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157 lines
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ReStructuredText
157 lines
4.5 KiB
ReStructuredText
.. _Adding_Trackbars:
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Adding a Trackbar to our applications!
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***************************************
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* In the previous tutorials (about *linear blending* and the *brightness and contrast adjustments*) you might have noted that we needed to give some **input** to our programs, such as :math:`\alpha` and :math:`beta`. We accomplished that by entering this data using the Terminal
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* Well, it is time to use some fancy GUI tools. OpenCV provides some GUI utilities (*highgui.h*) for you. An example of this is a **Trackbar**
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.. image:: images/Adding_Trackbars_Tutorial_Trackbar.png
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:alt: Trackbar example
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:align: center
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* In this tutorial we will just modify our two previous programs so that they get the input information from the trackbar.
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Goals
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======
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In this tutorial you will learn how to:
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* Add a Trackbar in an OpenCV window by using :create_trackbar:`createTrackbar <>`
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Code
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=====
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Let's modify the program made in the tutorial :ref:`Adding_Images`. We will let the user enter the :math:`\alpha` value by using the Trackbar.
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.. code-block:: cpp
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#include <cv.h>
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#include <highgui.h>
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using namespace cv;
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/// Global Variables
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const int alpha_slider_max = 100;
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int alpha_slider;
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double alpha;
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double beta;
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/// Matrices to store images
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Mat src1;
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Mat src2;
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Mat dst;
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/**
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* @function on_trackbar
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* @brief Callback for trackbar
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*/
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void on_trackbar( int, void* )
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{
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alpha = (double) alpha_slider/alpha_slider_max ;
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beta = ( 1.0 - alpha );
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addWeighted( src1, alpha, src2, beta, 0.0, dst);
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imshow( "Linear Blend", dst );
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}
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int main( int argc, char** argv )
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{
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/// Read image ( same size, same type )
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src1 = imread("../../images/LinuxLogo.jpg");
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src2 = imread("../../images/WindowsLogo.jpg");
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if( !src1.data ) { printf("Error loading src1 \n"); return -1; }
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if( !src2.data ) { printf("Error loading src2 \n"); return -1; }
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/// Initialize values
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alpha_slider = 0;
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/// Create Windows
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namedWindow("Linear Blend", 1);
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/// Create Trackbars
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char TrackbarName[50];
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sprintf( TrackbarName, "Alpha x %d", alpha_slider_max );
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createTrackbar( TrackbarName, "Linear Blend", &alpha_slider, alpha_slider_max, on_trackbar );
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/// Show some stuff
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on_trackbar( alpha_slider, 0 );
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/// Wait until user press some key
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waitKey(0);
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return 0;
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}
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Explanation
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============
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We only analyze the code that is related to Trackbar:
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#. First, we load 02 images, which are going to be blended.
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.. code-block:: cpp
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src1 = imread("../../images/LinuxLogo.jpg");
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src2 = imread("../../images/WindowsLogo.jpg");
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#. To create a trackbar, first we have to create the window in which it is going to be located. So:
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.. code-block:: cpp
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namedWindow("Linear Blend", 1);
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#. Now we can create the Trackbar:
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.. code-block:: cpp
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createTrackbar( TrackbarName, "Linear Blend", &alpha_slider, alpha_slider_max, on_trackbar );
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Note the following:
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* Our Trackbar has a label **TrackbarName**
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* The Trackbar is located in the window named **"Linear Blend"**
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* The Trackbar values will be in the range from :math:`0` to **alpha_slider_max** (the minimum limit is always **zero**).
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* The numerical value of Trackbar is stored in **alpha_slider**
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* Whenever the user moves the Trackbar, the callback function **on_trackbar** is called
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#. Finally, we have to define the callback function **on_trackbar**
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.. code-block:: cpp
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void on_trackbar( int, void* )
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{
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alpha = (double) alpha_slider/alpha_slider_max ;
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beta = ( 1.0 - alpha );
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addWeighted( src1, alpha, src2, beta, 0.0, dst);
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imshow( "Linear Blend", dst );
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}
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Note that:
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* We use the value of **alpha_slider** (integer) to get a double value for **alpha**.
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* **alpha_slider** is updated each time the trackbar is displaced by the user.
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* We define *src1*, *src2*, *dist*, *alpha*, *alpha_slider* and *beta* as global variables, so they can be used everywhere.
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Result
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=======
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* Our program produces the following output:
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.. image:: images/Adding_Trackbars_Tutorial_Result_0.jpg
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:alt: Adding Trackbars - Windows Linux
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:align: center
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* As a manner of practice, you can also add 02 trackbars for the program made in :ref:`Basic_Linear_Transform`. One trackbar to set :math:`\alpha` and another for :math:`\beta`. The output might look like:
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.. image:: images/Adding_Trackbars_Tutorial_Result_1.jpg
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:alt: Adding Trackbars - Lena
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:align: center
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