Find the Solidity and Equivalent Diameter of an Image Object Using OpenCV Python

Last Updated : 26 Apr, 2025

In this article, we will see how we can find the solidity and the equivalent diameter of an object present in an image with help of Python OpenCV.

Function to Find Solidity

The solidity of an image is the measurement of the overall concavity of a particle. We can define the solidity of an object as the ratio of the contour area to its convex hull area. The Contour area and convex hull area should be determined first in order to compute the solidity.

Python3

# function to find Solidity 
def find_solidity(count): 
    contourArea = cv2.contourArea(count) 
    convexHull = cv2.convexHull(count) 
    contour_hull_area = cv2.contourArea(convexHull) 
    solidity = float(contourArea)/contour_hull_area 
    return solidity 

Function to Find Equivalent Diameter

Also, the diameter of the circle whose area is equal to the contour area is known as the Equivalent Diameter. The Contour is an outline representing or bounding the shape or form of something.

Python3

# function to calculate the equivalent diameter 
def find_equi_diameter(count): 
    area = cv2.contourArea(count) 
    equi_diameter = np.sqrt(4*area/np.pi) 
    return equi_diameter 

Steps to find the Solidity and the Equivalent Diameter:

First, we need to import python’s OpenCV library and NumPy library.
Use the function cv2.imread() to read the desired image.
Create a binary image by applying thresholding on the grayscale image.
Use the cv2.findContours() function to find the contours in the image.
Compute the Solidity and the Equivalent Diameter for that we will priorly calculate the contour area and convex hull area.
Finally, print the Solidity and the Equivalent Diameter.

Solidity and Equivalent Diameter of an Image with Single Object

We will determine the solidity and equivalent diameter of a single object in the picture using below Python program. The solidity and equivalent diameter values for the item will be printed on the console.

Python3

# import required libraries 
import cv2 
import numpy as np 
  
# read the desired image 
# Give Path to the image 
img = cv2.imread(r"C:\Users\siddh\Downloads\star2.png") 
  
# convert the image to grayscale image 
grayScaleImg = cv2.cvtColor(img, cv2.COLOR_BGR2GRAY) 
  
# apply thresholding to convert 
# grayscale image to the binary image 
ret, thresh = cv2.threshold(grayScaleImg, 40, 255, 0) 
  
# find the contours 
contours, hierarchy = cv2.findContours(thresh, 
                                       cv2.RETR_TREE, 
                                       cv2.CHAIN_APPROX_SIMPLE) 
print(f"{len(contours)} objects detected") 
  
# select first contour 
count = contours[0] 
  
# find the solidity for this contour 
Solidity = find_solidity(count) 
Solidity = round(Solidity, 2) 
  
# find the equivalent diameter for this contour 
equi_diameter = find_equi_diameter(count) 
equi_diameter = round(equi_diameter, 2) 
  
print("Solidity - ", Solidity) 
print("Equivalent Diameter - ", equi_diameter) 

Input Image:

star.png

Output:

1 objects detected
Solidity -  0.48
Equivalent Diameter -  511.62

Solidity and Equivalent Diameter of an Image with Multiple Objects

We will determine the solidity and equivalent diameter of multiple objects in the picture using below Python program. The solidity and equivalent diameter values for the item will be printed on the console.

Python3

import cv2 
import numpy as np 
  
# read the desired image 
# Give Path to the image 
img = cv2.imread(r"C:\Users\siddh\Downloads\multshapes2.png") 
  
# convert the image to grayscale image 
gray = cv2.cvtColor(img, cv2.COLOR_BGR2GRAY) 
  
# apply thresholding to convert grayscale image to the binary image 
# and find the contours 
ret, thresh = cv2.threshold(gray, 100, 255, 0) 
contours, hierarchy = cv2.findContours(thresh, 1, 2) 
print("Number of objects detected:", len(contours)) 

Now we will iteratively find the solidity and the equivalent diameter for each of the objects which are detected in the image.

Python3

# iterate over the list 'contours' to get 
# solidity and Equivalent Diameter of each object 
for i, cnt in enumerate(contours): 
    Solidity = find_solidity(cnt) 
    Solidity = round(Solidity, 2) 
    equi_diameter = find_equi_diameter(cnt) 
    equi_diameter = round(equi_diameter, 2) 
    print(f"Solidity of object {i+1}: ", Solidity) 
    print(f"Equivalent Diameter of object {i+1}: ", equi_diameter) 

input Image:

multiple_shapes.png

Output:

Number of objects detected: 3
Solidity of object 1:  1.0
Equivalent Diameter of object 1:  90.93
Solidity of object 2:  0.98
Equivalent Diameter of object 2:  73.14
Solidity of object 3:  0.99
Equivalent Diameter of object 3:  93.43

Detecting objects of similar color in Python using OpenCV

siddheshsagar

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Find the Solidity and Equivalent Diameter of an Image Object Using OpenCV Python

Function to Find Solidity

Python3

Function to Find Equivalent Diameter

Python3

Solidity and Equivalent Diameter of an Image with Single Object

Python3

Solidity and Equivalent Diameter of an Image with Multiple Objects

Python3

Python3

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