OpenCV: Histogram not displaying correctly - c++

Question

I am trying to plot Histogram of lenna here the 8 bit single ch. gray scale image.
But it is not displaying the output correctly, as can be seen in the following output:
void show_histogram_image(Mat img1)
{
int sbins = 256;
int histSize[] = {sbins};
float sranges[] = { 0, 256 };
const float* ranges[] = { sranges };
cv::MatND hist;
int channels[] = {0};
cv::calcHist( &img1, 1, channels, cv::Mat(), // do not use mask
hist, 1, histSize, ranges,
true, // the histogram is uniform
false );
double maxVal=0;
minMaxLoc(hist, 0, &maxVal, 0, 0);
int xscale = 10;
int yscale = 10;
cv::Mat hist_image;
hist_image = cv::Mat::zeros(256, sbins*xscale, CV_8UC1);
for( int s = 0; s < sbins; s++ )
{
float binVal = hist.at<float>(s, 0);
int intensity = cvRound(binVal*255/maxVal);
rectangle( hist_image, cv::Point(s*xscale, 0),
cv::Point( (s+1)*xscale - 1, intensity),
cv::Scalar::all(255),
CV_FILLED );
}
imshow("Image1",hist_image);
waitKey(0);
}
Here is my main();
int main()
{
Mat img1 = imread("lena512.bmp", CV_8UC1);
if (img1.empty()) //check whether the image is valid or not
{
cout << "Error : Image cannot be read..!!" << endl;
system("pause"); //wait for a key press
return -1;
}
show_histogram_image(img1);
}
And here is the output Histogram image:
I tried changing the xscale even then it is not coming correctly.
Update
I made the following changes:
rectangle( hist_image, cv::Point(s*xscale, hist_image.rows),
cv::Point( (s+1)*xscale - 1, hist_image.rows - intensity),
cv::Scalar::all(255), CV_FILLED );
And now the output is:
It is much better , but I need lines and clearly visible bins. And it looks like some part is hidden on the right side.
Update 2
I changed CV_FILLED to '1' and now I have:

Answer 1

since the image origin in opencv is (0,0), and thus the y-axis is pointing downwards,
you will have to invert the y-values for your histogram-drawing:
rectangle( hist_image, cv::Point(s*xscale, hist_image.rows),
cv::Point( (s+1)*xscale - 1, hist_image.rows - intensity),
cv::Scalar::all(255),
CV_FILLED );