fixed GPU minMaxLoc test, updated docs

doc/gpu_matrix_reductions.tex

@@ -89,7 +89,7 @@ void minMax(const GpuMat\& src, double* minVal, double* maxVal,\par
 \cvarg{src}{Single-channel source image.}
 \cvarg{minVal}{Pointer to returned minimum value. \texttt{NULL} if not required.}
 \cvarg{maxVal}{Pointer to returned maximum value. \texttt{NULL} if not required.}
-\cvarg{mask}{Optional mask to select a sub-matrix.}
+\cvarg{mask}{Optional mask to select a sub-matrix. Please note the result is undefined in the case of empty mask.}
 \cvarg{buf}{Optional buffer to avoid extra memory allocations. It's resized automatically.}
 \end{description}
 
@@ -112,7 +112,7 @@ void minMaxLoc(const GpuMat\& src, double* minVal, double* maxVal,\par
 \cvarg{maxVal}{Pointer to returned maximum value. \texttt{NULL} if not required.}
 \cvarg{minValLoc}{Pointer to returned minimum location. \texttt{NULL} if not required.}
 \cvarg{maxValLoc}{Pointer to returned maximum location. \texttt{NULL} if not required.}
-\cvarg{mask}{Optional mask to select a sub-matrix.}
+\cvarg{mask}{Optional mask to select a sub-matrix. Please note the result is undefined in the case of empty mask.}
 \cvarg{valbuf}{Optional values buffer to avoid extra memory allocations. It's resized automatically.}
 \cvarg{locbuf}{Optional locations buffer to avoid extra memory allocations. It's resized automatically.}
 \end{description}

tests/gpu/src/arithm.cpp

@@ -829,6 +829,10 @@ struct CV_GpuMinMaxLocTest: public CvTest
         cv::Mat mask(src.size(), CV_8U);
         rng.fill(mask, RNG::UNIFORM, Scalar(0), Scalar(2));
 
+        // At least one of mask elements must be non zero as OpenCV returns 0
+        // in such case, our implementation returns max value
+        mask.at<unsigned char>(0, 0) = 1;
+
         double minVal, maxVal;
         cv::Point minLoc, maxLoc;
 
@@ -855,6 +859,10 @@ struct CV_GpuMinMaxLocTest: public CvTest
         cv::Point minLoc_, maxLoc_;        
         cv::gpu::minMaxLoc(cv::gpu::GpuMat(src), &minVal_, &maxVal_, &minLoc_, &maxLoc_, cv::gpu::GpuMat(mask), valbuf, locbuf);
 
+        cout << rows << " " << cols << " " << depth << endl;
+        cout << minVal << " " << minVal_ << endl;
+        cout << maxVal << " " << maxVal_ << endl;
+
         CHECK(minVal == minVal_, CvTS::FAIL_INVALID_OUTPUT);
         CHECK(maxVal == maxVal_, CvTS::FAIL_INVALID_OUTPUT);
         CHECK(0 == memcmp(src.ptr(minLoc.y) + minLoc.x * src.elemSize(), src.ptr(minLoc_.y) + minLoc_.x * src.elemSize(), src.elemSize()),