# VOXL2 NOT DETECTING APRIL TAG

Source: https://forum.modalai.com/topic/4720/voxl2-not-detecting-april-tag
Category: Autonomy: VIO, Mapping and Navigation (https://forum.modalai.com/category/19/autonomy-vio-mapping-and-navigation)
Tags: apriltag
Posted: 2025-09-01 17:13:46 UTC by taiwohazeez
Replies: 7 · Views: 4766

## taiwohazeez · 2025-09-01 17:13:46 UTC

Hello support,
I have an issue detecting apriltags with my voxl2 (starling 2), I looked at the documentation online to see how it works. 
I ran voxl-tag-detector, and i got this 

************voxl2:/$ voxl-tag-detector
loading tag-detector config file
loading apriltag config file
=================================================================**
detector #0
    enable:          1
    input_pipe:      tracking_front
    en_fast_mode:    1
    n_threads:       1
    en_undistortion: 1
    undistort_scale: 0.60
    overlay_name:    tracking_tag_overlay
    lens_cal_file:   /data/modalai/opencv_tracking_intrinsics.yml
    skip_n_frames:   5
detector #1
    enable:          0
    input_pipe:      stereo
    en_fast_mode:    1
    n_threads:       1
    en_undistortion: 1
    undistort_scale: 0.90
    overlay_name:    stereo_tag_overlay
    lens_cal_file:   /data/modalai/opencv_stereo_intrinsics.yml
    skip_n_frames:   5
detector #2
    enable:          0
    input_pipe:      extra
    en_fast_mode:    1
    n_threads:       1
    en_undistortion: 0
    undistort_scale: 1.00
    overlay_name:    extra_tag_overlay
    lens_cal_file:   /data/modalai/opencv_extra_intrinsics.yml
    skip_n_frames:   5
=================================================================**
default_size_m:        0.160
#0:
    id:              0
    name:            default_name
    loc_type:        unknown
    size_m:            0.400
    T_tag_wrt_fixed: 0.0  0.0  0.0 
    R_tag_to_fixed:  0.0 -1.0  0.0 
                     1.0  0.0  0.0 
                     0.0  0.0  1.0 
found cam matrix with name 'M' for detector #0
detector #0 is fisheye
waiting for cam pipe tracking_front
detector #0 connected to camera pipe tracking_front
lens #0: fx:460.04 fy:460.58 cx:666.72 cy:382.58
lens #0: fxrect:276.02 fyrect:276.35********. 

Then on another terminal, i ran voxl-inspect-detections -a tag_detections, but i keep getting unknown, can you help with what i might be doing wrong.

Meanwhile, I have ros2 run voxl_mpa_to_ros2 voxl_mpa_to_ros2_node running.

KIndly assist @tom @teddy-zaremba 
Thanks,
Taiwo

## Reply by Zachary Lowell 0 · 2025-09-02 21:28:19 UTC

Hi @taiwohazeez - can you paste the output? Usually you get NANS or unknown when the locations.txt file isnt filled out meaning the tag detector service is referencing an empty file. Once you paste the output of voxl-inspect-detections I will be able to give you better guidance.

Thanks!
Zach

## Reply by taiwohazeez · 2025-09-02 23:55:35 UTC (in reply to Zachary Lowell 0)

@Zachary-Lowell-0 thank you for your response, this is the output, |   
 Class Name    |    Camera    | Class Confidence | Detection Confidence | Frame ID |  Timestamp(|
|------------------|--------------|------------------|----------------------|----------|------------|
| unknown          |              |              nan |                  nan |       62 |447765889248|

Thanks,
Taiwo

## Reply by Zachary Lowell 0 · 2025-09-03 15:07:41 UTC

Class confidence and detection confidence are pertinent towards voxl tflite server not voxl-tag-detector - are you sure this is the right output?

## Reply by taiwohazeez · 2025-09-03 17:07:25 UTC (in reply to Zachary Lowell 0)

@Zachary-Lowell-0 yes, i copied that directly from the terminal after running this command voxl-inspect-detections -a tag_detections.

## Reply by taiwohazeez · 2025-09-08 17:06:05 UTC (in reply to taiwohazeez)

@taiwohazeez @Zachary-Lowell-0 , please i am still waiting for a response, an advice on how to go about Apriltag detection on starling 2 would be greatly appreciated.

Thanks

## Reply by Zachary Lowell 0 · 2025-09-08 18:20:18 UTC

@taiwohazeez  I am not fully convinced you are running the right command still

Running voxl-inspect-tag shoudl show the following headers:

ID | XYZ | RPY | SIZE_M | CAM

As shown in the code below:
`
		printf("\n");
		printf("id:%3d name: %s\n", d.id, d.name);
		printf("XYZ: %6.2f %6.2f %6.2f \n", (double)d.T_tag_wrt_cam[0], (double)d.T_tag_wrt_cam[1], (double)d.T_tag_wrt_cam[2]);
		printf("RPY: %6.1f %6.1f %6.1f \n", (double)roll, (double)pitch, (double)yaw);
		printf("size_m: %4.2f latency_ms: %6.2f\n", (double)d.size_m, latency_ms);
		printf("cam: %s type: %s\n", d.cam, pipe_tag_location_type_to_string(d.loc_type));
		printf("\n");
`

https://gitlab.com/voxl-public/voxl-sdk/services/voxl-tag-detector/-/blob/dev/clients/voxl-inspect-tags.c?ref_type=heads

Just run `voxl-inspect-tags` and that will output the right data.
Zach

## Reply by Zachary Lowell 0 · 2025-09-08 18:20:40 UTC

Detections is pertinent to voxl-tflite-server and onboard computer vision on yolo, NOT on april tags.