viso2 (visual odometry from mono_odometry)

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Thanks, but I don't need to build the map. I just need to know the position of a predefined tag (it's on the floor and I recognize it using ar_track_alvar), and what I want to do it see that tag, set the distance between that tag and my quadcopter (that distance is given by ar_track_alvar) and go there. I'm trying to use an EKF because the measurements of the tag were very noisy and the quadcopters moves a lot (just little movements but the errors that I can handle because of the relationship between the FOV and the size of the tag is really big (i.e, they're quite similar)), and I need some smoother movements. But to use the EKF I need the visual odometry, that's why I'm trying to use viso :)

Maybe something like SVO would be a better fit then? Viso2's mono odometer is not going to work for your use case as it needs a fixed distance from camera to the ground (see http://wiki.ros.org/viso2_ros#Monocul... ).

I read the documentation and it seems to be what I was looking for. Thanks! So, the output of the SVO is the Visual Odometry, and I can use that VO to feed an EKF (also fed by an accelerometer, a gyroscope, a sonar height, etc) to have a better state estimation of my quadcopter?

Another question: I read that the output of the SVO does not have a unit, so how can I make that feed the EKF (the idea is to do the EKF using robot_localization)

To get better answers to your add-on questions, I think it would be better to open new ones instead of commenting them here.

Thanks :))). One basic question, I see 3 folders: viso2, viso2_ros and libviso2. In the wiki page it says that viso2_ros is a ROS wrapper of the libviso2. I suppose I only have to modify the file that is inside the launch file in the viso2_ros, is that OK? Do I have to install anything?

You only have to write your own launch file. However, note what Stephan has said. Visio might not work for you.