RGB to Other Color Space Conversion

cvtcolor() converts an image from one color space to another.

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Syntax :
C++:void cvtColor(InputArray src, OutputArray dst, int code, int dstCn=0 )

Parameter:
src   :– input image: 8-bit unsigned, 16-bit unsigned ( CV_16UC... ), or single-precision floating-point.
dst    :– output image of the same size and depth as src.
code  :– color space conversion code (see the description below).
dstCn :– number of channels in the destination image; if the parameter is 0, the number of the channels is derived automatically from src and code.

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As it has already been mentioned that the functions converts an image form one color space to another, there are various types of conversion possible.

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Transformation	Syntax
RGB to YCrCb	CV_RGB2YCrCb
BGR to YCrCb	CV_BGR2YCrCb
YCrCb to RGB	CV_YCrCb2RGB
YCrCb to BGR	CV_YCrCb2BGR

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Transformation	Syntax
RGB to HSV	CV_RGB2HSV
BGR to HSV	CV_BGR2HSV
HSV to RGB	CV_HSV2RGB
HSV to BGR	CV_HSV2BGR

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Transformation	Syntax
RGB to CIE L*a*b*	CV_RGB2Lab
BGR to CIE L*a*b*	CV_BGR2Lab
CIE L*a*b* to RGB	CV_Lab2RGB
CIE L*a*b* to BGR	CV_Lab2BGR

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Transformation	Syntax
RGB to CIE L*u*v*	CV_RGB2Luv
BGR to CIE L*u*v*	CV_BGR2Luv
CIE L*u*v* to RGB	CV_Luv2RGB
CIE L*u*v* to BGR	CV_Luv2BGR

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Transformation	Syntax
RGB to CIE XYZ	CV_RGB2XYZ
BGR to CIE XYZ	CV_BGR2XYZ
CIE XYZ to RGB	CV_XYZ2RGB
CIE XYZ to BGR	CV_XYZ2BGR

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//OpenCV C++ Code for ColorSpace Conversion
#include "opencv2/core/core.hpp"
#include "opencv2/highgui/highgui.hpp"
#include "opencv2/imgproc/imgproc.hpp"
#include "iostream"
 
using namespace cv;
 
int main( )
{
// char* imageName = argv[1];
 
 Mat image;
 image = imread( "C:\\Users\\arjun\\Desktop\\color-image.png", 1 );
 
 Mat RGB2GRAY_image;
 cvtColor( image, RGB2GRAY_image, CV_RGB2GRAY );
 
 Mat BGR2GRAY_image;
 cvtColor( image, BGR2GRAY_image, CV_BGR2GRAY );
 
 Mat RGB2YCrCb_image;
 cvtColor( image, RGB2YCrCb_image, CV_RGB2YCrCb );
 
 Mat BGR2YCrCb_image;
 cvtColor( image, BGR2YCrCb_image, CV_BGR2YCrCb );
 
 Mat RGB2HSV_image;
 cvtColor( image, RGB2HSV_image, CV_RGB2HSV );
 
 Mat BGR2HSV_image;
 cvtColor( image, BGR2HSV_image, CV_BGR2HSV );
 
 Mat RGB2Lab_image;
 cvtColor( image, RGB2Lab_image, CV_RGB2Lab );
 
 Mat BGR2Lab_image;
 cvtColor( image, BGR2Lab_image, CV_BGR2Lab );
 
 Mat RGB2Luv_image;
 cvtColor( image, RGB2Luv_image, CV_RGB2Luv );
 
  Mat BGR2Luv_image;
 cvtColor( image, BGR2Luv_image, CV_BGR2Luv );
 
 Mat RGB2XYZ_image;
 cvtColor( image, RGB2XYZ_image, CV_RGB2XYZ );
 
