Open source Computer Vision (OpenCV) is a library of programming functions for real time computer vision. It is released under a BSD license and is free for both academic and commercial use on the Linux, Mac and Windows platforms. The OpenCV library has 500+ optimized algorithms. Example applications of the OpenCV library are Human-Computer Interaction (HCI); Object Identification, Segmentation and Recognition; Face Recognition; Gesture Recognition; Motion Tracking, Ego Motion, Motion Understanding; Structure From Motion (SFM); Stereo and Multi-Camera Calibration and Depth Computation; Mobile Robotics.
It is used around the world, with over 3M downloads, for purposes ranging from interactive art to mine inspection, stitching maps on the web through advanced robotics.
The upside is the open source nature of this project; the downside, is that the real substantive explanations are found in the book Learning OpenCV: Computer Vision with the OpenCV Library which is only available for purchase.