opencv/doc/tutorials/Drawing_2.rst

.. _Drawing_2:

Fancy Drawing!
****************

Goals
======

In this tutorial you will learn how to:

* Use the *Random Number generator class* (:rng:`RNG <>`) and how to get a random number from a uniform distribution.
* Display Text on an OpenCV window by using the function :put_text:`putText <>`

Code
=====
* In the previous tutorial we drew diverse geometric figures, giving as input parameters such as coordinates (in the form of :point:`Points <>`), color, thickness, etc. You might have noticed that we gave specific values for these arguments.
 
* In this tutorial, we intend to use *random* values for the drawing parameters. Also, we intend to populate our image with a big number of geometric figures. Since we will be initializing them in a random fashion, this process will be automatic and made by using *loops*

* You will find the code in the *samples/cpp* folder of your OpenCV distribution. The file is **drawing_2.cpp**

Explanation
============

#. Let's start by checking out the *main* function. We observe that first thing we do is creating a *Random Number Generator* object (RNG):

   .. code-block:: cpp

      RNG rng( 0xFFFFFFFF );

   RNG implements a random number generator. In this example, *rng* is a RNG element initialized with the value *0xFFFFFFFF*

#. Then we create a matrix initialized to *zeros* (which means that it will appear as black), specifying its height, width and its type:

   .. code-block:: cpp

      /// Initialize a matrix filled with zeros
      Mat image = Mat::zeros( window_height, window_width, CV_8UC3 );

      /// Show it in a window during DELAY ms
      imshow( window_name, image ); 

#. Then we proceed to draw crazy stuff. After taking a look at the code, you can see that it is mainly divided in 8 sections, defined as functions:

   .. code-block:: cpp

      /// Now, let's draw some lines
      c = Drawing_Random_Lines(image, window_name, rng);
      if( c != 0 ) return 0;

      /// Go on drawing, this time nice rectangles
      c = Drawing_Random_Rectangles(image, window_name, rng);
      if( c != 0 ) return 0;

      /// Draw some ellipses
      c = Drawing_Random_Ellipses( image, window_name, rng );
      if( c != 0 ) return 0;

      /// Now some polylines
      c = Drawing_Random_Polylines( image, window_name, rng );
      if( c != 0 ) return 0;

      /// Draw filled polygons
      c = Drawing_Random_Filled_Polygons( image, window_name, rng );
      if( c != 0 ) return 0;

      /// Draw circles
      c = Drawing_Random_Circles( image, window_name, rng );
      if( c != 0 ) return 0;

      /// Display text in random positions
      c = Displaying_Random_Text( image, window_name, rng );
      if( c != 0 ) return 0;

      /// Displaying the big end!
      c = Displaying_Big_End( image, window_name, rng );

   All of these functions follow the same pattern, so we will analyze only a couple of them, since the same explanation applies for all.

#. Checking out the function **Drawing_Random_Lines**:

   .. code-block:: cpp

      /**
       * @function Drawing_Random_Lines
       */
      int Drawing_Random_Lines( Mat image, char* window_name, RNG rng )
      {
        int lineType = 8;
        Point pt1, pt2;

	for( int i = 0; i < NUMBER; i++ )
  	{
    	 pt1.x = rng.uniform( x_1, x_2 );
    	 pt1.y = rng.uniform( y_1, y_2 );
    	 pt2.x = rng.uniform( x_1, x_2 );
    	 pt2.y = rng.uniform( y_1, y_2 );

         line( image, pt1, pt2, randomColor(rng), rng.uniform(1, 10), 8 );
    	 imshow( window_name, image );
         if( waitKey( DELAY ) >= 0 )
      	   { return -1; }
        }

        return 0;
      }

   We can observe the following:

   * The *for* loop will repeat **NUMBER** times. Since the function :line:`line <>` is inside this loop, that means that **NUMBER** lines will be generated.
   * The line extremes are given by *pt1* and *pt2*. For *pt1* we can see that:
  
     .. code-block:: cpp
         
        pt1.x = rng.uniform( x_1, x_2 );    
        pt1.y = rng.uniform( y_1, y_2 );

     * We know that **rng** is a *Random number generator* object. In the code above we are calling **rng.uniform(a,b)**. This generates a radombly uniformed distribution between the values **a** and **b** (inclusive in **a**, exclusive in **b**). 

     * From the explanation above, we deduce that the extremes *pt1* and *pt2* will be random values, so the lines positions will be quite impredictable, giving a nice visual effect (check out the Result section below).

     * As another observation, we notice that in the :line:`line <>` arguments, for the *color* input we enter:

       .. code-block:: cpp
	   
	  randomColor(rng)           
	   
       Let's check the function implementation:

       .. code-block:: cpp

	  static Scalar randomColor( RNG& rng )
          {
            int icolor = (unsigned) rng;
            return Scalar( icolor&255, (icolor>>8)&255, (icolor>>16)&255 );
	  }

       As we can see, the return value is an *Scalar* with 3 randomly initialized values, which are used as the *R*, *G* and *B* parameters for the line color. Hence, the color of the lines will be random too!

#. The explanation above applies for the other functions generating circles, ellipses, polygones, etc. The parameters such as *center* and *vertices* are also generated randomly. 

