compare histograms of grayscale images in opencv - c

Question

hi can anyone provide me with a simple open cv program to load two RGB images, convert it to Gray scale, calculate histogram and then compare their histograms. I saw a similar program done in the open cv site but they used HSV instead of Gray scale and it was a c++ program. I can look up the flow and everything...I don't know which functions to use and what their arguments will mean....
Regards,
Kiran

Answer 1

Here is the simple code snippet that does the thing. Since you not told how you want to compare histograms, I suggest to do it visually.
#include <opencv2/opencv.hpp>
void show_histogram(std::string const& name, cv::Mat1b const& image)
{
// Set histogram bins count
int bins = 256;
int histSize[] = {bins};
// Set ranges for histogram bins
float lranges[] = {0, 256};
const float* ranges[] = {lranges};
// create matrix for histogram
cv::Mat hist;
int channels[] = {0};
// create matrix for histogram visualization
int const hist_height = 256;
cv::Mat3b hist_image = cv::Mat3b::zeros(hist_height, bins);
cv::calcHist(&image, 1, channels, cv::Mat(), hist, 1, histSize, ranges, true, false);
double max_val=0;
minMaxLoc(hist, 0, &max_val);
// visualize each bin
for(int b = 0; b < bins; b++) {
float const binVal = hist.at<float>(b);
int const height = cvRound(binVal*hist_height/max_val);
cv::line
( hist_image
, cv::Point(b, hist_height-height), cv::Point(b, hist_height)
, cv::Scalar::all(255)
);
}
cv::imshow(name, hist_image);
}
int main (int argc, const char* argv[])
{
// here you can use cv::IMREAD_GRAYSCALE to load grayscale image, see image2
cv::Mat3b const image1 = cv::imread("C:\\workspace\\horse.png", cv::IMREAD_COLOR);
cv::Mat1b image1_gray;
cv::cvtColor(image1, image1_gray, cv::COLOR_BGR2GRAY);
cv::imshow("image1", image1_gray);
show_histogram("image1 hist", image1_gray);
cv::Mat1b const image2 = cv::imread("C:\\workspace\\bunny.jpg", cv::IMREAD_GRAYSCALE);
cv::imshow("image2", image2);
show_histogram("image2 hist", image2);
cv::waitKey();
return 0;
}
Result:

Answer 2

To compare histograms you can look at the last four points from this http://docs.opencv.org/doc/tutorials/imgproc/histograms/histogram_comparison/histogram_comparison.html.