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142 lines
4.2 KiB
142 lines
4.2 KiB
Adding a Trackbar to our applications! {#tutorial_trackbar} |
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- In the previous tutorials (about *linear blending* and the *brightness and contrast |
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adjustments*) you might have noted that we needed to give some **input** to our programs, such |
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as \f$\alpha\f$ and \f$beta\f$. 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*) |
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for you. An example of this is a **Trackbar** |
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- In this tutorial we will just modify our two previous programs so that they get the input |
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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 @ref cv::createTrackbar |
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Code |
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---- |
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Let's modify the program made in the tutorial @ref tutorial_adding_images. We will let the user enter the |
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\f$\alpha\f$ value by using the Trackbar. |
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@code{.cpp} |
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#include <opencv2/opencv.hpp> |
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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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@endcode |
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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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1. First, we load 02 images, which are going to be blended. |
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@code{.cpp} |
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src1 = imread("../../images/LinuxLogo.jpg"); |
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src2 = imread("../../images/WindowsLogo.jpg"); |
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@endcode |
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2. 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{.cpp} |
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namedWindow("Linear Blend", 1); |
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@endcode |
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3. Now we can create the Trackbar: |
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@code{.cpp} |
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createTrackbar( TrackbarName, "Linear Blend", &alpha_slider, alpha_slider_max, on_trackbar ); |
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@endcode |
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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 \f$0\f$ to **alpha_slider_max** (the minimum |
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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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4. Finally, we have to define the callback function **on_trackbar** |
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@code{.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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@endcode |
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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, |
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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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 |
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- As a manner of practice, you can also add 02 trackbars for the program made in |
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@ref tutorial_basic_linear_transform. One trackbar to set \f$\alpha\f$ and another for \f$\beta\f$. The output might |
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look like: |
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