add explanation to the Bounding Box and Circles tutorial

- also some minor changes to the respective sample cpp file

doc/tutorials/imgproc/shapedescriptors/bounding_rects_circles/bounding_rects_circles.markdown

@@ -22,6 +22,49 @@ This tutorial code's is shown lines below. You can also download it from
 Explanation
 -----------
 
+The main function is rather simple, as follows from the comments we do the following:
+-#  Open the image, convert it into grayscale and blur it to get rid of the noise.
+    @snippet samples/cpp/tutorial_code/ShapeDescriptors/generalContours_demo1.cpp setup
+-# Create a window with header "Source" and display the source file in it.
+    @snippet samples/cpp/tutorial_code/ShapeDescriptors/generalContours_demo1.cpp createWindow
+-# Create a trackbar on the source_window and assign a callback function to it
+   In general callback functions are used to react to some kind of signal, in our
+   case it's trackbar's state change.
+   @snippet samples/cpp/tutorial_code/ShapeDescriptors/generalContours_demo1.cpp taskbar
+-# Explicit one-time call of `thresh_callback` is necessary to display
+   the "Contours" window simultaniously with the "Source" window.
+   @snippet samples/cpp/tutorial_code/ShapeDescriptors/generalContours_demo1.cpp callback00
+-# Wait for user to close the windows.
+   @snippet samples/cpp/tutorial_code/ShapeDescriptors/generalContours_demo1.cpp waitForIt
+
+
+The callback function `thresh_callback` does all the interesting job.
+
+
+-# Writes to `threshold_output` the threshold of the grayscale picture (you can check out about thresholding @ref tutorial_threshold "here").
+   @snippet samples/cpp/tutorial_code/ShapeDescriptors/generalContours_demo1.cpp threshold
+-# Finds contours and saves them to the vectors `contour` and `hierarchy`.
+    @snippet samples/cpp/tutorial_code/ShapeDescriptors/generalContours_demo1.cpp findContours
+-# For every found contour we now apply approximation to polygons
+   with accuracy +-3 and stating that the curve must me closed.
+
+   After that we find a bounding rect for every polygon and save it to `boundRect`.
+
+   At last we find a minimum enclosing circle for every polygon and
+   save it to `center` and `radius` vectors.
+   @snippet samples/cpp/tutorial_code/ShapeDescriptors/generalContours_demo1.cpp allthework
+
+We found everything we need, all we have to do is to draw.
+
+-# Create new Mat of unsigned 8-bit chars, filled with zeros.
+   It will contain all the drawings we are going to make (rects and circles).
+   @snippet samples/cpp/tutorial_code/ShapeDescriptors/generalContours_demo1.cpp zeroMat
+-# For every contour: pick a random color, draw the contour, the bounding rectangle and
+   the minimal enclosing circle with it,
+   @snippet samples/cpp/tutorial_code/ShapeDescriptors/generalContours_demo1.cpp forContour
+-# Display the results: create a new window "Contours" and show everything we added to drawings on it.
+   @snippet samples/cpp/tutorial_code/ShapeDescriptors/generalContours_demo1.cpp showDrawings
+
 Result
 ------
 

samples/cpp/tutorial_code/ShapeDescriptors/generalContours_demo1.cpp

@@ -25,22 +25,34 @@ void thresh_callback(int, void* );
  */
 int main( int, char** argv )
 {
-  /// Load source image and convert it to gray
+  //![setup]
+  /// Load source image
   src = imread( argv[1], IMREAD_COLOR );
 
   /// Convert image to gray and blur it
   cvtColor( src, src_gray, COLOR_BGR2GRAY );
   blur( src_gray, src_gray, Size(3,3) );
+  //![setup]
 
+  //![createWindow]
   /// Create Window
   const char* source_window = "Source";
   namedWindow( source_window, WINDOW_AUTOSIZE );
   imshow( source_window, src );
+  //![createWindow]
 
+  //![taskbar]
   createTrackbar( " Threshold:", "Source", &thresh, max_thresh, thresh_callback );
-  thresh_callback( 0, 0 );
+  //![taskbar]
 
+  //![callback00]
+  thresh_callback( 0, 0 );
+  //![callback00]
+
+  //![waitForIt]
   waitKey(0);
+  //![waitForIt]
+
   return(0);
 }
 
@@ -53,10 +65,15 @@ void thresh_callback(int, void* )
   vector<vector<Point> > contours;
   vector<Vec4i> hierarchy;
 
+  //![threshold]
   /// Detect edges using Threshold
   threshold( src_gray, threshold_output, thresh, 255, THRESH_BINARY );
+  //![threshold]
+
+  //![findContours]
   /// Find contours
   findContours( threshold_output, contours, hierarchy, RETR_TREE, CHAIN_APPROX_SIMPLE, Point(0, 0) );
+  //![findContours]
 
   /// Approximate contours to polygons + get bounding rects and circles
   vector<vector<Point> > contours_poly( contours.size() );
@@ -64,24 +81,33 @@ void thresh_callback(int, void* )
   vector<Point2f>center( contours.size() );
   vector<float>radius( contours.size() );
 
+  //![allthework]
   for( size_t i = 0; i < contours.size(); i++ )
-     { approxPolyDP( Mat(contours[i]), contours_poly[i], 3, true );
-       boundRect[i] = boundingRect( Mat(contours_poly[i]) );
-       minEnclosingCircle( contours_poly[i], center[i], radius[i] );
-     }
-
+  {
+    approxPolyDP( Mat(contours[i]), contours_poly[i], 3, true );
+    boundRect[i] = boundingRect( Mat(contours_poly[i]) );
+    minEnclosingCircle( contours_poly[i], center[i], radius[i] );
+  }
+  //![allthework]
 
+  //![zeroMat]
   /// Draw polygonal contour + bonding rects + circles
   Mat drawing = Mat::zeros( threshold_output.size(), CV_8UC3 );
-  for( size_t i = 0; i< contours.size(); i++ )
-     {
-       Scalar color = Scalar( rng.uniform(0, 255), rng.uniform(0,255), rng.uniform(0,255) );
-       drawContours( drawing, contours_poly, (int)i, color, 1, 8, vector<Vec4i>(), 0, Point() );
-       rectangle( drawing, boundRect[i].tl(), boundRect[i].br(), color, 2, 8, 0 );
-       circle( drawing, center[i], (int)radius[i], color, 2, 8, 0 );
-     }
+  //![zeroMat]
 
+  //![forContour]
+  for( size_t i = 0; i< contours.size(); i++ )
+  {
+    Scalar color = Scalar( rng.uniform(0, 255), rng.uniform(0,255), rng.uniform(0,255) );
+    drawContours( drawing, contours_poly, (int)i, color, 1, 8, vector<Vec4i>(), 0, Point() );
+    rectangle( drawing, boundRect[i].tl(), boundRect[i].br(), color, 2, 8, 0 );
+    circle( drawing, center[i], (int)radius[i], color, 2, 8, 0 );
+  }
+  //![forContour]
+
+  //![showDrawings]
   /// Show in a window
   namedWindow( "Contours", WINDOW_AUTOSIZE );
   imshow( "Contours", drawing );
+  //![showDrawings]
 }