Can't detect image with blob

Question

Just tried using blobs to detect my image, using the example here: https://www.learnopencv.com/blob-detection-using-opencv-python-c/, but it just does not detect anything.

https://i.sstatic.net/yaw5P.jpg

I tried using the original Image, grey image, and thresholding it to only black and white, but none of them detect any blobs, and the keypoints always remain 0.

import numpy as np
import cv2


im_width = 320
im_height = 240

img = cv2.imread("D:\\20190822\\racket.bmp")
GreyImage=cv2.cvtColor(img,cv2.COLOR_BGR2GRAY)
ret,thresh=cv2.threshold(GreyImage,50,255,cv2.THRESH_BINARY)
detector = cv2.SimpleBlobDetector_create()
keypoints = detector.detect(thresh)
blobs = cv2.drawKeypoints(thresh, keypoints, np.array([]), (0,0,255), cv2.DRAW_MATCHES_FLAGS_DRAW_RICH_KEYPOINTS)
print(len(keypoints))
cv2.imshow("Keypoints", blobs)
cv2.waitKey(0)
cv2.destroyAllWindows()

Answer 1

I am not an expert on this, but it appears from the documentation that the default is to find circular blobs. You do not have circles except for some small dots. So you have to relax all the arguments to catch every shape. See

https://docs.opencv.org/3.4/d0/d7a/classcv_1_1SimpleBlobDetector.html

https://docs.opencv.org/3.4/d2/d29/classcv_1_1KeyPoint.html#a308006c9f963547a8cff61548ddd2ef2

https://craftofcoding.wordpress.com/tag/cv2/

So try this:

Input:

import numpy as np
import cv2
import math

img = cv2.imread("racket.png")
GreyImage=cv2.cvtColor(img,cv2.COLOR_BGR2GRAY)
ret,thresh=cv2.threshold(GreyImage,50,255,cv2.THRESH_BINARY)

# erode to one large blob
#thresh = cv2.erode(thresh, None, iterations=4)

cv2.imshow("Threshold", thresh)
cv2.waitKey(0)
cv2.destroyAllWindows()

# Set up the SimpleBlobdetector with default parameters.
params = cv2.SimpleBlobDetector_Params()

# Change thresholds
params.minThreshold = 0
params.maxThreshold = 256

# Filter by Area.
params.filterByArea = True
params.minArea = 0
params.maxArea = 100000000000

# Filter by Color (black)
params.filterByColor = True
params.blobColor = 0

# Filter by Circularity
params.filterByCircularity = True
params.minCircularity = 0
params.maxCircularity = 100000000

# Filter by Convexity
params.filterByConvexity = True
params.minConvexity = 0
params.maxConvexity = 100000000

# Filter by InertiaRatio
params.filterByInertia = True
params.minInertiaRatio = 0
params.maxInertiaRatio = 100000000

# Distance Between Blobs
params.minDistBetweenBlobs = 0

# Do detecting
detector = cv2.SimpleBlobDetector_create(params)

# Get keypoints
keypoints = detector.detect(thresh)

print(len(keypoints))
print('')

# Get keypoint locations and radius
for keypoint in keypoints:
   x = int(keypoint.pt[0])
   y = int(keypoint.pt[1])
   s = keypoint.size
   r = int(math.floor(s/2))
   print (x,y,r)
   #cv2.circle(img, (x, y), r, (0, 0, 255), 2)

# Draw blobs
blobs = cv2.drawKeypoints(thresh, keypoints, np.array([]), (0,0,255), cv2.DRAW_MATCHES_FLAGS_DRAW_RICH_KEYPOINTS)
cv2.imshow("Keypoints", blobs)
cv2.waitKey(0)
cv2.destroyAllWindows()

# Save result
cv2.imwrite("racket_blobs.jpg", blobs)

42

136 226 35
138 225 1
136 225 1
134 225 1
140 223 1
122 223 1
137 222 1
144 221 1
114 221 1
83 232 9
144 219 1
150 217 1
114 215 1
164 209 1
158 209 1
163 206 1
118 203 1
128 195 1
175 194 1
134 189 1
185 184 1
154 175 1
197 174 1
159 174 1
157 172 1
196 171 1
162 171 1
200 169 1
198 167 1
204 165 1
200 165 1
200 163 1
211 162 1
179 160 1
208 159 1
210 157 1
204 157 1
135 227 1
203 156 1
214 155 1
204 155 1
200 155 1

If you want to see the shapes, then you might be better off using contours rather than blobs.