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===== Tutorial 2 =====

 On this tutorial you will learn how to access a specific pixel in an image , and how to read the properties of this pixel ( RGB value and position ).

I recommend you to type the code on your own to get familiarized with the program language. If you have trouble , the original code is attached bellow ( Running on Visual Studio 2015 + OpenCV 3.1 ) * Check the installation guide to make sure that you linked all the OpenCV modules to your Visual Studio.

{{::pixel_value_and_location.rar|Pixel Value and Location}}



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===== Pixel Operations =====

{{ :pixel_value.jpg?direct |}}

<Code:c++ linenums:1>
#include <iostream>
#include <stdio.h>
#include <opencv2/opencv.hpp>

using namespace cv;
using namespace std;


Mat image;

//function to get the pixel RGB value at the given y column and x row
static void onMouse(int event, int x, int y, int f, void*) {

	Vec3b pix = image.at<Vec3b>(y, x);
	int B = pix.val[0];
	int G = pix.val[1];
	int R = pix.val[2];

	cout << "R:" << R << "   G:" << G << "   B:" << B << "  \t";
	cout << "Coordinate x:" << x << "     Coordinate y:" << y << "\n";

}



int main(int argc, char** argv)
{

	//Read the image from your archive and resize
	image = imread("C:/Users/jcunhamatos/Desktop/s63.jpg");
	resize(image, image, Size(640, 480));

	//show the image in a window
	imshow("Pixel position and RGB values", image);

	//use function setMouseCallback with our onMouse function to get the pixel properties
	//on the mouse pointer location
	setMouseCallback("Pixel position and RGB values", onMouse, 0);

	waitKey(0);
	return 0;
}
</Code>


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===== Understanding the Code =====

 Now that you have followed tutorial 1 , we already know how to open an image and how the program works. Lets analyze the new things on this tutorial.

<Code:c++ linenums:11>
//function to get the pixel RGB value at the given y column and x row
static void onMouse(int event, int x, int y, int f, void*) {

	Vec3b pix = image.at<Vec3b>(y, x);
	int B = pix.val[0];
	int G = pix.val[1];
	int R = pix.val[2];

	cout << "R:" << R << "   G:" << G << "   B:" << B << "  \t";
	cout << "Coordinate x:" << x << "     Coordinate y:" << y << "\n";

}
</Code>

 We can define a function to be called on the main program to get the RGB values (On OpenCV we always read BGR ) and the pixel's row(x) and column(y). **"Vec3b"** is the type of the pix variable , which will store the B[0], G[1] and R[2] color values from the image (which can be accessed using **".val[]"** ). The pix variable will receive the values from the Mat variable image - and using **"image.at<Vec3b>(y, x)"** we assign the pixel values from the image variable at the position (y,x) - which is where the mouse is - to the pix variable. Creating three integer variables we can access what is stored on the pix vector using ".val[]". This function will be called using the **"setMouseCallback"** command on the main program. This function will not return any value , it will just print the values on the CMD window opened when you run the program. To display any variables on the CMD Window we use the **"cout <<"** command. If we want to display a text message we need to use **" "** , and if it is a variable we just put the variable name . To separate the string from the variables we use **" << "** .


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<Code:c++ linenums:36>
//use function setMouseCallback with our onMouse function to get the piexel properties
	//on the mouse pointer location
	setMouseCallback("Pixel position and RGB values", onMouse, 0);

</Code>

On the main program we call our onMouse function using the setMouseCallback . It will set the mouse handler for the  first argument window (which we opened using the imread function ). On this case we don't set any event (just call if a certain mouse button is pressed ). If you want to call the function just in specific cases , you can find an example [[http://docs.opencv.org/2.4/modules/highgui/doc/user_interface.html|Here]] on the setMouseCallback section.
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The video below will demonstrate the program in real time.
{{youtube>rYMtRWtgx9Q?medium}}