.. _Drawing_2: Random generator and text with OpenCV ************************************* Goals ====== In this tutorial you will learn how to: .. container:: enumeratevisibleitemswithsquare * 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 ===== .. container:: enumeratevisibleitemswithsquare * In the previous tutorial (:ref:`Drawing_1`) 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* . * This code is in your OpenCV sample folder. Otherwise you can grab it from `here `_ . 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 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: .. container:: enumeratevisibleitemswithsquare * 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.jpg :alt: Drawing Tutorial 2 - Final Result 0 :align: center #. 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: .. image:: images/Drawing_2_Tutorial_Result_2.jpg :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.jpg :alt: Drawing Tutorial 2 - Final Result 3 :align: center #. Filled polygons (in this example triangles) will follow. #. The last geometric figure to appear: circles! .. image:: images/Drawing_2_Tutorial_Result_5.jpg :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. #. And the big end (which by the way expresses a big truth too): .. image:: images/Drawing_2_Tutorial_Result_7.jpg :alt: Drawing Tutorial 2 - Final Result 7 :align: center