OpenCV
3.4.1
Open Source Computer Vision

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 readonly 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 nonzero 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 nonzero 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 nonzero 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 squarelike contours the error will be minimal. For small and/or oblong contours the error can be quite large.