message: Migrated FAQ page to Github Wiki
diff: Frequently Asked Questions {#faq}
  ==========================
  -- **What is InputArray?**
  -
  - It can be seen that almost all OpenCV functions receive InputArray type.
  - What is it, and how can I understand the actual input types of parameters?
  -
  -
  - This is the proxy class for passing read-only input arrays into OpenCV functions.
  -
  - Inside a function you should use cv::_InputArray::getMat() method to construct
  -a matrix header for the array (without copying data). cv::_InputArray::kind() can be used to distinguish Mat from vector<> etc.
  -but normally it is not needed.
  -
  - for more information see cv::_InputArray
  -
  -- **Which is more efficient, use contourArea() or count number of ROI non-zero pixels?**
  -
  - In a case where you only want relative areas, which one is faster to compute:
  - calculate a contour area or count the number of ROI non-zero pixels?
  -
  - cv::contourArea() uses Green formula (http://en.wikipedia.org/wiki/Green's_theorem) to compute the area, therefore its complexity is O(contour_number_of_vertices). Counting non-zero pixels in the ROI is O(roi_width*roi_height) algorithm, i.e. much slower. Note, however, that because of finite, and quite low, resolution of the raster grid, the two algorithms will give noticeably different results. For large and square-like contours the error will be minimal. For small and/or oblong contours the error can be quite large.
  +Compatibility page. FAQ migrated to the project [wiki](https://github.com/opencv/opencv/wiki/FAQ).
  