 Mat BGR2XYZ_image;
 cvtColor( image, BGR2XYZ_image, CV_BGR2XYZ );
 
 namedWindow( "original image", CV_WINDOW_AUTOSIZE );
imshow( "original image", image );
 
namedWindow( "RGB2GRAY image", CV_WINDOW_AUTOSIZE );
imshow( "RGB2GRAY image",RGB2GRAY_image );
imwrite( "C:\\Users\\arjun\\Desktop\\RGB2GRAY.jpg", RGB2GRAY_image );
 
namedWindow( "BGR2GRAY image", CV_WINDOW_AUTOSIZE );
imshow( "BGR2GRAY image", BGR2GRAY_image );
imwrite( "C:\\Users\\arjun\\Desktop\\BGR2GRAY.jpg", BGR2GRAY_image );
 
namedWindow( "RGB2YCrCb image", CV_WINDOW_AUTOSIZE );
imshow( "RGB2YCrCb image", RGB2YCrCb_image );
imwrite( "C:\\Users\\arjun\\Desktop\\RGB2YCrCb.jpg", RGB2YCrCb_image );
 
 
namedWindow( "BGR2YCrCb image", CV_WINDOW_AUTOSIZE );
imshow( "BGR2YCrCb image", BGR2YCrCb_image );
imwrite( "C:\\Users\\arjun\\Desktop\\BGR2YCrCb.jpg", BGR2YCrCb_image );
 
namedWindow( "RGB2HSV image", CV_WINDOW_AUTOSIZE );
imshow( "RGB2HSV image", RGB2HSV_image );
imwrite( "C:\\Users\\arjun\\Desktop\\RGB2HSV.jpg", RGB2HSV_image );
 
namedWindow( "BGR2HSV image", CV_WINDOW_AUTOSIZE );
imshow( "BGR2HSV image", BGR2HSV_image );
imwrite( "C:\\Users\\arjun\\Desktop\\BGR2HSV.jpg", BGR2HSV_image );
 
namedWindow( "RGB2Lab image", CV_WINDOW_AUTOSIZE );
imshow( "RGB2Lab image", RGB2Lab_image );
imwrite( "C:\\Users\\arjun\\Desktop\\RGB2Lab.jpg", RGB2Lab_image );
 
namedWindow( "BGR2Lab image", CV_WINDOW_AUTOSIZE );
imshow( "BGR2Lab image", BGR2Lab_image );
imwrite( "C:\\Users\\arjun\\Desktop\\BGR2Lab.jpg", BGR2Lab_image );
 
namedWindow( "RGB2Luv image", CV_WINDOW_AUTOSIZE );
imshow( "RGB2Luv image", RGB2Luv_image );
imwrite( "C:\\Users\\arjun\\Desktop\\RGB2Luv.jpg", RGB2Luv_image );
 
namedWindow( "BGR2Luv image", CV_WINDOW_AUTOSIZE );
imshow( "BGR2Luv image", BGR2Luv_image );
imwrite( "C:\\Users\\arjun\\Desktop\\BGR2Luv.jpg", BGR2Luv_image );
 
namedWindow( "RGB2XYZ image", CV_WINDOW_AUTOSIZE );
imshow( "RGB2XYZ image", RGB2XYZ_image );
imwrite( "C:\\Users\\arjun\\Desktop\\RGB2XYZ.jpg", RGB2XYZ_image );
 
namedWindow( "BGR2XYZ image", CV_WINDOW_AUTOSIZE );
imshow( "BGR2XYZ image", BGR2XYZ_image );
imwrite( "C:\\Users\\arjun\\Desktop\\BGR2XYZ.jpg", BGR2XYZ_image );
 
 waitKey(0);
 
 return 0;
}

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Original Image:

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RGB to Grey Image:

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BGR to Grey Image:

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RGB to YCrCb Image:

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BGR to YCrCb Image:

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RGB to HSV Image:

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BGR to HSV Image:

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RGB to Lab Image:

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BGR to Lab Image:

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RGB to Luv Image:

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BGR to Luv Image:

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RGB to XYZ Image:

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BGR to XYZ Image:

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Reason for increase in the size of the image

In the previous tutorial we learnt how to save images in OpenCV, Refer:
http://opencv-code.blogspot.in/2016/12/how-to-write-and-dispay-image-in-opencv.html

This tutorial explains the reason behind increase in size of image in OpenCV.

