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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 belongs to a set of markers called ArUco markers dictionary.
Many ArUco dictionaries exist with properties concerning the format, the number of markers or the difference between each markers to avoid error 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 [ArUcoMarkers](../../argaze.md/#argaze.ArUcoMarkers.ArUcoMarker) pictures from a dictionary is illustrated in the code below:
```python
from argaze.ArUcoMarkers import ArUcoMarkersDictionary
# Create a dictionary of specific April tags
aruco_dictionary = ArUcoMarkersDictionary.ArUcoMarkersDictionary('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 **A4_DICT_APRILTAG_16h5_5cm_0-7.pdf** file located in *./src/argaze/ArUcoMarkers/utils/* folder ready to be printed on A4 paper sheet.
Let's print some of them before to go further.
!!! warning
Print markers with a blank zone around them to help in their detection.
## Describe ArUco markers place
Once [ArUcoMarkers](../../argaze.md/#argaze.ArUcoMarkers.ArUcoMarker) pictures are placed into a scene it is possible to describe their 3D places into a file.
Where ever the origin point is, all markers places need to be described in 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
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 common OBJ file features needed to describe ArUco markers places:
* Object lines (starting with *o* key) indicate markers dictionary and id by following this format: **DICTIONARY**#**ID**\_Marker.
* Vertice lines (starting with *v* key) indicate markers corners. The marker size will be automatically deducted from the geometry.
* Face (starting with *f* key) link vertices and normals indexes together.
!!! warning
Markers have to belong to the same dictionary.
!!! note
Markers can have different size.
### Edit JSON file description
JSON file format allows to describe markers 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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