How to compute distance of tag from camera ?

[pra-dan]

Hi, Thanks for this amazing resource !

I am able to get results (using test images and test YAML) nicely. The coordinates of the detected tags are obtained as shown in the pic.

I am unable to understand what do the parameters mean:

param	description
pose_R*	Rotation matrix of the pose estimate.
pose_t*	Translation of the pose estimate.
pose_err*	Object-space error of the estimation.

Also, how should I compute the distance of the tag from the camera ?

[pra-dan]

Ok, I get what rotation and translation matrix mean. Thanks to this StackOverflow answer and this paper.

I suppose, if in the camera projective, we define the center of the marker as (XW,YW,ZW) in the marker/tag coordinate system and (XC,YC,ZC) in the camera coordinate,

then,

d = ((XC - 0)2 + (YC - 0)2 + (ZC - 0)2)1/2

should correspond to the distance of the tag from the camera. Right ?

[fnoop]

Yes:
https://github.com/goodrobots/vision_landing/blob/master/src/track_targets.cpp#L639
https://en.wikipedia.org/wiki/Euclidean_distance

[pra-dan]

Hey @fnoop Thanks for confirming. Another thing is, the program returns the coordinates of the center of the detected tag as center = [399.22982016 718.50388705] i.e., only XW and YW. How to assume/find the ZW and hence compute the corresponding (XC,YC,ZC) ?

[fnoop]

Sorry I haven't used apriltags yet, I just happened to come across your issue while evaluating it!

[DanielChaseButterfield]

@pra-dan The center attribute is a reference to the center of the tag in the image, not in 3D space.