Draw contours around images of the same color with openCV python

I have this image with 3 channels RGB (a result of a VARI Index computation) and I would like to draw bounding boxes (rectangles) around the plants, represented in green here. What is the best and easiest way to do it with OpenCV / python?

I guess it’s an easy problem for OpenCV experts, but I could not find good tutorials online to do this for multiple objects.

The closest tutorial I found was: determining-object-color-with-opencv

The assumptions for the bounding boxes should/could be:

• green is the dominant color.
• it should be more than X pixels.

Thanks in advance!

Answer

Just answering my own question after stumbling upon this resource: https://docs.opencv.org/3.4/da/d0c/tutorial_bounding_rects_circles.html

May not be the best answer but it somehow solves my problem!

import cv2
import numpy as np

image = cv2.imread('vari3.png')

# https://www.pyimagesearch.com/2016/02/15/determining-object-color-with-opencv/
# https://docs.opencv.org/3.4/da/d0c/tutorial_bounding_rects_circles.html

gray = cv2.cvtColor(image, cv2.COLOR_BGR2GRAY)
lab = cv2.cvtColor(image, cv2.COLOR_BGR2LAB)

# mask: green is dominant.
thresh = np.array((image.argmax(axis=-1) == 1) * 255, dtype=np.uint8)

cnts = cv2.findContours(thresh.copy(), cv2.RETR_EXTERNAL, cv2.CHAIN_APPROX_SIMPLE)[0]

contours_poly = [None] * len(cnts)
boundRect = [None] * len(cnts)
for i, c in enumerate(cnts):
    contours_poly[i] = cv2.approxPolyDP(c, 3, True)
    boundRect[i] = cv2.boundingRect(contours_poly[i])

for i in range(len(cnts)):
    # cv2.drawContours(image, contours_poly, i, (0, 255, 0), thickness=2)
    pt1 = (int(boundRect[i][0]), int(boundRect[i][1]))
    pt2 = (int(boundRect[i][0] + boundRect[i][2]), int(boundRect[i][1] + boundRect[i][3]))
    if np.sqrt((pt2[1] - pt1[1]) * (pt2[0] - pt1[0])) < 30:
        continue
    cv2.rectangle(image, pt1, pt2, (0, 0, 0), 2)

cv2.imwrite('result.png', image)
cv2.imshow("Image", image)
cv2.waitKey(0)