Accessing Pixels and other basic necessary properties of an image

Accessing individual Pixels of an image

OpenCV Mat class provides .at() operator through which you can access and change individual pixels of a particular channel(B, G, or R channel).

Usage :

/* Read the image */
Mat in_image = imread("logo.png", CV_LOAD_IMAGE_COLOR);

/* Define the datatype for one pixel */
Vec3b pixel1 = in_image.at<Vec3b>(100,100));

/* Print BGR values of the pixel at 100th row and 100th col of the image */ 
cout << "Given pixel (B):" << (unsigned int)pixel1[0] << endl;
cout << "Given pixel (G):" << (unsigned int)pixel1[1] << endl;
cout << "Given pixel (R):" << (unsigned int)pixel1[2] << endl;

Get the Size of the image

OpenCV Mat class provides .size() operator through which you can get the size of the image.

Usage :

/* Read the image */
Mat in_image = imread("logo.png", CV_LOAD_IMAGE_COLOR);

/*Print size for the image read */
Size siz = in_image.size();

cout << "Size of the Image" << endl;
cout << "Width:" << siz.width << endl;
cout << "Height:" << siz.height << endl;

img.size() - will tell you the size of the image read

Get the number of channels in an image

OpenCV Mat class provides .channels() operator through which you can get the channels of the image.

/* Read the image */
Mat in_image = imread("logo.png", CV_LOAD_IMAGE_COLOR);

/*Print channels of the image read */
int ch = in_image.channels();
cout << "Image channels:" << ch << endl;

Get the Bit depth of the image

OpenCV Mat class provides .depth() operator through which you can get the bit depth of the image in terms of the CV_8U datatypes.

/* Read the image */
Mat in_image = imread("logo.png", CV_LOAD_IMAGE_COLOR);

/*Print pixel depth image read */
int d = in_image.depth();
cout << "Image depth:" << d << endl;

.clone()

Copy the image to another variable

OpenCV Mat class provides .clone() operator through which you can copy one image to another variable.

/* Read the image */
Mat in_image = imread("logo.png", CV_LOAD_IMAGE_COLOR);

/* Copy Input image to out variable */
Mat out = in_image.clone();

Color space conversion

OpenCV provides one function cvtColor() which can convert the colorspace of the image to predefined desired colorspaces.

You only need to change the 3rd argument to get the same image in different color spaces.

Usage : This example shows one image getting converted to 4 different color spaces

// Define variables to store the image 
Mat image, HSV, Luv, Lab, YCrCb, XYZ, Rimg, Bimg, Gimg;

//Read image
image = imread("02.jpg", CV_LOAD_IMAGE_COLOR);

//Convert RGB image to different color spaces
cvtColor(image, HSV, CV_BGR2HSV);
cvtColor(image, Luv, CV_BGR2Luv);
cvtColor(image, Lab, CV_BGR2Lab);
cvtColor(image, YCrCb, CV_BGR2YCrCb);
cvtColor(image, XYZ, CV_BGR2XYZ);

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