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Printing and Mounting AprilTags
Once you’ve picked your AprilTag family, you can pick out some tags to print (from the AprilTags-imgs github). Again, if possible, select tags that have a corner at the exact center. You’ll need to consider how many are required for your setup, at least three but probably more. (See the next section also.)
Important : You are picking different tags from the same family. Never make multiple tags with the same code value!
You’ll need to decide how big to make them, depending on their distance from the camera and the camera’s field of view. They should probably be 60-100 pixels or more across in the images (for clean images), roughly six-ten or more pixels per block in the tag.
Tip : As always, test before the day of the big shoot!
You can resize the tiny one pixel per block images using Photoshop or SynthEyes, using “nearest” interpolation, not filtering.
Tip : You may want to reduce the white level to a light gray, to avoid saturating the camera and having light spill into the black areas. This appeared to change the one-block border width on one of our shoots.
For smaller tags, inside rooms, say, you can print on a laser or photo printer.
Measure the sides of the square to ensure it really is square, as printers can have small scaling differences between the horizontal and vertical directions. You can adjust the scaling accordingly so that the square is square. That is a important, especially for extracting any 3-D information.
For larger tags, you can use a large format printer, such as those used for architectural drawings. The ability to produce an even 100% black is not required, there’s quite a bit of tolerance. But again, you should measure the sides of the square to ensure it is square.
Once you’ve printed your tags, you can mount them. It is important that the tags are flat. In an office-type set, you may be able to attach tags printed on pieces of paper directly to the walls, but otherwise they will need to be mounted to something, typically foam-core, using a spray adhesive such as Super-77.
For our larger outdoor tags, we have found it convenient to use blank “corrugated plastic” signs intended as hand-drawn yard sale signs, such as manufactured by Hillman and Everbilt and sold in home improvement stores including Home Depot and Lowes. Since the AprilTags are square, the smallest dimension controls, and signs that are nearly square are preferable, such as 14” by 18”, 20” by 24”, and 24” by 28”.
Measure before printing as dimensional accuracy is lacking.
We’ve observed that paper mounted on foam-core or corrugated plastic tends to absorb moisture and bubble up over time (unless kept flattened).
Accordingly, a deluxe approach for large outdoor tags might be to have a commercial sign-making operation print the tags onto a plastic sign, for example the Custom Yard Signs at Vistaprint.com .
You may want to record the family and tag number on the back of each tag for reference. You can put it on the front, if it’s not within the tag’s area (and not within the tag’s possible mandatory outer border).
In any case, you get to apply your maker skills to come up with something suitable. Let us know what you conjure!
©2024 Boris FX, Inc. — UNOFFICIAL — Converted from original PDF.