Python 3 script to take live video, detect the largest object, trace an outline (contour) and measure linear dimensions, using OpenCV

object-outline-and-dimensions-opencv.py

# in a terminal
# python -m pip install --user opencv-contrib-python numpy scipy matplotlib ipython jupyter pandas sympy nose


import cv2
import pandas as pd
import numpy as np
import imutils
from scipy.spatial import distance as dist
from imutils import perspective
from imutils import contours

# using cam built-in to computer
videocapture=cv2.VideoCapture(0)

# using IP camera address from my mobile phone, with Android 'IP Webcam' app over WiFi
# videocapture=cv2.VideoCapture("http://xxx.xxx.xxx.xxx:8080/video")

def safe_div(x,y): # so we don't crash so often
    if y==0: return 0
    return x/y

def nothing(x): # for trackbar
    pass

def rescale_frame(frame, percent=25):  # make the video windows a bit smaller
    width = int(frame.shape[1] * percent/ 100)
    height = int(frame.shape[0] * percent/ 100)
    dim = (width, height)
    return cv2.resize(frame, dim, interpolation=cv2.INTER_AREA)

if not videocapture.isOpened():
    print("can't open camera")
    exit()
    
windowName="Webcam Live video feed"

cv2.namedWindow(windowName)

# Sliders to adjust image
# https://medium.com/@manivannan_data/set-trackbar-on-image-using-opencv-python-58c57fbee1ee
cv2.createTrackbar("threshold", windowName, 75, 255, nothing)
cv2.createTrackbar("kernel", windowName, 5, 30, nothing)
cv2.createTrackbar("iterations", windowName, 1, 10, nothing)

showLive=True
while(showLive):
    
    ret, frame=videocapture.read()
    frame_resize = rescale_frame(frame)
    if not ret:
        print("cannot capture the frame")
        exit()
   
    thresh= cv2.getTrackbarPos("threshold", windowName) 
    ret,thresh1 = cv2.threshold(frame_resize,thresh,255,cv2.THRESH_BINARY) 
    
    kern=cv2.getTrackbarPos("kernel", windowName) 
    kernel = np.ones((kern,kern),np.uint8) # square image kernel used for erosion
    
    itera=cv2.getTrackbarPos("iterations", windowName) 
    dilation =   cv2.dilate(thresh1, kernel, iterations=itera)
    erosion = cv2.erode(dilation,kernel,iterations = itera) # refines all edges in the binary image

    opening = cv2.morphologyEx(erosion, cv2.MORPH_OPEN, kernel)
    closing = cv2.morphologyEx(opening, cv2.MORPH_CLOSE, kernel)  
    closing = cv2.cvtColor(closing,cv2.COLOR_BGR2GRAY)
    
    _,contours,hierarchy = cv2.findContours(closing,cv2.RETR_TREE,cv2.CHAIN_APPROX_NONE) # find contours with simple approximation cv2.RETR_TREE,cv2.CHAIN_APPROX_SIMPLE

    closing = cv2.cvtColor(closing,cv2.COLOR_GRAY2RGB)
    cv2.drawContours(closing, contours, -1, (128,255,0), 1)
    
    # focus on only the largest outline by area
    areas = [] #list to hold all areas

    for contour in contours:
      ar = cv2.contourArea(contour)
      areas.append(ar)

    max_area = max(areas)
    max_area_index = areas.index(max_area)  # index of the list element with largest area

    cnt = contours[max_area_index - 1] # largest area contour is usually the viewing window itself, why?

    cv2.drawContours(closing, [cnt], 0, (0,0,255), 1)
    
    def midpoint(ptA, ptB): 
      return ((ptA[0] + ptB[0]) * 0.5, (ptA[1] + ptB[1]) * 0.5)

    # compute the rotated bounding box of the contour
    orig = frame_resize.copy()
    box = cv2.minAreaRect(cnt)
    box = cv2.cv.BoxPoints(box) if imutils.is_cv2() else cv2.boxPoints(box)
    box = np.array(box, dtype="int")
    
