Frequently Asked Questions {#faq}
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-- **What is InputArray?**
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- 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?
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- This is the proxy class for passing read-only input arrays into OpenCV functions.
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- 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.
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- for more information see cv::_InputArray
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-- **Which is more efficient, use contourArea() or count number of ROI non-zero pixels?**
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- 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?
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- 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).