Tutorial 5 - Convert an Image Color OpenCV C++

Today I'm going to show you how to convert an image color using OpenCv.

Let's convert an image color using OpenCv in visual studio.

There are different color conversation in OpenCv. Before converting an image, we need to consider about the image types. Some common image color types and number of channels are as follow.

Image color type	No: of channels
BGR	3
RGB	3
BGRA	4
RGBA	4
GARY	1
XYZ	3
YCrCb	3
HSV	3
Lab	3
Luv	3
HLS	3
YUV	3

BGR image

BGR which means Blue Green Red, image is stored in a structure or unsigned integer. When it is about the occupied area Red occupying the least significant "area”, Green is next, and Blue is last.

RGB image

RGB which means Red Green Blue images are also same but only difference is order of occupying area. Blue occupying the least significant "area” Green the second least, and Red the third least.

BGRA image

RGBA which means BLUE GREEN RED ALPHA images are simply a use of extra alpha channel information in usual RGB color model.

RGBA image

Same as BGRA image only difference is significant order of color image

XYZ image

XYZ image is a 3- channel float image.

HSV image

HSV means Hue, Saturation Value images.

·         H (Hue): hue is a number from 0 to 360 degrees. It represents hues of red which starts at 0, yellow which starts at 60, green starts at 120, cyan starts at 180, blue starts at 240 and magenta starts at 300.

·         S (Saturation): Saturation is a percentage which represents the amount of gray in the color. (0 to 100)

·         V (Value): Value is represents the brightness.  It describes the brightness or intensity of the color in percent by conjunction with saturation. 

Lab image

This is three channel image. One channel for lightness (L) and other two channels for a & b.

The color space is much better to digital image manipulations than the RGB space. It is very common in many image editing programs such as sharpening images. 

 Example 1

#include "opencv2/core/core.hpp"

#include "opencv2/highgui/highgui.hpp"

#include "opencv2/imgproc/imgproc.hpp"

#include "iostream"

     using namespace cv;

using namespace std;

int main()

{

     Mat img;

     img = imread("C:/Users/hashan/Desktop/progtpoint.jpg", CV_LOAD_IMAGE_COLOR);

     if (img.empty())

     {

          cout << "Error loading img" << endl;

          return -1;

     }

     Mat grayimg;

     cvtColor(img, grayimg, CV_RGB2GRAY);

     namedWindow("Original_image", CV_WINDOW_AUTOSIZE);

     imshow("Original_image", img);

     namedWindow("Gray_color_image", CV_WINDOW_AUTOSIZE);

     imshow("Gray_color_image", grayimg);

     cout << "No: of channels(original image)   : " << img.channels() << endl;

     cout << "No: of channels(gray color image) : " << grayimg.channels() << endl;

     waitKey(0);

     return 0;

}

Explanation

You can check the available conversion types in OpenCv by right click on the conversion type and go to definition.

In this example I converted RGB image to Gray color image. Output is follow.

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