Now were getting to the fun part. I am going to start trying things, one by one, to try to find the lettering on the license plate. The first thing I want to try is template matching. From the very limited sample set, I can see that the license plate letters can be turned into a block of white (uising previous preprocessing combined with some morphology) like this:
and like this:
My thinking is that to start with a template that looks like the white rectangle in the image below. I generated it automatically based on the relative size of a license plate (12″x6″) and the width of a typical car (6′) and the distance I measured between the outsides of the brakelights.
I will have to rotate the template +/- five degrees and apply it multiple times to find the best match, but that will go pretty fast. My hope is that it will give me a repeatable, robust position of the center of the license plate. If it does, I will try to start separating the characters. If it does not, I will try a few more tricks.