It took me a while to figure out what was meant with

    OpenCV Error: Assertion failed (i < 0) in getMat

While searching for this error message I found [a list of error
messages](https://adventuresandwhathaveyou.wordpress.com/2014/03/14/opencv-error-messages-suck/)
which also explained what the problem was: The data type for `rvecs` was
not a simple `cv::Mat` but a `std::vector<cv::Mat>`.

After I fixed that, I got the next error message:

    OpenCV Error: Assertion failed (ni > 0 && ni == ni1) in
    collectCalibrationData, file
    /build/buildd/opencv-2.4.9+dfsg/modules/calib3d/src/calibration.cpp,
    line 3193

The problem here was that my data type for the `objectPoints` was just
`vector<Vec3f>` and not `vector<vector<Vec3f>>`.

In order to save other people the time looking for this, I added
explicit examples of the needed data types into the documentation of the
function. I had to re-read the current version a couple of times until I
can read the needed levels of `vector<>`. Having this example would have
really helped me there.