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Set up ArUco markers
====================
First of all, ArUco markers needs to be printed and placed into the scene.
Here is an example scene where markers are surrounding a workspace with two screens, a control panel and a window.
## Print ArUco markers from an ArUco dictionary
ArUco markers always belong to a set of markers called the ArUco marker dictionary.
Many ArUco marker dictionaries exist with properties concerning the format, the number of markers, or the difference between each marker to avoid errors in tracking.
Here is the documention [about ArUco markers dictionaries](https://docs.opencv.org/3.4/d9/d6a/group__aruco.html#gac84398a9ed9dd01306592dd616c2c975).
The creation of [ArUcoMarker](../../argaze.md/#argaze.ArUcoMarker.ArUcoMarker) pictures from a dictionary is illustrated in the code below:
```python
from argaze.ArUcoMarker import ArUcoMarkerDictionary
# Create a dictionary of specific April tags
aruco_dictionary = ArUcoMarkerDictionary.ArUcoMarkerDictionary('DICT_APRILTAG_16h5')
# Export marker n°5 as 3.5 cm picture with 300 dpi resolution
aruco_dictionary.create_marker(5, 3.5).save('./markers/', 300)
# Export all dictionary markers as 3.5 cm pictures with 300 dpi resolution
aruco_dictionary.save('./markers/', 3.5, 300)
```
!!! note
There is an **A4_DICT_APRILTAG_16h5_5cm_0-7.pdf** file located in *./src/argaze/ArUcoMarker/utils/* folder ready to be printed on an A4 paper sheet.
Let's print some of them before going further.
!!! warning
Print markers with a blank zone around them to help in their detection.
## Describe ArUco markers place
Once [ArUcoMarker](../../argaze.md/#argaze.ArUcoMarker.ArUcoMarker) pictures are placed into a scene, it is possible to describe their 3D places into a file.
Wherever the origin point is, all markers places need to be described on a [right-handed 3D axis](https://robotacademy.net.au/lesson/right-handed-3d-coordinate-frame/) where:
* +X is pointing to the right,
* +Y is pointing to the top,
* +Z is pointing to the backward.
!!! warning
All ArUco markers spatial values must be given in **centimeters**.
### Edit OBJ file description
The OBJ file format could be exported from most 3D editors.
``` obj
o DICT_APRILTAG_16h5#0_Marker
v 15.000000 0.378741 0.330527
v 20.000000 0.378741 0.330527
v 15.000000 5.120359 -1.255996
v 20.000000 5.120359 -1.255996
f 1 2 4 3
o DICT_APRILTAG_16h5#1_Marker
v 43.500000 31.428055 18.333317
v 48.500000 31.428055 18.333317
v 43.500000 36.428055 18.333317
v 48.500000 36.428055 18.333317
f 5 6 8 7
o DICT_APRILTAG_16h5#2_Marker
v 38.500000 2.678055 5.498381
v 43.500000 2.678055 5.498381
v 38.500000 5.178055 1.168253
v 43.500000 5.178055 1.168253
f 9 10 12 11
```
Here are some common OBJ file features needed to describe ArUco markers places:
* Object line (starting with the *o* key) indicates marker dictionary and id by following this format: **DICTIONARY**#**ID**\_Marker.
* Vertice line (starting with the *v* key) indicates marker corners. The marker size will be automatically deducted from the geometry.
* Face line (starting with the *f* key) links vertice and normal indexes together.
!!! warning
Markers have to belong to the same dictionary.
!!! note
Markers can have different sizes.
### Edit JSON file description
the JSON file format allows for the description of marker places using translation and Euler angle rotation vectors.
``` json
{
"dictionary": "DICT_APRILTAG_16h5",
"places": {
"0": {
"translation": [17.5, 2.75, -0.5],
"rotation": [-18.5, 0, 0],
"size": 5
},
"1": {
"translation": [46, 34, 18.333],
"rotation": [0, 70, 0],
"size": 5
},
"2": {
"translation": [41, 4, 3.333],
"rotation": [-60, 0, 0],
"size": 5
}
}
}
```
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