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Why does the size of the image increases when i save the same image again with a different name in OpenCV?
Solution:
There exists various methods of compression in JPEG.
So your OpenCV used a different compression technique for jpeg image than that used by an original image.
By Default OPENCV uses the compression number of 95 while dealing with JPEG images.
The higher the number , lesser compression we would obtain.

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To manually pass the compression factor refer the code below:
Note: Here i have used the compression number of 25 and 100 as the compression number for JPEG image

#include "opencv2/highgui/highgui.hpp"
#include "opencv2/imgproc/imgproc.hpp"
#include <iostream>
#include <stdio.h>
using namespace cv;
using namespace std;
 
 
int main(int argc, char **argv)
{
     Mat image1;
     const int JPEG_QUALITY = 25;
     const int JPEG_QUALITY2 = 25;

    // Read the file
    image1 = imread("C:\\Users\\arjun\\Desktop\\opencv-logo.jpg",CV_LOAD_IMAGE_COLOR);

    // Check for invalid input
    if(! image1.data )                              
    {
        cout << "Could not open or find the image" << std::endl ;
        return -1;
    } 

    vector<int> params;
    params.push_back(CV_IMWRITE_JPEG_QUALITY);
    params.push_back(JPEG_QUALITY);

    //Write the File
    imwrite( "C:\\Users\\arjun\\Desktop\\opencvlogo-new1.jpg",image1, params);

    vector<int> params2;
    params2.push_back(CV_IMWRITE_JPEG_QUALITY);
    params2.push_back(JPEG_QUALITY2);

    //Write the File
    imwrite( "C:\\Users\\arjun\\Desktop\\opencvlogo-new2.jpg",image1, params2);
  
    //Dispay the Image in the window 
    namedWindow("Image1");
    imshow("Image1",image1);
 
 waitKey(0);
}

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Note:- Now compare the size of the original image and the two new images.

Write and Dispay Image in OpenCV

In my previous tutorial i have explained how to read images in OpenCV.
Refer : http://opencv-code.blogspot.in/2016/12/how-to-read-and-display-image-in-opencv.html

This tutorial is about how to read, write and display images in Opencv.

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Syntax:
C++:bool imwrite(const string& filename, InputArray img, const vector<int>& params=vector<int>() )
Saves an image to a specified file.

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Parameters:           
filename – Name of file to be loaded.
image – Image to be saved.
params –
CV_LOAD_IMAGE_ANYDEPTH - If set, return 16-bit/32-bit image when the input has the corresponding depth, otherwise convert it to 8-bit.
CV_LOAD_IMAGE_COLOR - If set, always convert image to the color one
CV_LOAD_IMAGE_GRAYSCALE - If set, always convert image to the grayscale one

>0 Return a 3-channel color image.
=0 Return a grayscale image.
<0 Return the loaded image as is (with alpha channel).

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//OpenCV C++ code for reading, writing and displaying image
#include "opencv2/core/core.hpp"
#include "opencv2/highgui/highgui.hpp"
#include "iostream"
 
using namespace cv;
using namespace std;
 
int main( int argc, char** argv )
{
 Mat image1,image2;
 
// Read the file
image1 = imread("C:\\Users\\arjun\\Desktop\\opencv-logo.jpg",CV_LOAD_IMAGE_COLOR);

// Check for invalid input
 if(! image1.data )                              
    {
        cout << "Could not open or find the image" << std::endl ;
        return -1;
    } 
 
//Write the File
imwrite( "C:\\Users\\arjun\\Desktop\\opencv-logo-new.jpg",image1);
 
// Read the Writen File
image2 =imread("C:\\Users\\arjun\\Desktop\\opencv-logo-new.jpg",CV_LOAD_IMAGE_COLOR);  
    
//Window for displaying both the images
 namedWindow("Image1");
 imshow("Image1",image1);
 
 namedWindow("Image2");
 imshow("Image2",image2);

 waitKey(0);
 
}

Note:- Compare the size of the two images.(opencv-logo.jpg & opencv-logo-new.jpg).
To know the reason about the increase in size of the image refer:
http://opencv-code.blogspot.in/2016/12/why-does-size-of-images-increases-opencv-imwrite.html