Fill contours with alternating colors in Python

Question

I'm trying to get an algorithm that fills the contours of an image with alternating colors: first white, then black, then white again, then black again... like how the following picture shows:

What I've achieved so far was getting to fill the contours of an image with white, and then leaving the contours inside them with black, with the following code:

import numpy as np
import cv2

# Load the PNG image
img = cv2.imread('slice.png')

# Convert the image to grayscale
gray = cv2.cvtColor(img, cv2.COLOR_BGR2GRAY)

# Threshold the image to create a binary image
ret, thresh = cv2.threshold(gray, 0, 255, cv2.THRESH_BINARY)

# Find the outer contours in the binary image (using cv2.RETR_EXTERNAL)
contours, hierarchy = cv2.findContours(thresh, cv2.RETR_EXTERNAL, cv2.CHAIN_APPROX_SIMPLE)

# Create a blank image with the same dimensions as the original image
filled_img = np.zeros(img.shape[:2], dtype=np.uint8)

# Fill the outer contour with white color
cv2.drawContours(filled_img, contours, -1, 255, cv2.FILLED)

# Find contours with hierarchy, this time use cv2.RETR_TREE
contours, hierarchy = cv2.findContours(thresh, cv2.RETR_TREE, cv2.CHAIN_APPROX_SIMPLE)

# Iterate over the contours and their hierarchies
for i, contour in enumerate(contours):
    has_grandparent = False
    has_parent = hierarchy[0][i][3] >= 0
    if has_parent:
        # Check if contour has a grandparent
        parent_idx = hierarchy[0][i][3]
        has_grandparent = hierarchy[0][parent_idx][3] >= 0

    # Draw the contour over temporary image first (for testing if it has black pixels inside).
    tmp = np.zeros_like(thresh)
    cv2.drawContours(tmp, [contour], -1, 255, cv2.FILLED)
    has_innder_black_pixels = (thresh[tmp==255].min() == 0)  # If the minimum value is 0 (value where draw contour is white) then the contour has black pixels inside

    if hierarchy[0][i][2] < 0 and has_grandparent and has_innder_black_pixels:
        # If contour has no child and has a grandparent and it has black inside, fill the contour with black color
        cv2.drawContours(filled_img, [contour], -1, 0, cv2.FILLED)

# Display the result
cv2.imshow('Original Image', img)
cv2.imshow('Filled Regions', filled_img)
cv2.waitKey(0)
cv2.destroyAllWindows()

But once I get several contours, it doesn't do the job

I've seen another question about something similar (Find all pixels bounded between two contours in a binary image in Python), but I can't get to find a generalisation of the problem

Rotem · Accepted Answer

Different approach:
We may use cv2.floodFill operations instead of cv2.findContours:

Using flood filling the corner with white color, we can "peel" the contours from the outer contour toward the inner ones.

Since we want to keep every second contour with white color, we may apply the following two steps each iteration:

First step: fill, XOR, negate

 cv2.floodFill(thresh, None, (0, 0), 255, loDiff=0, upDiff=0)  # Fill the outer background with white color.
 out = out ^ thresh
 thresh = 255 - thresh  # Invert thresh

Second step: fill negate, XOR cv2.floodFill(thresh, None, (0, 0), 255, loDiff=0, upDiff=0) # Fill the outer background with white color.
thresh = 255 - thresh # Invert thresh out = out ^ thresh

Using XOR operation turns white and white into black... that way we the result is alternating black and white.

At the end, we have to negate the output if the number of iterations is odd.

Code sample:

import numpy as np
import cv2

img = cv2.imread('slice.png')  # Load the PNG image
gray = cv2.cvtColor(img, cv2.COLOR_BGR2GRAY)  # Convert the image to grayscale
thresh = cv2.threshold(gray, 0, 255, cv2.THRESH_BINARY)[1]  # Threshold the image to create a binary image

orig_thresh = thresh.copy()  # Save original thresh.
out = thresh.copy()  # Copy thresh to out (out is the final result).

i = 0
while np.any(thresh != 0):
    # First step: fill, XOR, negate
    cv2.floodFill(thresh, None, (0, 0), 255, loDiff=0, upDiff=0)  # Fill the outer background with white color.
    out = out ^ thresh
    thresh = 255 - thresh  # Invert thresh

    # Second step: fill negate, XOR
    cv2.floodFill(thresh, None, (0, 0), 255, loDiff=0, upDiff=0)  # Fill the outer background with white color.    
    thresh = 255 - thresh  # Invert thresh
    out = out ^ thresh

    i += 1  # Count iterations


if i % 2 == 1:
    out = 255 - out  # Negate output if number of iterations is odd

out[orig_thresh == 255] = 255  # Set all original white pixels to white

cv2.imshow('out', out)  # Show the output for testing
cv2.waitKey()
cv2.destroyAllWindows()

Example for inputs and outputs:

=>

==>

Fill contours with alternating colors in Python

Answers (2)

Related Questions