OpenCV Mat element types and their sizes

Question

I\'m confused by the OpenCV Mat element types. This is from the docs:

There is a limited fixed set of primitive data types the library can operate on.
That i         


        

Answer 1

While C++ doesn't define the size of an element, the question is hypothetical: for systems OpenCV is run on, the sizes are known. Given

cv::Mat m(32,32,CV_32SC1, cv:Scalar(0));
std::cout << "size of the element in bytes: " << m.depth() << std::endl;
std::cout << "or " << m.step.p[ m.dims-1 ]/m.channels() << std::endl;

So how can you be sure it is int?

An attempt to call

int pxVal = m.at(0,0);

will

CV_DbgAssert( elemSize()==sizeof(int) );

Where the left hand is defined via the cv::Mat::flags -- in this example as the predefined depth of the CV_32SC1 equal to

CV_DbgAssert( m.depth() == sizeof(int) )

or

CV_DbgAssert( 4 == sizeof(int) )

So if you succeeded you are left only the endianness. And that was checked when the cvconfig.h was generated (by CMake).

TL;DR, expect the types given in the header and you'll be fine.