Merge remote-tracking branch 'upstream/3.4' into merge-3.4

samples/cpp/tutorial_code/videoio/video-input-psnr-ssim/video-input-psnr-ssim.cpp

@@ -132,6 +132,7 @@ int main(int argc, char *argv[])
     return 0;
 }
 
+// ![get-psnr]
 double getPSNR(const Mat& I1, const Mat& I2)
 {
     Mat s1;
@@ -152,6 +153,9 @@ double getPSNR(const Mat& I1, const Mat& I2)
         return psnr;
     }
 }
+// ![get-psnr]
+
+// ![get-mssim]
 
 Scalar getMSSIM( const Mat& i1, const Mat& i2)
 {
@@ -205,3 +209,4 @@ Scalar getMSSIM( const Mat& i1, const Mat& i2)
     Scalar mssim = mean(ssim_map);   // mssim = average of ssim map
     return mssim;
 }
+// ![get-mssim]