Modifying a particular pixel value of an Image

In the previous tutorials we learnt how to access a pixel value of a particular co-ordinate,
Refer :
http://opencv-code.blogspot.in/2016/12/how-to-access-extract-pixel-value-particular-location-image.html

This,

OpenCV C++ tutorial

is about accessing and changing the pixel value at a particular co-ordinate of an Image.
Here is the code below:

/*Displaying the Pixel value of the whole Image using Loops*/
 
#include <opencv2/core/core.hpp>  
#include <opencv2/highgui/highgui.hpp>  
#include <iostream> 
 
  using namespace std;  
  using namespace cv;  
 
int main() 
  {  
    Mat image1,image2; 
    //Reading the color image 
    image1 = imread("C:\\Users\\arjun\\Desktop\\image003.png", CV_LOAD_IMAGE_COLOR);  
 
    //If image1 not found 
    if (!image1.data)                                                                          
    {  
     cout << "No image data \n";  
     return -1;  
    } 
 
    //Display the original image
    namedWindow("Original Image");               
    imshow("Original Image", image1);
 
    //Changing the pixel value at just a particular point(100,200)
     Vec3b color = image1.at<Vec3b>(Point(100,200));
      color.val[0] = 100;
      color.val[1] = 0;
      color.val[2] = 0;
    image1.at<Vec3b>(Point(100,200)) = color;
 
    //Save the modified image
    imwrite("C:\\Users\\arjun\\Desktop\\mod_image.png",image1);
    //Reading the modifed image
    image2 = imread("C:\\Users\\arjun\\Desktop\\mod_image.png", CV_LOAD_IMAGE_COLOR);  
 
   //If image2 not found 
     if (!image2.data)                                                                          
       {  
        cout << "No image data \n";  
        return -1;  
       } 
 
    //Display the modified image
    namedWindow("Modified Image");               
    imshow("Modified Image", image2); 
    waitKey(0);
    return 0;
   }

Input:

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Modified Image:

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Accessing Pixel Value at a Location(x,y)

Let us consider a 3 channel image of BGR color ordering
(The BGR color ordering is the default order returned  by imread)
Here the order of the channel is reverse

(We generally use RGB color model while describing about an image.In BGR the color model is same except the order of the channel is reverse)

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We use :
Vec3b imagepixel = image.at(x,y);

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/*Reading the pixel value of an image at a particular location*/
#include <opencv2/core/core.hpp>  
#include <opencv2/highgui/highgui.hpp>  
#include <iostream> 
 
  using namespace std;  
  using namespace cv;  
 
  int main() 
  {  
    Mat image; 
    //Reading the color image 
    image = imread("C:\\Users\\arjun\\Desktop\\image003.png", CV_LOAD_IMAGE_COLOR);  
 
    //If image not found
     if (!image.data)                                                                          
     {  
      cout << "No image data \n";  
      return -1;  
     } 
    
     
     //Reading pixel value at location (i,j)
     Vec3b imagepixel = image.at<Vec3b>(250,500);

     //Displaying the pixel value  
     cout<<"imagepixel(BGR)="<<imagepixel<<"\n" ;  
        
     //Display the original image
     namedWindow("Display Image");               
     imshow("Display Image", image);  
  
     waitKey(0);
     return 0;
   }

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Input:

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Output:

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/*Reading the pixel value of an image at a particular location*/
 
 
#include <opencv2/core/core.hpp>  
#include <opencv2/highgui/highgui.hpp>  
#include <iostream> 
 
  using namespace std;  
  using namespace cv;  
 
int main() 
  {  
    Mat image; 
    //Reading the color image 
    image = imread("C:\\Users\\arjun\\Desktop\\image003.png", CV_LOAD_IMAGE_COLOR);  
 
     //If image not found  
       if (!image.data)                                                             
     {  
      cout << "No image data \n";  
      return -1;  
     } 
 
 
    while(1)
    {
     //Taking inputs from the user for the co-ordinates of the image 
      int i,j;
      cout<<"Enter the co-ordinates of the image where you want to find the pixel value (i,j): \n";
      cout<<"i<"<<image.rows<<"\t"<<"&"<<"\t"<<"j<"<<image.cols<<"\n";
      
     cout<<"i= ";  cin>>i;
     cout<<"j= ";  cin>>j;
     
     if(i < image.rows) 
      { 
        if(j < image.cols)
          {
           //Reading pixel value at location (i,j)
            Vec3b imagepixel = image.at<Vec3b>(i,j); 
           //Displaying the pixel value                                                        
           cout<<"imagepixel(BGR)="<<imagepixel<<"\n" ;
          }  
        }
      else
        { 
           cout<<"Image Co-ordinates value out of range \n"; 
        }
 
     }
        return 0; 
  }

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Input:

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Output:

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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

---

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

---

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.

Read and Display Image in OpenCV

Syntax:
C++: Mat imread(const string& filename, int flags=1 )

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Parameters:           
filename – Name of file to be loaded.
flags –Flags specifying the color type of a loaded image:-

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++ Example of Reading 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;
 
// 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;
    } 
 
//Dispay the Image in the window 
namedWindow("Image1");
 imshow("Image1",image1);

 waitKey(0);
}

Note the Location of the image is:
C:\Users\arjun\Desktop\a.jpg where "opencv-logo.jpg" .
( Here we append additional "\" after each file or folder name.)

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Image Formats supported by OpenCV:

Windows bitmaps	*.bmp, *.dib
JPEG files	*.jpeg, *.jpg, *.jpe
JPEG 2000 files	*.jp2
Portable Network Graphics	*.png
Portable image format	*.pbm, *.pgm, *.ppm
Sun rasters	*.sr, *.ras
TIFF files	*.tiff, *.tif