OpenCV Histogram Tutorial

Histogram is a graphical representation of the intensity distribution of an image.

[caption id="attachment_741" align="alignnone" width="291"] Original Image[/caption]

[caption id="attachment_742" align="alignnone" width="286"] Grayscale Image[/caption]

[caption id="attachment_743" align="alignnone" width="289"] Histogram[/caption]

[sourcecode language="c"]
#include "stdafx.h"
#include<opencv\cv.h>
#include<opencv\cxcore.h>
#include<opencv\highgui.h>

int _tmain(int argc, _TCHAR* argv[])
{
IplImage *img = cvLoadImage("C:\\Users\\gamma.jpg");

IplImage *gray = cvCreateImage(cvSize(img->width,img->height),8,1);

cvCvtColor(img,gray,CV_RGB2GRAY);
cvNamedWindow("Image:",1);
cvShowImage("Image:", img);

cvNamedWindow("Gray:",1);
cvShowImage("Gray:", gray);

//create a rectangular area
CvRect rect = cvRect(0,0,500,600);
//CvRect rect = cvRect(x,y,width,height)

//apply the rectangular to the image and establish a region of interest
cvSetImageROI(gray,rect);
//gray=pointer to the image(gray pointer)
//rect=nameROI

float range[]={0,255};
float *ranges[]={range};
int hist_size = 256;

//create an image to hold the histogram
IplImage*histImage = cvCreateImage(cvSize(320,200),8,1);
//create a histogram to the information from the image
CvHistogram *hist = cvCreateHist(1,&hist_size,CV_HIST_ARRAY,ranges,1);
//1=no of dimentions
//&hist_size=size of dimention
//CV_HIST_ARRY=how to store data in histogram(multi dimentional array)
//ranges=ranges of values for dimention(0,255)
//1=uniform flag(sub intervals should be same)

//calculate the histogram
cvCalcHist(&gray,hist,0,NULL);
//&gray=source
//hist=pointer to the histogram
//0=accumilation flag(if set,hist is not clear at the begining)
//NULL=operational mask

float min_value,max_value=0;
int min_idx,max_idx=0;
//Grab Min/Max Values
cvGetMinMaxHistValue(hist,&min_value,&max_value,&min_idx,&max_idx);
//hist=pointer to the histogram

//Scale the bin values to fit to image representation
//remove higher values
cvScale(hist->bins, hist->bins,((double)histImage->height)/max_value,0);

//hist->bins=source to be scared
//hist->bins=destination
//use of same vales is manupilate source values and put them in the same location

//Set Up Factors For Visualization(set all histogram values to 255)
cvSet(histImage,cvScalarAll(255),0);
//create a factor for scaling along the width
int bin_w=cvRound((double)histImage->width/hist_size);

//draw the values
int i;
for(i=0;i<hist_size;i++)
{
//draw the histogram data on to the histogram image
cvRectangle(histImage,cvPoint(i*bin_w,histImage->height),cvPoint((i+1)*bin_w,histImage->height-cvRound(cvGetReal1D(hist->bins,i))),cvScalarAll(0),-1,8,0);
//cvScalarAll(0)=draw in black color
//-1=thickness of the line(filled rectangal)
//8=line type(connected)
//0=no of fractional points
}

cvNamedWindow("Histogram:",1);
cvShowImage("Histogram:", histImage);

cvWaitKey(0);
cvDestroyWindow("Image:");
cvReleaseImage(&img);

cvDestroyWindow("Gray:");
cvReleaseImage(&gray);

cvDestroyWindow("Histogram:");
cvReleaseImage(&histImage);

return 0;

}
[/sourcecode]