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Imagine having an image of circles of different colors on a background of one color. What would be the easiest way to find the coordinates of the circles' centers (of course programmatically)?

I felt like doing it in Python with OpenCV as well, using the same starting image as my other answer.

The code looks like this:

#!/usr/bin/env python3



import numpy as np

import cv2



# Load image

im = cv2.imread('start.png')



# Convert to grayscale and threshold

imgray = cv2.cvtColor(im,cv2.COLOR_BGR2GRAY)

ret,thresh = cv2.threshold(imgray,1,255,0)



# Find contours, draw on image and save

im2, contours, hierarchy = cv2.findContours(thresh,cv2.RETR_TREE,cv2.CHAIN_APPROX_SIMPLE)

cv2.drawContours(im, contours, -1, (0,255,0), 3)

cv2.imwrite('result.png',im)



# Show user what we found

for cnt in contours:

   (x,y),radius = cv2.minEnclosingCircle(cnt)

   center = (int(x),int(y))

   radius = int(radius)

   print('Contour: centre {},{}, radius {}'.format(x,y,radius))

That gives this on the Terminal:

Contour: centre 400.0,200.0, radius 10

Contour: centre 500.0,200.0, radius 80

Contour: centre 200.0,150.0, radius 90

Contour: centre 50.0,50.0, radius 40

And this as the result image:

There's a very easy way with ImageMagick which is free and installed on most Linux distros and is available for macOS and Windows - no programming required!

Let's start with this image:

Now you just run this in Terminal or Command Prompt:

magick input.png -define connected-components:verbose=true -connected-components 8 -auto-level output.png

Output

Objects (id: bounding-box centroid area mean-color):

  0: 600x300+0+0 297.4,145.3 128391 srgb(0,0,0)          <--- black background

  2: 181x181+110+60 200.0,150.0 25741 srgb(0,0,255)      <--- blue circle

  3: 161x161+420+120 500.0,200.0 20353 srgb(255,0,255)   <--- magenta circle

  1: 81x81+10+10 50.0,50.0 5166 srgb(0,255,0)            <--- green circle

  4: 21x21+390+190 400.0,200.0 349 srgb(255,255,0).      <--- yellow circle

I added the comments above after <---.

Looking at the blue circle, you can see its colour is srgb(0,0,255) which is blue and it measures 181x181 pixels - so its radius is 90 pixels. The top-left corner of the containing rectangle is at [110,60] so the centre is at [200,150], which matches the 200.00,150.00 given for the centroid.

Likewise, looking at the yellow circle, its colour is srgb(255,255,0) which is yellow. Its height and width are 21 pixels which means the radius is 10. The top-left corner of the containing square is at [390,190] which means the centre is at [400,200], matching the centroid given as 400.0,200.0.