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TagSLAM

C. Making and Using AprilTags

At the moment, the only tags supported by TagSLAM are the popular AprilTags.

What tag family to use

The AprilTags come in several different variants (families), depending on how many bits a tag can represent. Some tag families have large, coarse bit blocks that can be seen from far, but can only cover a small number space. The most popular one is the family “36h11”, which has a 6x6 bit pattern. It can cover the numbers 0 to 586, and is rather robust to false positive detections. If you don’t need the full number space, and must work with a lower resolution camera, consider the “16h5” family, which can represent 0 to 29. Be warned however: you may have more false positive detections when using this tag. This means the AprilTag library may mistake some rectangular objects on the image as tags! This can be a real nuisance when using TagSLAM.

tag 36h11 tag 16h5
Tag family 36h11 (preferred). Small blocks, harder to detect, can represent larger numbers. Tag family 16h5. Large blocks, better visible, but susceptible to false positives.

What material to use

An ideal AprilTag should be:

  • perfectly flat, without any warp
  • matte (not glossy!)
  • high contrast: pitch black on pure white
  • rigid and sturdy
  • light weight

tag and board

Here are two popular options: foam core board (left), or DiBond aluminum (right)

DiBond aluminum

For very sturdy tags, consider having them printed on DiBond aluminum, a material used for making advertisement signs. The company FoamCorePrint.com has been used for this before. Make sure to select 1/4” thick material (the 1/8” boards only come glossy, not matte), and specify explicitly that you want it matte, and no rescaling of the image. You must supply them with a pdf that exactly fills the size of the board. For a 12"x12” board, the cost is currently about $15 without shipping. The material is quite heavy, so not easily attached to walls with duct tape. However, it’s very rugged and weather resistant, i.e. suitable for permanent installation outdoors.

DiBond is also great material for a calibration board that gets heavy use. Due to popular demand, here are links to the front and back of the calibration board in the image. You can use them directly for printing on 1/4"x16"x12” DiBond at FoamCorePrint.com.

Foam core

For a do-it-yourself approach, you can produce tags on foam core board. Avoid the “Readi” board sold by e.g. Dollar Tree stores. It warps badly! Rather get a board of 3/16” ProFoam from Jerry’s Artarama for about $6, from which you can cut 12 tags of size 10"x10”.

Use an inkjet printer to print the tag on a matte clear adhesive label like the Avery 8665, which go for about $1 for an 8.5"x11” page. Stick the label on the foam board, done. The board is significantly cheaper (about $1.50/tag DIY, or $4/tag + shipping if you have it printed) and lighter than DiBond aluminum, but also much more fragile. Be careful not to touch the printed surface, or the color will come off over time.

An even better solution is to use a laser printer and these labels for about $0.45/sheet.

Paper

OK when used against flat surfaces like walls, or put on the ground. Good for some initial experimentation. Make sure the tag is really not warped, as this will greatly affect the pose estimation.

How to print your own AprilTags

There is a handy little program in the TagSLAM repo (courtesy of the Kalibr folks) with which you can generate a pdf of an AprilTag:

sudo apt install python-pyx
rosrun tagslam make_tag.py --nx 1 --ny 1 --marginx 0.00 --marginy 0.00 --tsize 0.16 --tspace 0.0 --startid 4 --tfam t36h11 --borderbits 1 

This will produce a pdf file target.pdf of a single tag with id 4, size 16cm, and a 1 bit wide border. When you generate tags, you obviously want to print them as big as possible, but you must leave a large white frame around the tag! Make it about 2 bits wide (a bit corresponds to one of the small squares that make up the tag’s code). The white border around the tag is crucial for detection!

The borderbits parameter determines the size of the black border on the outside of the AprilTag. You will generally want to use a 1 bit wide border, although you can see that the calibration board in the image has tags with 2 bit borders (the corner squares between the tags are for more accurate localization of the tag corners, TagSLAM is not using them).

To verify that the tag will actually have the size you want:

pdfinfo target.pdf | grep -n 'Page size'

output:

Page size:      455.543 x 455.543 pts

Divide this by 72 to get the physical size in inches, then multiply by 0.0254 to get meters. In this case it works out to 0.1607, so actually a little off (why???).