How to use ar_track_alvar for detecting markers using Kinect ?

answered 2015-02-16 22:52:30 -0500

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You can indeed use ar_track_alvar to detect ar_markers.

If you are using openni_launch or freenect_launch packages to get the images from your kinect, you should have a bunch of topics published.
Camera image topic stands for the topic where your kinect image is published, should look like: /camera/rgb/image_raw
Camera info topic stands for the topic where the information of your camera are published, shoud look like: /camera/rgb/camera_info
The output frame parameter means the link relative to which you want the position of the detected ar marjers will be published (for example base_link) or whatever link suits your application. This must be a link known by tf, otherwise ROS won't be able to compute a transformation.
Documentation can be found here
Examples can be found here

The detected markers will be published on /ar_pose_marker (ar_track_alvar/AlvarMarkers) as an array of AlvarMarkers You can also visualize them in rviz, you just have to add a marker plugin and select the correspondig topic (/visualization_marker).

Hope this helps,

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currently I'm using /camera/rgb/image_color for Camera image topic to make the camera receive the coloured image . and for the output frame I'm using camera_link(not sure its right). but I can't see any output yet and I can't manage to show it on rviz. What am I doing wrong here ?

MSYazeed ( 2015-02-24 04:38:47 -0500 )edit

I also should note that I ran roscore first then openni_launch then the ar_track_alvar and rviz. it gives me weird error saying '[1424775599.179064296]: Client [/marker_detect] wants topic /camera/rgb/image_color to have datatype/md5sum [~], but our version has [~]. Dropping connection.

MSYazeed ( 2015-02-24 05:13:28 -0500 )edit

This is the full error: [ERROR] [1424775599.179064296]: Client [/marker_detect] wants topic /camera/rgb/image_color to have datatype/md5sum [sensor_msgs/PointCloud2/1158d486dd51d683ce2f1be655c3c181], but our version has [sensor_msgs/Image/060021388200f6f0f447d0fcd9c64743]. Dropping connection.

MSYazeed ( 2015-02-24 05:13:50 -0500 )edit

ok, here the error says that this node requires a sensor_msgs/PointCloud2 and not a color image. So you should use depth_registered (camera/depth_registered/image_raw). If you want to check the type of a message sent on a given topic you can use: rostopic info TOPICNAME

marguedas ( 2015-02-25 01:24:17 -0500 )edit

Thank you so much its working now and can detect the the marker, but some how it shows the marker on Rivz for a second then it gives me the following error

MSYazeed ( 2015-03-02 07:43:06 -0500 )edit

/usr/include/eigen3/Eigen/src/Core/DenseStorage.h:69: Eigen::internal::plain_array<t, size,="" matrixorarrayoptions,="" 16="">::plain_array() [with T = float, int Size = 4, int MatrixOrArrayOptions = 0]: Assertion `(reinterpret_cast<size_t>(array) & 0xf) == 0 && "this assertion is explained here: " "http://e

MSYazeed ( 2015-03-02 07:43:26 -0500 )edit

No idea, Sorry. I've never face this issue. But it seems that several people got this problem recently: http://answers.ros.org/question/20369... Try to track these threads and check on the internet, the solution will pop up in the eventually.

marguedas ( 2015-03-02 08:26:02 -0500 )edit

Thank you so much for your help

MSYazeed ( 2015-03-02 08:43:13 -0500 )edit

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How to use ar_track_alvar for detecting markers using Kinect ?

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How to use ar_track_alvar for detecting markers using Kinect ?