Cv2.CalcHist doesn't return histogram

I calculate histogram of RGB image with this func:

static void calc_hist_bgr(Mat bgr,  Mat hist)
        {
           int[] channels = new int[3] {0, 1, 2}; 
           const int b_bins = 8; 
           const int g_bins = 8; 
           const int r_bins = 8;
           int[] hist_size = {b_bins, g_bins, r_bins};
           float[] branges = {0, 256};
           float[] granges = {0, 256};
           float[] rranges = {0, 256};
           float[][] ranges = {branges, granges, rranges};
           Mat mask = new Mat();
           const int dims = 3;
           Mat[] srcs = {bgr};         
           Cv2.CalcHist(srcs, channels, mask, hist, dims, hist_size, ranges, true, false);
        }

My bgr image is: Mat [77x59xCV_8UC3,…] But i got hist image: Mat [-1x-1xCV_32FC1…]

What was i wrong ? Thanks

Sorry, i have tried to insert the code format but i couldn’t. I don’t know why

[fdncred]

You might want to look at HistSample.cs and KAZESample2.cs for tips. When I had to use it, I modeled mine off HistSample.cs (below) and it worked fine.

    public void Run()
    {
        Mat src = Cv2.ImRead(FilePath.Image.Lenna, ImreadModes.GrayScale);

        // Histogram view
        const int Width = 260, Height = 200;
        Mat render = new Mat(new Size(Width, Height), MatType.CV_8UC3, Scalar.All(255));

        // Calculate histogram
        Mat hist = new Mat();
        int[] hdims = {256}; // Histogram size for each dimension
        Rangef[] ranges = { new Rangef(0,256), }; // min/max 
        Cv2.CalcHist(
            new Mat[]{src}, 
            new int[]{0}, 
            null,
            hist, 
            1, 
            hdims, 
            ranges);

        // Get the max value of histogram
        double minVal, maxVal;
        Cv2.MinMaxLoc(hist, out minVal, out maxVal);

        Scalar color = Scalar.All(100);
        // Scales and draws histogram
        hist = hist * (maxVal != 0 ? Height / maxVal : 0.0);
        for (int j = 0; j < hdims[0]; ++j)
        {
            int binW = (int)((double)Width / hdims[0]);
            render.Rectangle(
                new Point(j * binW, render.Rows),
                new Point((j + 1) * binW, render.Rows - (int)(hist.Get<float>(j))),
                color, 
                -1);
        }

        using (new Window("Image", WindowMode.AutoSize | WindowMode.FreeRatio, src))
        using (new Window("Histogram", WindowMode.AutoSize | WindowMode.FreeRatio, render))
        {
            Cv2.WaitKey();
        }
    }

[nanoMV]

It did not work !

Just simple sample:

Mat src = new Mat(@"C:\desert.jpg", ImreadModes.Color); Mat hist = new Mat(); int[] hdims = {256,256,256}; Rangef[] ranges = {new Rangef(0,256),new Rangef(0,256),new Rangef(0,256) }; Cv2.CalcHist(new Mat[]{src}, new int[]{0,1,2}, null, hist, 3, hdims, ranges); I got the hist with size of (-1,-1);

But if with one channel, it works: Mat src = new Mat(@"C:\desert.jpg"); Mat hist = new Mat(); int[] hdims = {256}; Rangef[] ranges = {new Rangef(0,256),}; Cv2.CalcHist(new Mat[]{src}, new int[]{0}, null, hist, 1, hdims, ranges); I got the hist with size of (256,1)