title: opencv sliding window
body: <p>Is there any built in library for sliding a window (custom size) over an image in opencv version 2.x?
  I tried to write the algorithm by myself but I found it very painful and probably error-prone.
  I need to slide over an image and create histogram for the input of svm.
  there is one for HOG Descriptor, which calculates HOG features but I have my own feature set so I just need an algorithm to let me slide over an image.</p>