some improvements on tutorials

samples/cpp/tutorial_code/ImgProc/BasicLinearTransforms.cpp

@@ -15,7 +15,7 @@ using namespace cv;
  * @function main
  * @brief Main function
  */
-int main( int, char** argv )
+int main( int argc, char** argv )
 {
     //! [basic-linear-transform-parameters]
     double alpha = 1.0; /*< Simple contrast control */
@@ -24,7 +24,12 @@ int main( int, char** argv )
 
     /// Read image given by user
     //! [basic-linear-transform-load]
-    Mat image = imread( argv[1] );
+    String imageName("../data/lena.jpg"); // by default
+    if (argc > 1)
+    {
+        imageName = argv[1];
+    }
+    Mat image = imread( imageName );
     //! [basic-linear-transform-load]
     //! [basic-linear-transform-output]
     Mat new_image = Mat::zeros( image.size(), image.type() );

samples/cpp/tutorial_code/ImgProc/Morphology_2.cpp

@@ -29,10 +29,15 @@ void Morphology_Operations( int, void* );
 /**
  * @function main
  */
-int main( int, char** argv )
+int main( int argc, char** argv )
 {
   //![load]
-  src = imread( argv[1], IMREAD_COLOR ); // Load an image
+  String imageName("../data/baboon.jpg"); // by default
+  if (argc > 1)
+  {
+  imageName = argv[1];
+  }
+  src = imread(imageName, IMREAD_COLOR); // Load an image
 
   if( src.empty() )
     { return -1; }

samples/cpp/tutorial_code/ImgProc/Threshold.cpp

@@ -30,10 +30,15 @@ void Threshold_Demo( int, void* );
 /**
  * @function main
  */
-int main( int, char** argv )
+int main( int argc, char** argv )
 {
   //! [load]
-  src = imread( argv[1], IMREAD_COLOR ); // Load an image
+  String imageName("../data/stuff.jpg"); // by default
+  if (argc > 1)
+  {
+      imageName = argv[1];
+  }
+  src = imread( imageName, IMREAD_COLOR ); // Load an image
 
   if( src.empty() )
     { return -1; }

samples/cpp/tutorial_code/ImgProc/changing_contrast_brightness_image/changing_contrast_brightness_image.cpp

@@ -58,9 +58,16 @@ void on_gamma_correction_trackbar(int, void *)
 }
 }
 
-int main( int, char** argv )
+int main( int argc, char** argv )
 {
-    img_original = imread( argv[1] );
+
+    String imageName("../data/lena.jpg"); // by default
+    if (argc > 1)
+    {
+        imageName = argv[1];
+    }
+
+    img_original = imread( imageName );
     img_corrected = Mat(img_original.rows, img_original.cols*2, img_original.type());
     img_gamma_corrected = Mat(img_original.rows, img_original.cols*2, img_original.type());