count colored dots in image

Question

First of all, sorry if this topic already exists (I think this is a common task, but couldn\'t find anything).

The point, is that I have an image who shows different

Answer 1

Here is a sample solution based on OpenCV 3.2 and Python 2.7.

To count the colored dots, repeat below 4 steps once per color type.

1. Apply median filter to reduce noise - cv2.medianBlur().
2. Apply color threshold to segment the colored dots - use cv2.inRange().
3. Use Hough Circle Transform to detect the circles - use circles = cv2.HoughCircles(mask,cv2.HOUGH_GRADIENT,...)
4. Loop through each detected circles to draw its center and a circle around it, and count the numbers of colored dots.

Sample images of dots detected:

Red - 10 dots

Green - 39 dots

Yellow - 30 dots

Take note that the last yellow dots at the right side with less than half a circle hasn't been detected. This is likely a limitation of the Hough Circle Transform cv2.HoughCircles(). So you need to decide how to handle this type of issue if it happens.

Here is the sample code:

import cv2
import numpy

red = [(0,0,240),(10,10,255)] # lower and upper 
green = [(0,240,0),(10,255,10)]
yellow = [(0,240,250),(10,255,255)]
dot_colors = [red, green, yellow]
    
img = cv2.imread('./imagesStackoverflow/count_colored_dots.jpg')   
# apply medianBlur to smooth image before threshholding
blur= cv2.medianBlur(img, 7) # smooth image by 7x7 pixels, may need to adjust a bit

for lower, upper in dot_colors:
    output = img.copy()
    # apply threshhold color to white (255,255, 255) and the rest to black(0,0,0)
    mask = cv2.inRange(blur,lower,upper) 

    circles = cv2.HoughCircles(mask,cv2.HOUGH_GRADIENT,1,20,param1=20,param2=8,
                               minRadius=0,maxRadius=60)    
    index = 0
    if circles is not None:
        # convert the (x, y) coordinates and radius of the circles to integers
        circles = numpy.round(circles[0, :]).astype("int")

        # loop over the (x, y) coordinates and radius of the circles
        for (x, y, r) in circles:
            # draw the circle in the output image, 
            #   then draw a rectangle corresponding to the center of the circle
            cv2.circle(output, (x, y), r, (255, 0, 255), 2)
            cv2.rectangle(output, (x - 5, y - 5), (x + 5, y + 5), (255, 0, 255), -1)

            index = index + 1
            #print str(index) + " : " + str(r) + ", (x,y) = " + str(x) + ', ' + str(y)
        print 'No. of circles detected = {}'.format(index)

Hope this help.