April Tags

1. Make a printable grid of april tags in pdf form
2. Setting up a test workspace
3. Combining april tags with the rest of robot control

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Make a printable grid of april tags in pdf form

There are a few PDFs floating around (like this one) with one april tag per page. This makes sense if you are going to hang them on a wall and use them for SLAM, but what if you want to print smaller tags to use with smaller-scale applications (like object tracking during manipulation for example)?

Solution: A bash script

I wrote a bash script to generate pdfs with grids of april tags at a specified real-world size:

resize_tags.sh

Using the script

1. Download the pre-made apriltag images from thier repo. These images are all tiny (each square in the tag is one pixel)
2. Fix ImageMagick’s pdf write permissions per these instructions
   • sudo gedit /etc/ImageMagick-6/policy.xml
   • Change PDF rights to rights="read|write"
3. Increase ImageMagick’s memory allowance per these instructions
   • sudo gedit /etc/ImageMagick-6/policy.xml
   • Change memory value to value="32GB". This should give you enough RAM allowance for lots of high-res images. (Even if you don’t have that amount of ram, your computer should be fine.)
4. Download my “resize_tags.sh” file, and move it into the root folder of the apriltag-imgs repo.
5. Open “resize_tags.sh” in a text editor and set the parameters at the top.

    # Set the folder to use
    FILES=tag36h11/*.png  # Set the folder to convert from
   
    # Set the desired width of your tags
    width=30.0         # [mm]  Set the desired width in real units
    dpi=300            # [ppi] Set the dpi (to control the print quality)
    extra_margin=1     # [px]  Add an extra margin around your tags
    default_width=10.0 # [px]  The pixel width of the original images
   
    # Set the grid size your want to make
    grid_h=4           # [num] Number of rows
    grid_w=6           # [num] Number of columns
    grid_filetype=pdf  # [png, jpg, pdf] Set the filetype for the grids
   
    # Decide whether to keep intermediate files
    keep_single_tag_images=false
   

6. Run the script from a terminal.
   • bash resize_tags.sh
   • A new folder (named by the tag dimension) will be generated inside the folder of whichever tag type you set to resize.
   • Larger images of all individual tags are stored in the new folder.
   • Printable grids are also generated according to your settings.
7. Print grids. When printing the grid pdfs, be sure to turn off “auto-scaling” and “document scaling” everywhere in the print dialog to ensure all the tags come out the correct real-world size.

Result: PDFs for the standard tags

Since I use the “tag36h11” variant often, I have already generated sets of printable tags in PDF form at various useful dimensions.

30 mm Side Length 60 mm Side Length 100 mm Side Length

Setting up a test workspace

Building the catkin workspace

1. Build the workspace: catkin_make_isolated

2. Add the source to the ~/.bashrc file: source devel_isolated/setup.sh

Run the apriltag detection algorithm

roslaunch usb_cam usb_cam-test.launch roslaunch apriltag_ros continuous_detection.launch

1. Calibrate the camera (luckily NVidia has the characteristics for the Logitech C920 webcam)

Combining april tags with the rest of robot control

You need to use a separate workspace for apriltags due to some conflicts in the way the apriltags ros package needs to be built.

You can see how to do that in the ROS page

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