TF convention tag and bundles

[miguelcastillon]

Hi!

Following this issue, I would kindly recommend to clarify the convention used when defining the tag TF.

The only info I could find is here and says:

The tag's coordinate frame is centered at the center of the tag, with x-axis to the right, y-axis down, and z-axis into the tag.

However, this contradicts the observations when running the apriltag_ros detector.

When I print a tag and look at it with a camera, the tag TF from the position of the camera is:

• x towards the right of the camera
• y upwards
• z outwards towards the camera.

Likewise, when I create a tag bundle with several tags, the coordinate system used by the detector is:

• x towards the right side of the paper
• y upwards

This is very important when defining the position of each tag in the tag_bundles of the yaml file. Following a wrong convention leads to a completely wrong pose estimation.

[zgxsin]

I experienced this issue as well. I would appreciate it if the maintainer could specify this coordinate convention wherever applicable.