Detect colorful dots in image in python?

Question

I’m trying to detect colorful dots on a white/gray background. The dots are 3 different colors (yellow, purple, blue) of different sizes. Here is the original image:

I converted the image to HSV and found lower and upper bounds for each image then applied contour detection to find those dots. The following code detects most of the dots:

import cv2
import numpy as np
from matplotlib import pyplot as plt
img = cv2.imread('image1_1.png')

hsv = cv2.cvtColor(img, cv2.COLOR_BGR2HSV)

lower_yellow = np.array([22,25,219])
upper_yellow = np.array([25,75,225])

lower_purple = np.array([141,31,223])
upper_purple = np.array([143,83,225])

lower_blue = np.array([92,32,202])
upper_blue = np.array([96,36,208])

mask_blue = cv2.inRange(hsv, lower_blue, upper_blue)
mask_purple = cv2.inRange(hsv, lower_purple, upper_purple)
mask_yellow = cv2.inRange(hsv, lower_yellow, upper_yellow)

res_blue = cv2.bitwise_and(img,img, mask=mask_blue)
res_purple = cv2.bitwise_and(img,img, mask=mask_purple)
res_yellow = cv2.bitwise_and(img,img, mask=mask_yellow)

gray_blue = cv2.cvtColor(res_blue, cv2.COLOR_BGR2GRAY)
gray_purple = cv2.cvtColor(res_purple, cv2.COLOR_BGR2GRAY)
gray_yellow = cv2.cvtColor(res_yellow, cv2.COLOR_BGR2GRAY)

_,thresh_blue = cv2.threshold(gray_blue,10,255,cv2.THRESH_BINARY)
_,thresh_purple = cv2.threshold(gray_purple,10,255,cv2.THRESH_BINARY)
_,thresh_yellow = cv2.threshold(gray_yellow,10,255,cv2.THRESH_BINARY)

contours_blue, hierarhy1 = cv2.findContours(thresh_blue,cv2.RETR_TREE,cv2.CHAIN_APPROX_SIMPLE)
contours_purple, hierarhy2 = cv2.findContours(thresh_purple,cv2.RETR_TREE,cv2.CHAIN_APPROX_SIMPLE)
contours_yellow, hierarhy3 = cv2.findContours(thresh_yellow,cv2.RETR_TREE,cv2.CHAIN_APPROX_SIMPLE)

result = img.copy()
cv2.drawContours(result, contours_blue, -1, (0, 0, 255), 2)
cv2.drawContours(result, contours_purple, -1, (0, 0, 255), 2)
cv2.drawContours(result, contours_yellow, -1, (0, 0, 255), 2)
cv2.imwrite("_allContours.jpg", result)

Here are the detected contours:

The problem is that some of the colored dots are not detected. I understand by fine-tuning the color ranges (lower and upper) it’s possible to detect more dots. But that is very time consuming and not generalizable to similar images. For example the following image looks similar to the first image above and has the same colorful dots but the background is slightly different, once I ran it through above code it was not able to detect even one of the dots. Am I on the right track? Is there a more scalable and reliable solution with less need to tune color parameters in order to solve this problem? Here is the other image I tried:

Asked By: Mohammad

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Source

Answer 1

I would suggest simply using adaptiveThreshold in Python/OpenCV

import cv2
import numpy as np

# read image
img = cv2.imread("dots.png")

# convert img to grayscale
gray = cv2.cvtColor(img, cv2.COLOR_BGR2GRAY)

# do adaptive threshold on gray image
thresh = cv2.adaptiveThreshold(gray, 255, cv2.ADAPTIVE_THRESH_MEAN_C, cv2.THRESH_BINARY, 25, 6)

# write results to disk
cv2.imwrite("dots_thresh.jpg", thresh)

# display it
cv2.imshow("thresh", thresh)
cv2.waitKey(0)

Answered By: fmw42

Detect colorful dots in image in python?

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