Explain usage when called without arguments

CGAL_ImageIO/include/CGAL/Image_3.h

@@ -165,10 +165,6 @@ public:
                                          vx,vy,vz,offset));
   }
 
-#if 0
-  bool read_vtk_image_data(vtkImageData*);
-#endif // CGAL_USE_VTK
-
   // implementation in src/CGAL_ImageIO/Image_3.cpp
   void gl_draw(const float point_size,
                const unsigned char r,

Mesh_3/examples/Mesh_3/mesh_3D_gray_vtk_image.cpp

@@ -37,7 +37,10 @@ using namespace CGAL::parameters;
 int main(int argc, char* argv[])
 {
   // Loads image
-   
+  if(argc == 1){
+    std::cerr << "Usage:  " << argv[0] << " <directory with dicom data>\n";
+    return 0;
+  }
 
   vtkDICOMImageReader*dicom_reader = vtkDICOMImageReader::New();
   dicom_reader->SetDirectoryName(argv[1]);