Find and draw the largest contour in opencv on a specific color (Python)

问题

Im trying to get the largest contour of a red book. I've got a little problem with the code because its getting the contours of the smallest objects (blobs) instead of the largest one and I can't seem to figure out why this is happening

The code I use:

camera = cv2.VideoCapture(0)
kernel = np.ones((2,2),np.uint8)

while True:
    #Loading Camera
    ret, frame = camera.read()

    blurred = cv2.pyrMeanShiftFiltering(frame, 3, 3)
    hsv = cv2.cvtColor(blurred, cv2.COLOR_BGR2HSV)

    lower_range = np.array([150, 10, 10])
    upper_range = np.array([180, 255, 255])
    mask = cv2.inRange(hsv, lower_range, upper_range)

    dilation = cv2.dilate(mask,kernel,iterations = 1)

    closing = cv2.morphologyEx(dilation, cv2.MORPH_GRADIENT, kernel)
    closing = cv2.morphologyEx(dilation, cv2.MORPH_CLOSE, kernel)

    #Getting the edge of morphology
    edge = cv2.Canny(closing, 175, 175)
    _, contours,hierarchy = cv2.findContours(edge, cv2.RETR_TREE, cv2.CHAIN_APPROX_SIMPLE)

    # Find the index of the largest contour
    areas = [cv2.contourArea(c) for c in contours]
    max_index = np.argmax(areas)
    cnt=contours[max_index]

    x,y,w,h = cv2.boundingRect(cnt)
    cv2.rectangle(frame,(x,y),(x+w,y+h),(0,255,0),2)


    cv2.imshow('threshold', frame)
    cv2.imshow('edge', edge)

    if cv2.waitKey(1) == 27:
        break


camera.release()
cv2.destroyAllWindows()

As you can see on this picture

Hopefully there is someone who can help

回答1:

You can start by defining a mask in the range of the red tones of the book you are looking for.

Then you can just find the contour with the biggest area and draw the rectangular shape of the book.

import numpy as np
import cv2

# load the image
image = cv2.imread("path_to_your_image.png", 1)

# red color boundaries [B, G, R]
lower = [1, 0, 20]
upper = [60, 40, 200]

# create NumPy arrays from the boundaries
lower = np.array(lower, dtype="uint8")
upper = np.array(upper, dtype="uint8")

# find the colors within the specified boundaries and apply
# the mask
mask = cv2.inRange(image, lower, upper)
output = cv2.bitwise_and(image, image, mask=mask)

ret,thresh = cv2.threshold(mask, 40, 255, 0)
im2,contours,hierarchy = cv2.findContours(thresh, cv2.RETR_EXTERNAL, cv2.CHAIN_APPROX_NONE)

if len(contours) != 0:
    # draw in blue the contours that were founded
    cv2.drawContours(output, contours, -1, 255, 3)

    #find the biggest area
    c = max(contours, key = cv2.contourArea)

    x,y,w,h = cv2.boundingRect(c)
    # draw the book contour (in green)
    cv2.rectangle(output,(x,y),(x+w,y+h),(0,255,0),2)

# show the images
cv2.imshow("Result", np.hstack([image, output]))

cv2.waitKey(0)

Using your image:

If you want the book to rotate you can use rect = cv2.minAreaRect(cnt) as you can find it here.

来源：https://stackoverflow.com/questions/44588279/find-and-draw-the-largest-contour-in-opencv-on-a-specific-color-python

标签

python

draw

contour