OpenCV

OpenCV is a an open source computer vision library available at opencv.org. Version 2.x of OpenCV has C++, C, Java and Python interfaces and supports several platforms (Windows, Linux, Mac OS, iOS and Android). Version 3.x add C#, JavaScript, Haskell and D interfaces and adds WinRT & QNX (BlackBerry) as additional supported platforms.

For ease of access, a copy of OpenCV V2.4.10 (for Windows) is provided here.

OpenCV 4.1.0 is now available at opencv.org.

Online documentation is quite extensive. There is also some support specifically for Ubuntu Linux.