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114 lines
3.9 KiB
Markdown
114 lines
3.9 KiB
Markdown
# ArUco marker detection
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> **Note** The following applies to [image versions](image.md) **0.22** and up. Older documentation is still available for [for version **0.20**](https://github.com/CopterExpress/clover/blob/v0.20/docs/en/aruco_marker.md).
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<!-- -->
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> **Info** Marker detection requires the camera module to be correctly plugged in and [configured](camera.md).
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`aruco_detect` module detects ArUco markers and publishes their positions in ROS topics and as [TF frames](frames.md).
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This is useful in conjunction with other positioning systems, such as [GPS](gps.md), [Optical Flow](optical_flow.md), PX4Flow, visual odometry, ultrasonic ([Marvelmind](https://marvelmind.com)) or UWB-based ([Pozyx](https://www.pozyx.io)) localization.
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Using this module along with [map-based navigation](aruco_map.md) is also possible.
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## Setup
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Set the `aruco` argument in `~/catkin_ws/src/clover/clover/launch/clover.launch` to `true`:
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```xml
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<arg name="aruco" default="true"/>
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```
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For enabling detection set the `aruco_detect` argument in `~/catkin_ws/src/clover/clover/launch/aruco.launch` to `true`:
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```xml
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<arg name="aruco_detect" default="true"/>
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```
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For the module to work correctly the following arguments should also be set:
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```xml
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<arg name="placement" default="floor"/> <!-- markers' placement, explained below -->
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<arg name="length" default="0.33"/> <!-- length of a single marker, in meters (excluding the white border) -->
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```
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`placement` argument should be set to:
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* `floor` if *all* markers are on the ground;
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* `ceiling` if *all* markers are on the ceiling;
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* an empty string otherwise.
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You may specify length for each marker individually by using the `length_override` parameter of the node `aruco_detect`:
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```xml
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<param name="length_override/3" value="0.1"/> <!-- marker with id=3 has a side of 0.1m -->
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<param name="length_override/17" value="0.25"/> <!-- marker with id=17 has a side of 0.25m -->
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```
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## Coordinate system
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Each marker has its own coordinate systems. It is aligned as follows:
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* the **<font color=red>x</font>** axis points to the right side of the marker;
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* the **<font color=green>y</font>** axis points to the top side of the marker;
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* the **<font color=blue>z</font>** axis points outwards from the plane of the marker
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<img src="../assets/aruco-axis.png" width="300">
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## Working with detected markers
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Navigation within the marker-based TF frames is possible with `simple_offboard` node.
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Sample code to fly to a point 1 metre above marker with id 5:
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```python
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navigate(frame_id='aruco_5', x=0, y=0, z=1)
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```
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Sample code to fly to a point 1 metre to the left and 2 metres above marker with id 7:
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```python
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navigate(frame_id='aruco_7', x=-1, y=0, z=2)
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```
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Note that if the required marker isn't detected for 0.5 seconds after the `navigate` command, the command will be ignored.
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These frames may also be used in other services that accept TF frames (like `get_telemetry`). The following code will get the drone's position relative to the marker with id 3:
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```python
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telem = get_telemetry(frame_id='aruco_3')
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```
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Note that if the required marker isn't detected for 0.5 seconds, the `telem.x`, `telem.y`, `telem.z`, `telem.yaw` fields will contain `NaN`.
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## Handling marker detection in Python
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The following snippet shows how to read the `aruco_detect/markers` topic in Python:
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```python
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import rospy
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from aruco_pose.msg import MarkerArray
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rospy.init_node('my_node')
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# ...
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def markers_callback(msg):
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print('Detected markers:'):
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for marker in msg.markers:
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print('Marker: %s' % marker)
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# Create a Subscription object. Each time a message is posted in aruco_detect/markers, the markers_callback function is called with this message as its argument.
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rospy.Subscriber('aruco_detect/markers', MarkerArray, markers_callback)
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# ...
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rospy.spin()
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```
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Each message contains the marker ID, its corner points on the image and its position relative to the camera.
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---
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Suggested reading: [map-based navigation](aruco_map.md)
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