Random generator and text with OpenCV {#tutorial_random_generator_and_text}
=====================================

@tableofcontents

@prev_tutorial{tutorial_basic_geometric_drawing}
@next_tutorial{tutorial_gausian_median_blur_bilateral_filter}

|    |    |
| -: | :- |
| Original author | Ana Huamán |
| Compatibility | OpenCV >= 3.0 |

Goals
-----

In this tutorial you will learn how to:

-   Use the *Random Number generator class* (@ref cv::RNG ) and how to get a random number from a
    uniform distribution.
-   Display text on an OpenCV window by using the function @ref cv::putText

Code
----

-   In the previous tutorial (@ref tutorial_basic_geometric_drawing) we drew diverse geometric figures, giving as input
    parameters such as coordinates (in the form of @ref cv::Point), 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* .
-   This code is in your OpenCV sample folder. Otherwise you can grab it from
    [here](https://github.com/opencv/opencv/blob/5.x/samples/cpp/tutorial_code/ImgProc/basic_drawing/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{.cpp}
    RNG rng( 0xFFFFFFFF );
    @endcode
    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{.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 );
    @endcode
-#  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{.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 );
    @endcode
    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{.cpp}
    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;
    }
    @endcode
    We can observe the following:

    -   The *for* loop will repeat **NUMBER** times. Since the function @ref cv::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{.cpp}
        pt1.x = rng.uniform( x_1, x_2 );
        pt1.y = rng.uniform( y_1, y_2 );
        @endcode
        -   We know that **rng** is a *Random number generator* object. In the code above we are
            calling **rng.uniform(a,b)**. This generates a randomly 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 @ref cv::line arguments, for the *color*
            input we enter:
            @code{.cpp}
            randomColor(rng)
            @endcode
            Let's check the function implementation:
            @code{.cpp}
            static Scalar randomColor( RNG& rng )
              {
              int icolor = (unsigned) rng;
              return Scalar( icolor&255, (icolor>>8)&255, (icolor>>16)&255 );
              }
            @endcode
            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, polygons,
    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{.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;
    }
    @endcode
    Everything looks familiar but the expression:
    @code{.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);
    @endcode
    So, what does the function @ref cv::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: \f$[0, 8>\f$.
    -   The scale of the font is denoted by the expression **rng.uniform(0, 100)x0.05 + 0.1**
        (meaning its range is: \f$[0.1, 5.1>\f$)
    -   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{.cpp}
    int Displaying_Big_End( Mat image, char* window_name, RNG rng )
    {
      Size textsize = getTextSize("OpenCV forever!", 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, FONT_HERSHEY_COMPLEX, 3,
               Scalar(i, i, 255), 5, lineType );

        imshow( window_name, image2 );
        if( waitKey(DELAY) >= 0 )
          { return -1; }
      }

      return 0;
    }
    @endcode
    Besides the function **getTextSize** (which gets the size of the argument text), the new
    operation we can observe is inside the *foor* loop:
    @code{.cpp}
    image2 = image - Scalar::all(i)
    @endcode
    So, **image2** is the subtraction of **image** and **Scalar::all(i)**. In fact, what happens
    here is that every pixel of **image2** will be the result of subtracting 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 subtraction 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:

    ![](images/Drawing_2_Tutorial_Result_0.jpg)

-#  Then, a new set of figures, these time *rectangles* will follow.
-#  Now some ellipses will appear, each of them with random position, size, thickness and arc
    length:

    ![](images/Drawing_2_Tutorial_Result_2.jpg)

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

    ![](images/Drawing_2_Tutorial_Result_3.jpg)

-#  Filled polygons (in this example triangles) will follow.
-#  The last geometric figure to appear: circles!

    ![](images/Drawing_2_Tutorial_Result_5.jpg)

-#  Near the end, the text *"Testing Text Rendering"* will appear in a variety of fonts, sizes,
    colors and positions.
-#  And the big end (which by the way expresses a big truth too):

    ![](images/Drawing_2_Tutorial_Result_big.jpg)
