I'm trying to use Hough Transform during number plate localization process. I have seen some articles and ideas about finding rectangles with that, still almost every example was quite simple - one rectangle on image, usually game card or TV. When I want to implement that in my system, it's not working well. I'm finding usually more then 3000 lines, and much more intersections. I'm using Canny edge filter. I tested that with some different parameters (both, Canny Filter and HoughLinesP function) and always got very huge numer of points. Is it possible to find that plate, when we are having a lot of environment information on our image? Or are there any other options to achieve some good results? I would appreciate any answers and ideas. Some code samples in OpenCV will be very usefull too.
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Detecting many line segments is typical for the Hough transform. E.g. the letters on the plates might contain straight line segments, the surroundings of the plate (a car?) and whatever.
So, you should try utilizing more context information on your plate detection such as
- background color of the plate (e.g. is it white? or black or yellow or whatever? are your image data colored?) So, try filtering for the color
- what size is a typical plate on the image? is it always roughly the same size? Then you could filter the found Hough segments by their length, respectively. Look for sets of colinear line segments, which might be the parts of a single but broken line.
- What orientation have the plates? Parallel to the image main axes? Or can they be rotated or even warped by depth projection? In the first case of axe-parallel plates, restrict to all Hough line with angle orientations of 0° or 90°.
Have you applied contrast normalization on the original image? What do the Canny edge images look like, are they already suited for finding plates? Can You see the plates on the edge images or are they hidden between so many edgels or got split apart too much? What about the thresholds for Canny detector?
Finally, have you googled for papers about plate-finding algorithms?
