Why won't OpenCV SimpleBlobDetector detect the blobs in this image?

Question

I am trying to detect blobs in images like the following one:

Nonetheless, no blob is detected. I checked on some test images and the following code is working fine. I also tried tweaking the parameters, without any success. Any idea what I am doing wrong?

This is the code I am using:

procpred = cv2.bitwise_not(procpred)
blur = cv2.blur(procpred, (15,15), 0)

params = cv2.SimpleBlobDetector_Params() 
# Change thresholds
params.minThreshold = 10
params.maxThreshold = 200

# Filter by Area.
params.filterByArea = False
params.minArea = 10

# Filter by Circularity
params.filterByCircularity = False
params.minCircularity = 0.1

# Filter by Convexity
params.filterByConvexity = False
params.minConvexity = 0.87

# Filter by Inertia
params.filterByInertia = False
params.minInertiaRatio = 0.01

# Create a detector with the parameters
ver = (cv2.__version__).split('.')
if int(ver[0]) < 3 :
    detector = cv2.SimpleBlobDetector(params)
else: 
    detector = cv2.SimpleBlobDetector_create(params)

# Detect blobs.
keypoints = detector.detect(blur)
print(keypoints)

#Draw detected blobs as red circles.
#cv2.DRAW_MATCHES_FLAGS_DRAW_RICH_KEYPOINTS ensures the size of the circle corresponds to the size of blob
im_with_keypoints = cv2.drawKeypoints(procpred, keypoints, np.array([]), (0,0,255), cv2.DRAW_MATCHES_FLAGS_DRAW_RICH_KEYPOINTS)

plt.imshow(im_with_keypoints)

Answer 1

Not sure what you do at the beginning of your code, but I did this and it seems to work fine:

#!/usr/bin/env python3

import cv2
import numpy as np

# Read image
im = cv2.imread("blobs.png", cv2.IMREAD_GRAYSCALE)

params = cv2.SimpleBlobDetector_Params() 
# Change thresholds
params.minThreshold = 10
params.maxThreshold = 200

# Filter by Area.
params.filterByArea = False
params.minArea = 10

# Filter by Circularity
params.filterByCircularity = False
params.minCircularity = 0.1

# Filter by Convexity
params.filterByConvexity = False
params.minConvexity = 0.87

# Filter by Inertia
params.filterByInertia = False
params.minInertiaRatio = 0.01

# Create a detector with the parameters
ver = (cv2.__version__).split('.')
if int(ver[0]) < 3 :
    detector = cv2.SimpleBlobDetector(params)
else: 
    detector = cv2.SimpleBlobDetector_create(params)

# Detect blobs.
keypoints = detector.detect(im)
print(keypoints)
im_with_keypoints = cv2.drawKeypoints(im, keypoints, np.array([]), (0,0,255), cv2.DRAW_MATCHES_FLAGS_DRAW_RICH_KEYPOINTS)
cv2.imwrite("result.png",im_with_keypoints)