ValueError: need more than 2 values to unpack

Question

I am doing image processing on Raspberry Pi 3, Model B, using Python and OpenCv.

I need to follow an orange object by drawing a circle around it. Here is the code.

    # import the necessary packages
    from picamera.array import PiRGBArray
    from picamera import PiCamera
    import time
    import cv2
    import numpy as np

    # initialize the camera and grab a reference to the raw camera capture
    camera = PiCamera()
    camera.resolution = (640, 480)
    camera.framerate = 30
    camera.hflip = True

    rawCapture = PiRGBArray(camera, size=(640, 480))

    # allow the camera to warmup
    time.sleep(0.1)

    # capture frames from the camera
    for frame in camera.capture_continuous(rawCapture, format="bgr", use_video_port=True):
            # grab the raw NumPy array representing the image, then initialize the timestamp
            # and occupied/unoccupied text
            image = frame.array

            blur = cv2.blur(image, (3,3))

            #hsv to complicate things, or stick with BGR
            hsv = cv2.cvtColor(blur,cv2.COLOR_BGR2HSV)
            thresh = cv2.inRange(hsv,np.array((0, 150, 150)), np.array((40, 220, 220)))

            #lower = np.array([0,100,200],dtype="uint8")
            #upper = np.array([90,180,255], dtype="uint8")

            #thresh = cv2.inRange(blur, lower, upper)
            thresh2 = thresh.copy()

            # find contours in the threshold image
            image, contours,hierarchy = cv2.findContours(thresh,cv2.RETR_LIST,cv2.CHAIN_APPROX_SIMPLE)

            # finding contour with maximum area and store it as best_cnt
            max_area = 0
            best_cnt = 1
            for cnt in contours:
                    area = cv2.contourArea(cnt)
                    if area > max_area:
                            max_area = area
                            best_cnt = cnt

            # finding centroids of best_cnt and draw a circle there
            M = cv2.moments(best_cnt)
            cx,cy = int(M['m10']/M['m00']), int(M['m01']/M['m00'])
            print("cx: " + str(cx) + " cy: " + str(cy))
            #if best_cnt>1:
            cv2.circle(blur,(cx,cy),40,(0,255,0),4)
            # show the frame
            cv2.imshow("Frame", blur)
            cv2.imshow('thresh',thresh2)
            key = cv2.waitKey(1) & 0xFF

            # clear the stream in preparation for the next frame
            rawCapture.truncate(0)

            # if the `q` key was pressed, break from the loop
            if key == ord("q"):
                    break

This code is succesfully compiled when I use python colourdetection.py on the termminal. However, When I try to compile this code by using sudo python colourdetection.py , I get the following error:

Traceback (most recent call last): File "colourdetection.py", line 38, in image, contours,hierarchy = cv2.findContours(thresh,cv2.RETR_LIST,cv2.CHAIN_APPROX_SIMPLE) ValueError: need more than 2 values to unpack

I need to be able to compile this code using sudo, because I will have to combine it with a motor driving code, which is compiled with sudo pyhton motordrive.py

Any help ?

Answer 1

In opencv2, findContours returns just two values, contours and hierarchy. Therefore line 38 should be corrected to be:

contours, hierarchy = cv2.findContours(thresh, cv2.RETR_EXTERNAL, cv2.CHAIN_APPROX_NONE)