#. Before finishing, we also should take a look at the functions *Display_Random_Text* and *Displaying_Big_End*, since they both have a few interesting features:

#. **Display_Random_Text:**

   .. code-block:: cpp

      int Displaying_Random_Text( Mat image, char* window_name, RNG rng )
      {
        int lineType = 8;

        for ( int i = 1; i < NUMBER; i++ )
        {
          Point org;
          org.x = rng.uniform(x_1, x_2);
          org.y = rng.uniform(y_1, y_2);

          putText( image, "Testing text rendering", org, rng.uniform(0,8),
                   rng.uniform(0,100)*0.05+0.1, randomColor(rng), rng.uniform(1, 10), lineType);
        
          imshow( window_name, image );
          if( waitKey(DELAY) >= 0 )
            { return -1; }
        }

        return 0;
      }

   Everything looks familiar but the expression:

   .. code-block:: cpp

      putText( image, "Testing text rendering", org, rng.uniform(0,8),
               rng.uniform(0,100)*0.05+0.1, randomColor(rng), rng.uniform(1, 10), lineType);   


   So, what does the function :put_text:`putText <>` do? In our example:

   * Draws the text **"Testing text rendering"** in **image**
   * The bottom-left corner of the text will be located in the Point **org**
   * The font type is a random integer value in the range: :math:`[0, 8>`.
   * The scale of the font is denoted by the expression **rng.uniform(0, 100)x0.05 + 0.1** (meaning its range is: :math:`[0.1, 5.1>`)
   * The text color is random (denoted by **randomColor(rng)**)
   * The text thickness ranges between 1 and 10, as specified by **rng.uniform(1,10)**

   As a result, we will get (analagously to the other drawing functions) **NUMBER** texts over our image, in random locations.

#. **Displaying_Big_End**

   .. code-block:: cpp

      int Displaying_Big_End( Mat image, char* window_name, RNG rng )
      {
        Size textsize = getTextSize("OpenCV forever!", CV_FONT_HERSHEY_COMPLEX, 3, 5, 0);
  	Point org((window_width - textsize.width)/2, (window_height - textsize.height)/2);
  	int lineType = 8;
    
	Mat image2;

  	for( int i = 0; i < 255; i += 2 )
  	{
    	  image2 = image - Scalar::all(i);
    	  putText( image2, "OpenCV forever!", org, CV_FONT_HERSHEY_COMPLEX, 3,
          	   Scalar(i, i, 255), 5, lineType );
        
    	  imshow( window_name, image2 );
    	  if( waitKey(DELAY) >= 0 )
       	  { return -1; }
  	}

        return 0;
      }

   Besides the function **getTextSize** (which gets the size of the argument text), the new operation we can observe is inside the *foor* loop:

   .. code-block:: cpp

      image2 = image - Scalar::all(i)

   So, **image2** is the substraction of **image** and **Scalar::all(i)**. In fact, what happens here is that every pixel of **image2** will be the result of substracting every pixel of **image** minus the value of **i** (remember that for each pixel we are considering three values such as R, G and B, so each of them will be affected)

  Also remember that the substraction operation *always* performs internally a **saturate** operation, which means that the result obtained will always be inside the allowed range (no negative and between 0 and 255 for our example).   
 

Result
========

As you just saw in the Code section, the program will sequentially execute diverse drawing functions, which will produce:

#. First a random set of *NUMBER* lines will appear on screen such as it can be seen in this screenshot:

   .. image:: images/Drawing_2_Tutorial_Result_0.png
      :height: 300px
      :alt: Drawing Tutorial 2 - Final Result 0
      :align: center 

#. Then, a new set of figures, these time *rectangles* will follow:

   .. image:: images/Drawing_2_Tutorial_Result_1.png
      :height: 300px
      :alt: Drawing Tutorial 2 - Final Result 1
      :align: center 

#. Now some ellipses will appear, each of them with random position, size, thickness and arc length:

   .. image:: images/Drawing_2_Tutorial_Result_2.png
      :height: 300px
      :alt: Drawing Tutorial 2 - Final Result 2
      :align: center 

#. Now, *polylines* with 03 segments will appear on screen, again in random configurations.

   .. image:: images/Drawing_2_Tutorial_Result_3.png
      :height: 300px
      :alt: Drawing Tutorial 2 - Final Result 3
      :align: center 

#. Filled polygons (in this example triangles) will follow:

   .. image:: images/Drawing_2_Tutorial_Result_4.png
      :height: 300px
      :alt: Drawing Tutorial 2 - Final Result 4
      :align: center 

#. The last geometric figure to appear: circles!

   .. image:: images/Drawing_2_Tutorial_Result_5.png
      :height: 300px
      :alt: Drawing Tutorial 2 - Final Result 5
      :align: center 

#. Near the end, the text *"Testing Text Rendering"* will appear in a variety of fonts, sizes, colors and positions.

   .. image:: images/Drawing_2_Tutorial_Result_6.png
      :height: 300px
      :alt: Drawing Tutorial 2 - Final Result 6
      :align: center 

#. And the big end (which by the way expresses a big truth too):

   .. image:: images/Drawing_2_Tutorial_Result_7.png
      :height: 300px
      :alt: Drawing Tutorial 2 - Final Result 7
      :align: center