Building OpenCV's HTML documentation

OpenCV has become the de facto standard vision processing library, but the project remains surprisingly obscure in several areas, particularly in relation to building and installation tasks. For example, it took me a couple hours to discover how to build the project's HTML documentation (though to be fair, I was looking into it concurrently with other work).

For those also struggling with this, see this question in the OpenCV Q&A site. In short, it gives the following instructions:

1. Install Sphinx;
2. Run cmake on your OpenCV source base as explained in the installation instructions, perhaps adding the -D BUILD_DOCS=ON option for good measure;
3. Run make html_docs to build the documentation in HTML format.

If everything goes well the HTML documentation will be built to the subfolder doc/_html/ inside the build output folder. If not, check if the output of the cmake call reported items Build Documentation and Sphinx both set to YES. You may also check the cmake script cmake/OpenCVDetectPython.cmake inside your OpenCV source base for potential problems.