Why is OpenCV blob detection not working?

Question

I am using EmguCV for C# and no matter what configuration I try I can not figure out why my blob detection is not working. Here is my code:

public static void Process(string videoPath, string folderSavePath)
{
    // Create video capture with memory safety
    using VideoCapture videoCapture = new(videoPath);

    // Get properties of video
    int fps = (int)videoCapture.Get(CapProp.Fps);
    int frameCount = (int)videoCapture.Get(CapProp.FrameCount);
    Size size = new(videoCapture.Width, videoCapture.Height);
    Size processSize = size / 4;

    // Determine new path for video output
    string videoName = Path.GetFileNameWithoutExtension(videoPath) + "_out" + ".mp4";
    string videoSavePath = Path.Combine(folderSavePath, videoName);

    // Create video writer with memory safety
    using VideoWriter videoWriter = new(videoSavePath, VideoWriter.Fourcc('H', '2', '6', '4'), fps, size, true);
    
    // Set up background subtraction
    BackgroundSubtractorMOG2 backgroundSubtractor = new();
    backgroundSubtractor.History = 30;
    backgroundSubtractor.VarThreshold = 40;

    // Set up blob detection
    SimpleBlobDetectorParams blobDetectorParams = new()
    {
        FilterByArea = true,
        FilterByCircularity = false,
        FilterByConvexity = false,
        FilterByInertia = false,
        MinArea = 1,
        MaxArea = 100_000
    };
    SimpleBlobDetector blobDetector = new(blobDetectorParams);
    int blobs = 0;

    // Read frames from video capture
    Mat[] frames = new Mat[frameCount];
    Mat sourceFrame = new();
    while (videoCapture.Read(sourceFrame))
    {
        // Resize frame
        Mat downsizedFrame = new();
        CvInvoke.Resize(sourceFrame, downsizedFrame, processSize, interpolation: Inter.Linear);

        // Subtract the background
        Mat foregroundFrame = new();
        backgroundSubtractor.Apply(downsizedFrame, foregroundFrame);

        // Remove noise
        Mat noiselessFrame = new();
        CvInvoke.MedianBlur(foregroundFrame, noiselessFrame, 5);

        // Convert to binary video
        Mat binaryFrame = new Mat();
        CvInvoke.Threshold(noiselessFrame, binaryFrame, 127, 255, ThresholdType.Binary);

        // Blob detection
        VectorOfKeyPoint keypoints = new VectorOfKeyPoint();
        blobDetector.Detect(binaryFrame, keypoints);

        // Draw blobs
        Mat blobsFrame = new Mat();
        Features2DToolbox.DrawKeypoints(binaryFrame, keypoints, blobsFrame, new Bgr(0, 255, 0));
        blobs += keypoints.Size;
        
        // Resize again
        Mat upsizedFrame = new();
        CvInvoke.Resize(blobsFrame, upsizedFrame, size, interpolation: Inter.Cubic);

        // Write frame to video
        videoWriter.Write(upsizedFrame);
    }

    Debug.Print($"Detected {blobs} blobs.");
}

What is so frustrating is that the video is outputting clear shapes. My background subtraction and noise removal seem to be working pretty flawlessly. Yet, still blob detection fails.

By the way, I understand I am drawing green keypoints to a binary image, which I believe may not actually draw. That can be ignored. I will draw them to one of my non-binary frames once it is working. I have been using my print statement to determine if any blobs were found, not the actual output video itself.

Answer 1

As it turns out I simply needed to invert my frame with ThresholdType.BinaryInv and use blobDetector.DetectRaw as opposed to blobDetector.Detect.