Why do I get bad results with calcBackProject in opencv? - c++

Question

I create a project to reduce noise in the target image. I want to do something on Histograms, so I use calcHist to get the histogram of the image. When I use calcBackProject to recover images, the results are very different from the source image that I can't understand. The results show that the histogram of the two images is significant differences. Are there some problems with histogram processing?
By the way, my OpenCV version is 4.5.0.
int main()
{
Mat src = imread("./newpaperback.jpg");
imshow("src", src);
Mat gray;
cvtColor(src, gray, COLOR_BGR2GRAY);
imshow("gray", gray);
const int channels[] = {0};
Mat hist;
int dims = 1;
const int histSize[] = {256};;
float granges[] = {0, 256};
const float* ranges[] = { granges };
calcHist(&gray, 1, channels, Mat(), hist, dims, histSize, ranges, true, false);
int hist_height = 256;
Mat hist_image = Mat::zeros(hist_height, hist_height * 2, CV_8UC3);
double max_val;
minMaxLoc(hist, 0, &max_val, 0, 0);
for(int i = 0; i < hist_height; i++)
{
float bin_val = hist.at<float>(i);
segment[i] = bin_val * hist_height / max_val;
int intensity = bin_val * hist_height / max_val;
rectangle(hist_image, Point(i * 2, hist_height - 1), Point((i + 1) * 2 - 1, hist_height - intensity), Scalar(255, 255, 255));
}
imshow("hist", hist_image);
Mat back_img;
calcBackProject(&gray, 1, channels, hist, back_img, ranges, 1, true);
imshow("back", back_img);
waitKey(0);
return 0;
}