Update README.md

examples/zed_ar_track_alvar_example/README.md

@@ -1,32 +1,42 @@
 # Stereolabs ZED Camera - Alvar AR tag example
 
-This package shows how to use the ZED Wrapper with [ar_track_alvar](http://wiki.ros.org/ar_track_alvar)
+This package shows how to use the ZED Wrapper with [ar_track_alvar](http://wiki.ros.org/ar_track_alvar) to detect AR markers and localize them in the space.
+
+The [ar_track_alvar](http://wiki.ros.org/ar_track_alvar) package is a ROS wrapper for [Alvar](http://virtual.vtt.fi/virtual/proj2/multimedia/index.html), an open source AR tag tracking library.
+
+## Install the ar_track_alvar package
+
+Open a terminal and launch
+
+    $ sudo apt install ros-kinetic-ar-track-alvar
 
 ## Run the program
 
 To launch the example, open a terminal and launch:
 
     $ roslaunch zed_ar_track_alvar_example zed_ar_track_alvar.launch
 
-The launch file automatically starts the ZED node, the ar_track_alvar node and RVIZ with a preconfigured view:
+The launch file automatically starts the `zed_wrapper` node, the `ar_track_alvar` node and `RVIZ` with a preconfigured view:
 
 ![Example of indoor 3D mapping](images/ar_track_alvar.png)
 
-The `Left Image` view shows the RGB stream from the left camera of the ZED.
-The `Left Camera` view shows the "world" as seen by the left camera. The `TF` and the `Tags` are selected to be shown in 3D projection
-The 3D view shows the ZED camera localized in the space and the position of the `Tags` as coloured markers.
+* the `Left Image` view shows the RGB stream from the left camera of the ZED.
+* the `Left Camera` view shows the "world" as seen by the left camera. The `TF` and the `Tags` are selected to be shown in 3D projection
+* the 3D view shows the ZED camera localized in the space and the position of the `Tags` as coloured markers.
 
 *Note* It is important to subscribe the `/zed/pose` topic to be able to localize the camera in the space.
 
-*Note* the tags used in the example are available following this [link](http://wiki.ros.org/ar_track_alvar?action=AttachFile&do=get&target=markers0to8.png)
+*Note* the tags used in the example are available following this [link](images/markers0to8.png)
 
 ## The launch file explained
 
-To correctly use the ZED wrapper with the `ar_track_alvar` node we need to match the following ar_track_alvar parameters:
+To use the ZED wrapper with the `ar_track_alvar` node we need to correctly set the following ar_track_alvar parameters:
 
-- `cam_image_topic` -> topic of the color image used to detect the tags
-- `cam_info_topic` -> topic of camera parameters used to create the association of each color pixel to the relative 3D point
-- `camera_frame` -> the name of the TF frame associated to the camera position
+- `camera_image` -> the name of the topic that provides camera frames for detecting the AR tags.
+- `camera_info` -> the name of the topic of camera parameters used to correctly localize the tag in the 3D world
+- `output_frame` -> the name of the TF frame associated to the camera position
+
+It is important that the `zed_wrapper` node and the `ar_track_alvar` node use the same values for the these three parameters to be able to correctly interface.
 
 The values associated to the above parameters are the following:
 
@@ -48,9 +58,26 @@ The corresponding parameters of the `rtabmap_ros` node are the following:
 - `cam_info_topic` -> `camera_info`
 - `camera_frame` -> `output_frame`
 
-It is also important to set the correct `marker_size` in millimeters. Better if measured after printing them:
+It is also important to set the correct `marker_size` in centimeters. Better if the real size of the markers is measured after printing them:
 
     <arg name="marker_size" default="5.0" />
+
+Other parameters that can be tuned:           
+* `max_new_marker_error` (double) -- A threshold determining when new markers can be detected under uncertainty
+
+    <arg name="max_new_marker_error" default="0.08" />
+
+* `max_track_error` (double) -- A threshold determining how much tracking error can be observed before an tag is considered to have disappeared
+
+    <arg name="max_track_error" default="0.2" />
+
+## Markers
+
+Following two examples of markers with ID numbers from 0 to 17:
+
+![Markers 0-8](images/markers0to8.png)
+![Markers 9-17](images/markers9to17.png)
+