a part of PR #11364 (extended findNonZero & PSNR) (#11837)

* a part of https://github.com/opencv/opencv/pull/11364 by Tetragramm. Rewritten and extended findNonZero & PSNR to support more types, not just 8u.

* fixed compile & doxygen warnings

* fixed small bug in findNonZero test

modules/core/include/opencv2/core.hpp

@@ -599,7 +599,7 @@ or
     // access pixel coordinates
     Point pnt = locations[i];
 @endcode
-@param src single-channel array (type CV_8UC1)
+@param src single-channel array
 @param idx the output array, type of cv::Mat or std::vector<Point>, corresponding to non-zero indices in the input
 */
 CV_EXPORTS_W void findNonZero( InputArray src, OutputArray idx );
@@ -699,7 +699,8 @@ CV_EXPORTS double norm( const SparseMat& src, int normType );
 
 /** @brief Computes the Peak Signal-to-Noise Ratio (PSNR) image quality metric.
 
-This function calculates the Peak Signal-to-Noise Ratio (PSNR) image quality metric in decibels (dB), between two input arrays src1 and src2. Arrays must have depth CV_8U.
+This function calculates the Peak Signal-to-Noise Ratio (PSNR) image quality metric in decibels (dB),
+between two input arrays src1 and src2. The arrays must have the same type.
 
 The PSNR is calculated as follows:
 
@@ -707,13 +708,15 @@ The PSNR is calculated as follows:
 \texttt{PSNR} = 10 \cdot \log_{10}{\left( \frac{R^2}{MSE} \right) }
 \f]
 
-where R is the maximum integer value of depth CV_8U (255) and MSE is the mean squared error between the two arrays.
+where R is the maximum integer value of depth (e.g. 255 in the case of CV_8U data)
+and MSE is the mean squared error between the two arrays.
 
 @param src1 first input array.
 @param src2 second input array of the same size as src1.
+@param R the maximum pixel value (255 by default)
 
   */
-CV_EXPORTS_W double PSNR(InputArray src1, InputArray src2);
+CV_EXPORTS_W double PSNR(InputArray src1, InputArray src2, double R=255.);
 
 /** @brief naive nearest neighbor finder