how to recognize a zebra crossing from top view using opencv ??

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if you want use the canny algorithm in the outdoor condition you will need to estimate the threshold1 & threshold2 for canny algorithm in different lighting condition by neural network(train process).

There really huge amount of approaches for automatic threshold detection. I once saw a survey that mentioned ~300 different methods. Personally I don't really like neural networks because they are complicate, relativelly slow and require learning. My favorite method is: 1 - calculate gradients in X and Y directions (Sobel), 2 - calculate gradient magnitude in each pixel, 3 - set lower threshold to be average gradient, and higher threshold to be average + standard deviation.

thank you @michael i have already done that but the problem is the stripes are not exactly rectangle it will be like oval white stripes on black background path any idea on that ie. how to find shapes that are slightly oval ?? see that pdf document for image of the crossing!!

Extracting contours will never give you exact rectangle even for rectangular stripes. This is the world of natural images. So you should collect objects whose features are "acceptable". For example their average color is "acceptably" close to white. Or their area is "acceptably" close to expected area. Or the object covexity is "acceptable". Or proportions of minimum bounding rectangle are "acceptable". And so on. You would have to define what is "acceptable" according to real images that you may get in your application.