Merge pull request #2387 from alalek:fix_ximgproc_slic_empty_mat

* ximgproc(test): createSuperpixelSLIC smoke test

* ximgproc: don't use 'operator+()' with empty cv::Mat

modules/ximgproc/src/slic.cpp

@@ -247,14 +247,14 @@ void SuperpixelSLICImpl::initialize()
     // intitialize label storage
     m_klabels = Mat( m_height, m_width, CV_32S, Scalar::all(0) );
 
-    // storage for edge magnitudes
-    Mat edgemag = Mat( m_height, m_width, CV_32F, Scalar::all(0) );
-
     // perturb seeds is not absolutely necessary,
     // one can set this flag to false
     bool perturbseeds = true;
 
-    if ( perturbseeds ) DetectChEdges( edgemag );
+    // storage for edge magnitudes
+    Mat edgemag;
+    if (perturbseeds)
+      DetectChEdges(edgemag);
 
     if( m_algorithm == SLICO )
       GetChSeedsK();
@@ -268,7 +268,8 @@ void SuperpixelSLICImpl::initialize()
     m_numlabels = (int)m_kseeds[0].size();
 
     // perturb seeds given edges
-    if ( perturbseeds ) PerturbSeeds( edgemag );
+    if (perturbseeds)
+      PerturbSeeds(edgemag);
 
     if( m_algorithm == MSLIC )
     {
@@ -569,12 +570,21 @@ inline void SuperpixelSLICImpl::DetectChEdges( Mat &edgemag )
         Sobel( m_chvec[c], dy, CV_32F, 0, 1, 1, 1.0f, 0.0f, BORDER_DEFAULT );
 
         // acumulate ^2 derivate
-        S_dx = S_dx + dx.mul(dx);
-        S_dy = S_dy + dy.mul(dy);
-
+        MatExpr dx2 = dx.mul(dx);
+        MatExpr dy2 = dy.mul(dy);
+        if (S_dx.empty())
+        {
+            S_dx = dx2;
+            S_dy = dy2;
+        }
+        else
+        {
+            S_dx += dx2;
+            S_dy += dy2;
+        }
     }
     // total magnitude
-    edgemag += S_dx + S_dy;
+    edgemag = S_dx + S_dy;
 }
 
 /*

modules/ximgproc/test/test_slic.cpp

@@ -0,0 +1,22 @@
+// This file is part of OpenCV project.
+// It is subject to the license terms in the LICENSE file found in the top-level directory
+// of this distribution and at http://opencv.org/license.html.
+#include "test_precomp.hpp"
+
+namespace opencv_test { namespace {
+
+TEST(ximgproc_SuperpixelSLIC, smoke)
+{
+    Mat img = imread(cvtest::findDataFile("cv/shared/lena.png"), IMREAD_COLOR);
+    Mat labImg;
+    cvtColor(img, labImg, COLOR_BGR2Lab);
+    Ptr< SuperpixelSLIC> slic = createSuperpixelSLIC(labImg);
+    slic->iterate(5);
+    Mat outLabels;
+    slic->getLabels(outLabels);
+    EXPECT_FALSE(outLabels.empty());
+    int numSuperpixels = slic->getNumberOfSuperpixels();
+    EXPECT_GT(numSuperpixels, 0);
+}
+
+}} // namespace