tool is statistical in nature, so it may flag trackers that are actually fine (or of course miss trackers that have different problems). To make the detected trackers easier for you to find and review, the tool optionally sets the erratic trackers to a color you have selected (magenta by default). Turn on View/Group by Color to have them grouped together on the graph editor, for example.

If you run the tool repeatedly, the Find Erratic Trackers tool changes any trackers that have your Bad Tracker Color, but are no longer detected as bad, back to the default uncolored state, to correctly indicate that they are no longer considered to be bad. This means that there can be some confusion if you change the Bad Tracker Color in between runs of the tool. You may want to select all the trackers and reset their color on the Tracker panel.

If you have your own color scheme for trackers, this tool can overwrite it. To avoid that, you can tell the tool not to change the colors by clearing the color selection field on the tool. More subtly, you can turn on View/Tracker Appearance/Use alternate colors so that this tool overwrites the alternate color, and when you're done with the tool, turn Use alternate colors back off to make your original colors visible again.

You can run this tool before solving (as intended), but you can also run the tool after solving if you forgot to run it earlier. That is also useful if you want to see how well the tool does, by putting the graph editor into sort-by-error mode and seeing where the selected trackers fall in the list. Conceptually they'd be all at the top, but that's not necessarily the case.

Here are a few limitations of the tool:

 It needs many trackers to work with, and you may not have enough if you are doing supervised tracking. You may be able to change the kernel settings for to accommodate fewer trackers in borderline situations

 It doesn't look in the interior lifetime of the tracker for problems, it only looks for long- term problems.

 It relies on there being minimal lens distortion and rolling shutter. If you have known lens distortion, remove it from the tracker data before running this tool.

 360VR shots will need many more trackers visible on each frame than regular shots, because erratic trackers can only be detected in a comparatively narrow field of view.

 Experience shows that real 360VR camera data typically has too much rolling shutter and camera field boundary mismatch (ie between the fields of a 2- or 6- camera set) to effectively find erratic trackers.

The indication of 360VR problems is a bad kernel histogram, where the count values decrease steadily as the number of kernels increases, rather than having an empty region in the middle. As a result, the number of trackers flagged will be very high!

Note that the Find Erratic Trackers tool currently uses a script to provide its user interface; the script can be found on the Scripts menu.

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