Generating a histogram in OpenCV is very straightforward.
C++ Generate histogram opencv
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#include "opencv2/highgui/highgui.hpp"
#include "opencv2/imgproc/imgproc.hpp"
#include
#include
using namespace std;
using namespace cv;
/**
* @function main
*/
int main( int argc, char** argv )
{
Mat src, dst;
/// Load image
src = imread( argv[1], 1 );
if( !src.data )
{ return -1; }
/// Separate the image in 3 places ( B, G and R )
vectorMat> bgr_planes;
split( src, bgr_planes );
/// Establish the number of bins
int histSize = 256;
/// Set the ranges ( for B,G,R) )
float range[] = { 0, 256 } ;
const float* histRange = { range };
bool uniform = true; bool accumulate = false;
Mat b_hist, g_hist, r_hist;
/// Compute the histograms:
calcHist( &bgr_planes[0], 1, 0, Mat(), b_hist, 1, &histSize, &histRange, uniform, accumulate );
calcHist( &bgr_planes[1], 1, 0, Mat(), g_hist, 1, &histSize, &histRange, uniform, accumulate );
calcHist( &bgr_planes[2], 1, 0, Mat(), r_hist, 1, &histSize, &histRange, uniform, accumulate );
// Draw the histograms for B, G and R
int hist_w = 512; int hist_h = 400;
int bin_w = cvRound( (double) hist_w/histSize );
Mat histImage( hist_h, hist_w, CV_8UC3, Scalar( 0,0,0) );
/// Normalize the result to [ 0, histImage.rows ]
normalize(b_hist, b_hist, 0, histImage.rows, NORM_MINMAX, -1, Mat() );
normalize(g_hist, g_hist, 0, histImage.rows, NORM_MINMAX, -1, Mat() );
normalize(r_hist, r_hist, 0, histImage.rows, NORM_MINMAX, -1, Mat() );
/// Draw for each channel
for( int i = 1; i histSize; i++ )
{
line( histImage, Point( bin_w*(i-1), hist_h - cvRound(b_hist.atfloat>(i-1)) ) ,
Point( bin_w*(i), hist_h - cvRound(b_hist.atfloat>(i)) ),
Scalar( 255, 0, 0), 2, 8, 0 );
line( histImage, Point( bin_w*(i-1), hist_h - cvRound(g_hist.atfloat>(i-1)) ) ,
Point( bin_w*(i), hist_h - cvRound(g_hist.atfloat>(i)) ),
Scalar( 0, 255, 0), 2, 8, 0 );
line( histImage, Point( bin_w*(i-1), hist_h - cvRound(r_hist.atfloat>(i-1)) ) ,
Point( bin_w*(i), hist_h - cvRound(r_hist.atfloat>(i)) ),
Scalar( 0, 0, 255), 2, 8, 0 );
}
/// Display
namedWindow("calcHist Demo", CV_WINDOW_AUTOSIZE );
imshow("calcHist Demo", histImage );
waitKey(0);
return 0;
}
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Draw histogram in Python:
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# draw histogram in python. talkera.org/opencv
import cv2
import numpy as np
img = cv2.imread('cat.jpg')
h = np.zeros((300,256,3))
bins = np.arange(256).reshape(256,1)
color = [ (255,0,0),(0,255,0),(0,0,255) ]
for ch, col in enumerate(color):
hist_item = cv2.calcHist([img],[ch],None,[256],[0,255])
cv2.normalize(hist_item,hist_item,0,255,cv2.NORM_MINMAX)
hist=np.int32(np.around(hist_item))
pts = np.column_stack((bins,hist))
cv2.polylines(h,[pts],False,col)
h=np.flipud(h)
cv2.imshow('colorhist',h)
cv2.waitKey(0)
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Output: