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.
Conflicts:
modules/calib3d/include/opencv2/calib3d.hpp
Updated the documents to give warning to the users of `findHomography` that the function may return an empty matrix in some cases.
The user must take care of checking that.
In the function cvInitIntrinsicParams2D the principal point for
normalized image coordinates is set to 0/0. This updates the function
to initialize the principal point at 0.5/0.5.