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drawing a rectangle around a color as shown?

asked 2018-10-10 05:08:01 -0500

rohan__007 gravatar image

updated 2019-12-04 10:00:45 -0500

supra56 gravatar image

I want to detect the color, which I have done with the following code:

import cv2 as cv
import numpy as np

cap = cv.VideoCapture(0)


    _, frame =
    # Convert BGR to HSV
    hsv = cv.cvtColor(frame, cv.COLOR_BGR2HSV)
    # define range of blue color in HSV
    lower_blue = np.array([100,55,55])
    upper_blue = np.array([105,255,255])
    # Threshold the HSV image to get only blue colors
    mask = cv.inRange (hsv, lower_blue, upper_blue)
    # Bitwise-AND mask and original image
    res = cv.bitwise_and(frame,frame, mask= mask)
    k = cv.waitKey(5) & 0xFF
    if k == 27:

but want to know how to draw a rectangle around it?

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somewhat better, than your last question. however, please spare us duplicate ones to the same topic, thank. you !

berak gravatar imageberak ( 2018-10-10 05:13:52 -0500 )edit

and what you really need, is a tour through the tutorials

berak gravatar imageberak ( 2018-10-10 05:15:08 -0500 )edit

Esp this one

"...Contours can be explained simply as a curve joining all the continuous points (along the boundary), having same color or intensity..."

Sound good to me. Should work.

holger gravatar imageholger ( 2018-10-10 05:31:25 -0500 )edit

I just noticed you already extracted the bbox(ignore comment above) - maybe all you asking for is how to draw rectangles at all? For drawing functions look here

holger gravatar imageholger ( 2018-10-10 05:38:47 -0500 )edit

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answered 2019-03-16 09:12:06 -0500

supra56 gravatar image

I am using raspberry pi using linux. Btw, I used hsv colour, Fortunately, on your side, you may used your hsv colour.

import numpy as np
import cv2  

cap = cv2.VideoCapture(0)

while True:
    _, frame =
    # Convert BGR to HSV
    hsv = cv2.cvtColor(frame, cv2.COLOR_BGR2HSV)
    # define range of blue color in HSV
    lower_blue = np.array([100,50,50])
    upper_blue = np.array([130,255,255])
    # Threshold the HSV image to get only blue colors
    mask = cv2.inRange (hsv, lower_blue, upper_blue)
    bluecnts = cv2.findContours(mask.copy(),

    if len(bluecnts)>0:
        blue_area = max(bluecnts, key=cv2.contourArea)
        (xg,yg,wg,hg) = cv2.boundingRect(blue_area)
        cv2.rectangle(frame,(xg,yg),(xg+wg, yg+hg),(0,255,0),2)


    k = cv2.waitKey(5) 
    if k == 27:


Output: blue_rect

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Hi may I know that what is the meaning of the line "key=cv2.contourArea" in the max() function. Thank you for reading this post.

MingCheng gravatar imageMingCheng ( 2019-12-12 02:25:39 -0500 )edit

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Asked: 2018-10-10 05:08:01 -0500

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Last updated: Dec 04 '19