Recently released versions are called 0.9, 0.99 and 0.999. Another popular method to set the number is the semantic versioning. Even HBase, one of the Hadoop ecosystem projects, that has been approaching version 1.0 for a long time names its recent versions as 0.9x.x.

OpenCV updates its version in a header file for the users to know the current version they are using. It also enables CMake to automatically version some other files.