Histograms of Color Images using OpenCV

This is another simple example to view Histograms of various channels of an Image

Download:
Download the Project folder and the source code [1.29 Mb].



Source code:
//______________________________________________________________________________________
// OpenCV Histograms of Color Images
// Author: Bharath Prabhuswamy
//______________________________________________________________________________________
//______________________________________________________________________________________

#include "cv.h"
#include "highgui.h"

int main()
{
    IplImage* img  = cvLoadImage("plastics.jpg");

 /* Always check if the program can find a file */
 if( !img )
   return -1;

 IplImage* channel = cvCreateImage( cvGetSize(img), 8, 1 );
        IplImage *hist_img = cvCreateImage(cvSize(300,240), 8, 3);
 cvSet( hist_img, cvScalarAll(255), 0 );

 CvHistogram *hist_red;
 CvHistogram *hist_green;
 CvHistogram *hist_blue;

    int hist_size = 256;      
    float range[]={0,256};
    float* ranges[] = { range };
    
    float max_value = 0.0;
    float max = 0.0;
    float w_scale = 0.0;

 /* Create a 1-D Arrays to hold the histograms */
 hist_red = cvCreateHist(1, &hist_size, CV_HIST_ARRAY, ranges, 1);
 hist_green = cvCreateHist(1, &hist_size, CV_HIST_ARRAY, ranges, 1);
 hist_blue = cvCreateHist(1, &hist_size, CV_HIST_ARRAY, ranges, 1);

 /* Set image to obtain RED as Channel of Interest (COI) */
  cvSetImageCOI(img,3);
  cvCopy(img,channel);
  cvResetImageROI(img);

 /* Calculate histogram of the Image and store it in the array */
  cvCalcHist( &channel, hist_red, 0, NULL );

 /* Calculate and Plot the histograms Green and Blue channels as well */
 /* Green channel */
  cvSetImageCOI(img,2);
  cvCopy(img,channel);
  cvResetImageROI(img);

  cvCalcHist( &channel, hist_green, 0, NULL );
 
 /* Blue channel */
  cvSetImageCOI(img,1);
  cvCopy(img,channel);
  cvResetImageROI(img);

  cvCalcHist( &channel, hist_blue, 0, NULL );


        /* Find the minimum and maximum values of the histograms */
        cvGetMinMaxHistValue( hist_red, 0, &max_value, 0, 0 );
 cvGetMinMaxHistValue( hist_green, 0, &max, 0, 0 );

 max_value = (max > max_value) ? max : max_value;

 cvGetMinMaxHistValue( hist_blue, 0, &max, 0, 0 );

 max_value = (max > max_value) ? max : max_value;    
                        // The variable max_value has the maximum of the three histograms

 /* Using the maximum value, Scale/Squeeze the histogram (to fit the image) */
        cvScale( hist_red->bins, hist_red->bins, ((float)hist_img->height)/max_value, 0 );
 cvScale( hist_green->bins, hist_green->bins, ((float)hist_img->height)/max_value, 0 );
 cvScale( hist_blue->bins, hist_blue->bins, ((float)hist_img->height)/max_value, 0 );

 printf("Scale: %4.2f pixels per 100 units\n", max_value*100/((float)hist_img->height));                         
                                                   //A scale to estimate the number of pixels

    /* Scale/Squeeze the histogram range to image width */
    w_scale = ((float)hist_img->width)/hist_size;

 /* Plot the Histograms */
    for( int i = 0; i < hist_size; i++ )
    {
      cvRectangle( hist_img, cvPoint((int)i*w_scale , hist_img->height),
        cvPoint((int)(i+1)*w_scale, hist_img->height - cvRound(cvGetReal1D(hist_red->bins,i))),
        CV_RGB(255,0,0), -1, 8, 0 );
      cvRectangle( hist_img, cvPoint((int)i*w_scale , hist_img->height),
       cvPoint((int)(i+1)*w_scale, hist_img->height - cvRound(cvGetReal1D(hist_green->bins,i))),
       CV_RGB(0,255,0), -1, 8, 0 );
      cvRectangle( hist_img, cvPoint((int)i*w_scale , hist_img->height),
       cvPoint((int)(i+1)*w_scale, hist_img->height - cvRound(cvGetReal1D(hist_blue->bins,i))),
       CV_RGB(0,0,255), -1, 8, 0 );
    }

    cvNamedWindow( "Image", 1 );
    cvShowImage( "Image",img);

    /* create a window to show the histogram of the image */
    cvNamedWindow("Histogram", 1);
    cvShowImage( "Histogram", hist_img);

    cvWaitKey(0);

 cvDestroyWindow( "Image" );
 cvDestroyWindow( "Histogram" );
    cvReleaseImage( &img );
    return 0;
}


Image:



8 comments:

  1. Hi !
    Great tutorial , it helps me a lot , but i have a question ( my be a stupid question )

    what is a histogram and what i should do with the result of histogram of a picture ???

    ReplyDelete
  2. Good Article about
    Histograms of Color Images using OpenCV

    Post by
    Term Papers

    ReplyDelete
  3. Thank you. It's really useful to me.

    ReplyDelete
  4. Thank you. Just a small question ...is it possible to get the complete histogram (not split it into 3 channels) like in the link but using opencv --> http://www.kenrockwell.com/tech/yrgb.htm

    I am working on a project where we need to get the histogram of a picture and separate the colours by setting some threshold values. Can you please give me some idea on how to carry on? thank you once again
    cheers,
    Sneha

    ReplyDelete