Apriltag specs#

What you will need

What you will get

  • The city is ready to be used for localization

Why do we need ground Apritags#

Watchtowers can move around very easily. Collisions with duckiebots and humans are happening all the time. To create a robust way of having a global localization in the autolab, we decide not to fix the Watchtowers.

Instead, we fix ground Apriltags on the ground, and we measure and record their positions in the map. This provides enough ground truth for the localization system.

Making the ground Apriltags#

As a reminder, the Apriltags already have predetermined sets of use case. The ranges of tags are specified in Apriltag ID ranges.

For localization, the ground Apriltags are in range 300 to 399.

Note

For the Autobots themselves, the tags are in range 400 to 439.

Table 1 Apriltag ID ranges#

Purpose

Size

Family

ID Range

Traffic signs

6.5cm x 6.5cm

36h11

1-199

Traffic lights

6.5cm x 6.5cm

36h11

200-299

Localization

6.5cm x 6.5cm

36h11

300-399

Autobots

6.5cm x 6.5cm

36h11

400-439

Street Name Signs

6.5cm x 6.5cm

36h11

440-587

To print the ones you need, you can find a PDF here

Placing the ground Apriltags and the Watchtowers#

At this point, you have already decided where to place your Watchtowers.

Now, you need to place the ground apriltags. Once they are printed, place them with double sided tape respecting the following convention:

  • The ground Apriltags are outside of roads

  • The ground Apriltags are near the side of the road, and the name of the tag is oriented toward the road (see figure).

  • The ground Apriltag’s orientation is a multiple of 45°, meaning there is only eights possible orientations.

  • The only ground Apriltags not oriented in multiple of 90° are the ones on the outside of curve lanes. (see figure)

TODO

TODO: add figure(s)

Tip

Try to put at least two ground Apriltag in the field of view of each watchtower (see Watchtower view straight line.), and try to have Apriltags that are seen by multiple Watchtowers at the same time. This will improve the robustness of the localization graph.

Generating the map in duckietown-world#

You should already have a map as explained in Map. If not, go back and do it, as it will be necessary for the following instructions.

Adding the ground Apriltags localization to your map#

Before measuring#

This is the important part of the Apriltag specifications. You need to make sure that:

  • The ground Apriltags are indeed 6.5 cm x 6.5 cm. If they are not, the localization system will get flawed data and thus will be useless.

  • The ground Apriltags are very well fixed to the ground (using tape and nails). Once they are measured they should not move at all.

  • The ground Apriltags angles must be multiples of 45°. This will make the following much easier.

Measuring#

  • The measure of the ground Apriltags needs to be very precise. You should have a meter with millimeter precision.

  • The tiles have an internal and an external border, because of the interlocking slots. In the following, as for the map, take the inside bottom left as reference for a tile. (see We measure from the inside band from the bottom left inner corner, defined as the origin of the tile)

  • Each Apriltag placement will be measured relatively to the tile it is on, from the above described origin.

  • Always measure the center of the April tag itself.

Attention

In the map you created before, the origin is the bottom-left corner.

How to measure:#

In the following, you will be asked 5 numbers for each Apriltag:

  • The x coordinate of the tile : it is the number (starting at 0 at the first bottom left origin tile) of tile along the x-axis (bottom axis)

  • The y coordinate of the tile : same as for x, it starts at 0

  • The x measure of the Apriltag on the tile : you can get it by blocking your meter in the interior left edge and measuring (in meters) the distance from the interior edge to the center of the Apriltag.

  • The y measure of the Apriltag on the tile : you can get it by blocking your meter in the interior bottom edge and measuring (in meters) the distance from the interior edge to the center of the Apriltag.

  • The orientation of the Apriltag (in degrees) : it is 0 if the Apriltag’s name is aligned normally with the x-axis (e.g. readable from the “bottom” of the map). Then it is defined with the trigonometric convention (counter-clockwise). This should normally always be in multiples of 45°.

_images/apriltag_measure_x.png

Fig. 55 We measure from the inside band from the bottom left inner corner, defined as the origin of the tile#

TODO

TODO: Add more pictures

Filling the map in#

Once you are sure of your positioning of the Apriltags, you can start measuring them. To do so:

  • Open a terminal inside the duckietown-world directory, as you did to create your map. Your map should still be in the src/duckietown_world/data/gd1/maps folder.

Inside the duckietown-world directory, run the following command:

python3 src/apriltag_measure/measure_ground_apriltags.py ![MAP_NAME]

  • Follow the instructions in the terminal : choose an Apriltag number, and fill in the 5 numbers required, as described above.

  • If an Apriltag was already filled in before (if you are changing your map for instance), you will be asked to confirm the overwriting of the positioning. As everything is versioned in GitHub, you can always go back to find the previous positions if need be.

  • If you try recording an Apriltag number than is not in the allocated range (300- 399), the script will also ask to confirm.

  • At the end, just confirm the saving. The resulting map will be where it was before, with now the Apriltag measures added to it.

  • As described in the Map page, you should recompile your map and visualize the apriltags on it (easy debug to find obvious mistakes).