Producing Planar Trackers

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‌Producing Planar Trackers

Clearly if we’re producing trackers from tags, producing a planar tracker should be an option, and it is. We might want a planar tracker to drive a downstream visual effect. A SynthEyes planar tracker affords the possibility of more accuracy and higher rejection of obstructing clutter, since the planar tracker is based on the entire area, while AprilTags relies only on the square, ignoring additional detail potentially contributed by the data bits. There are some subtle details to planar tracking of the AprilTags.

First, while the 4 corner trackers (if produced) go on the corners of the AprilTag’s square, the planar tracker will always encompass the entire area of the tag, including any empty border pixels or square-external data bits. The planar tracker must be able to see the outer edges of the square, and not have to just look at the internal data bit content. (This is true when placing any planar tracker by hand as well, of course.) The AprilTag detector’s script bumps up the size of the planar tracker automatically.

Next, observe that you have a choice of planar tracker types to pick from via the AprilTags Detector control panel. Those include both 2-D and 3-D variants to choose from according to your needs. The 3-D options illustrate some complexity, with options

for known or varying field of view (FOV), and a known aspect ratio or not. (We do, tags are square!)

A small tag can’t accurately determine the field of view, and if you have multiple tags, they must all have the same field of view, which won’t happen accidentally. In such circumstances, if you want a 3-D planar tracker, you should 3-D solve the scene first (or at least estimate a field of view), then use “Known FOV, Aspect” mode for the planar tracker(s).

Lastly, if you’re producing 3-D planar trackers and want the proper 3-D scaling, you’ll need to enter the correct size (in your choice of units) into the Tag Size field. You should measure the side of the printed tag’s main square; use the green arrow below, not the red one that is the printed size:

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