AprilTags library

Detect April tags (2D bar codes) in images; reports unique ID of each
detection, and optionally its position and orientation relative to a
calibrated camera.

See examples/apriltags_demo.cpp for a simple example that detects
April tags (see tags/pdf/tag36h11.pdf) in laptop or webcam images and
marks any tags in the live image.

Ubuntu dependencies:
sudo apt-get install subversion cmake libopencv-dev libeigen3-dev libv4l-dev

Mac dependencies:
sudo port install pkgconfig opencv eigen3

Uses the pods build system in connection with cmake, see:
http://sourceforge.net/p/pods/

Michael Kaess
October 2012

----------------------------

AprilTags were developed by Professor Edwin Olson of the University of
Michigan.  His Java implementation is available on this web site:
  http://april.eecs.umich.edu.

Olson's Java code was ported to C++ and integrated into the Tekkotsu
framework by Jeffrey Boyland and David Touretzky.

See this Tekkotsu wiki article for additional links and references:
  http://wiki.tekkotsu.org/index.php/AprilTags

----------------------------

This C++ code was further modified by
Michael Kaess (kaess@mit.edu) and Hordur Johannson (hordurj@mit.edu)
and the code has been released under the LGPL 2.1 license.

- converted to standalone library
- added stable homography recovery using OpenCV
- robust tag code table that does not require a terminating 0
  (omission results in false positives by illegal codes being accepted)
- changed example tags to agree with Ed Olson's Java version and added
  all his other tag families
- added principal point as parameter as in original code - essential
  for homography
- added some debugging code (visualization using OpenCV to show
  intermediate detection steps)
- added fast approximation of arctan2 from Ed's original Java code
- using interpolation instead of homography in Quad: requires less
  homography computations and provides a small improvement in correct
  detections

todo:
- significant speedup could be achieved by performing image operations
  using OpenCV (Gaussian filter, but also operations in
  TagDetector.cc)
- replacing arctan2 by precomputed lookup table
- converting matrix operations to Eigen (mostly for simplifying code,
  maybe some speedup)




#####################
  What are AprilTags?

AprilTags are 2D barcodes developed for robotics applications by Ed Olson. The library detects any April tags in a given image, provides the unique ID of the tag as well as its location in the image. If the camera is calibrated and the physical size of the tag known, also provides the relative transform between tag and camera.


Examples of different tag families (number of bits versus minimum Hamming distance between valid tags).


Apriltag detections in an image (see examples/apriltags_demo.cpp).

Copyright and License

This library is based on the original Java code by Ed Olson (used with permission) available in his April library, and on the original C++ port by Jeffrey Boyland and David Touretzky as part of Tekkotsu.
The AprilTags C++ library is available as open source under GNU LGPL version 2.1.

Download and Install

AprilTags mainly depends on OpenCV and Eigen3. All dependencies can be installed in Linux (tested on Ubuntu 12.04) with the following command:

sudo apt-get install subversion cmake libopencv-dev libeigen3-dev libv4l-dev
and in Mac OS X (tested on 10.8.2) with:

sudo port install pkgconfig opencv eigen3
You can check out AprilTags from our public svn repository:

svn co https://svn.csail.mit.edu/apriltags
The AprilTags library uses the pods build system in connection with cmake. Compile with

cd apriltags
make
After compiling, run the example program

./build/bin/apriltags_demo
which detects AprilTags visible in laptop or webcam images and marks any tags in the live image. See examples/apriltags_demo.cpp for the source code.
Publication

Ed Olson, AprilTag: A robust and flexible visual fiducial system, Proceedings of the IEEE International Conference on Robotics and Automation (ICRA), 2011

Last updated: Mar 27, 2013 by kaess@mit.edu
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more information: http://people.csail.mit.edu/kaess/apriltags/