Calculate the Horizon and Tilt in pure C code for a sequence of RAW Video frames

We are looking for software written in C only (no libraries) to efficiently do the following:
a) Take RAW Video Streams and use the Frames to calculate Horizon and Vertical coordinates in each frame, i.e. find the horizon in each frame and the tilt etc. This should be an average of previous frames. The code should be very tight. We will provide sample streams.