    # order the points in the contour such that they appear
    # in top-left, top-right, bottom-right, and bottom-left
    # order, then draw the outline of the rotated bounding
    # box
    box = perspective.order_points(box)
    cv2.drawContours(orig, [box.astype("int")], -1, (0, 255, 0), 1)
 
    # loop over the original points and draw them
    for (x, y) in box:
      cv2.circle(orig, (int(x), int(y)), 5, (0, 0, 255), -1)

    # unpack the ordered bounding box, then compute the midpoint
    # between the top-left and top-right coordinates, followed by
    # the midpoint between bottom-left and bottom-right coordinates
    (tl, tr, br, bl) = box
    (tltrX, tltrY) = midpoint(tl, tr)
    (blbrX, blbrY) = midpoint(bl, br)
     
    # compute the midpoint between the top-left and top-right points,
    # followed by the midpoint between the top-righ and bottom-right
    (tlblX, tlblY) = midpoint(tl, bl)
    (trbrX, trbrY) = midpoint(tr, br)
     
    # draw the midpoints on the image
    cv2.circle(orig, (int(tltrX), int(tltrY)), 5, (255, 0, 0), -1)
    cv2.circle(orig, (int(blbrX), int(blbrY)), 5, (255, 0, 0), -1)
    cv2.circle(orig, (int(tlblX), int(tlblY)), 5, (255, 0, 0), -1)
    cv2.circle(orig, (int(trbrX), int(trbrY)), 5, (255, 0, 0), -1)
     
    # draw lines between the midpoints
    cv2.line(orig, (int(tltrX), int(tltrY)), (int(blbrX), int(blbrY)),(255, 0, 255), 1)
    cv2.line(orig, (int(tlblX), int(tlblY)), (int(trbrX), int(trbrY)),(255, 0, 255), 1)
    cv2.drawContours(orig, [cnt], 0, (0,0,255), 1)
    
    # compute the Euclidean distance between the midpoints
    dA = dist.euclidean((tltrX, tltrY), (blbrX, blbrY))
    dB = dist.euclidean((tlblX, tlblY), (trbrX, trbrY))

    # compute the size of the object
    pixelsPerMetric = 1 # more to do here to get actual measurements that have meaning in the real world
    dimA = dA / pixelsPerMetric
    dimB = dB / pixelsPerMetric
 
    # draw the object sizes on the image
    cv2.putText(orig, "{:.1f}mm".format(dimA), (int(tltrX - 15), int(tltrY - 10)), cv2.FONT_HERSHEY_SIMPLEX, 0.65, (255, 255, 255), 2)
    cv2.putText(orig, "{:.1f}mm".format(dimB), (int(trbrX + 10), int(trbrY)), cv2.FONT_HERSHEY_SIMPLEX, 0.65, (255, 255, 255), 2)

    # compute the center of the contour
    M = cv2.moments(cnt)
    cX = int(safe_div(M["m10"],M["m00"]))
    cY = int(safe_div(M["m01"],M["m00"]))
 
    # draw the contour and center of the shape on the image
    cv2.circle(orig, (cX, cY), 5, (255, 255, 255), -1)
    cv2.putText(orig, "center", (cX - 20, cY - 20), cv2.FONT_HERSHEY_SIMPLEX, 0.5, (255, 255, 255), 2)
 
    cv2.imshow(windowName, orig)
    cv2.imshow('', closing)
    if cv2.waitKey(30)>=0:
        showLive=False
        

        
videocapture.release()
cv2.destroyAllWindows()

Hi
I got this error.

thresh= cv2.getTrackbarPos("threshold", windowName)
cv2.error: OpenCV(4.3.0) /io/opencv/modules/highgui/src/window_QT.cpp:462: error: (-27:Null pointer) NULL window handler in function 'icvFindTrackBarByName'

A related method is here: https://www.digitalarchns.com/measureartifacts/