Why the output of Dense Optical Flow in OpenCV is not continuous？ - opencv

Question

I want to track whether worker throw garbage into the truck manually in the video.
Q1: Is Dense Optical Flow in OpenCV a good solution for me?
Q2: I tried to code a sample. But why the flow image is not continuous?
frame 41 and 43 is good , but frame 42 is black
Q3: Can I spy on the color change in a small area to track if garbage through ?
small area like this:
here is my code. you can run my code and video in my github repository
https://github.com/Pinocchio2018/QuestionHelper/blob/main/openCV_related/dence_optical_flow_problem/test.py
import numpy as np
import cv2 as cv
def put_frame_no(image, frame_no):
# font
font = cv.FONT_HERSHEY_SIMPLEX
# org
org = (50, 450)
# fontScale
font_scale = 2
# Blue color in BGR
color = (0, 0, 255)
# Line thickness of 2 px
thickness = 2
# Using cv2.putText() method
image = cv.putText(image, "frame no: " + str(frame_no), org, font,
font_scale, color, thickness, cv.LINE_AA)
return image
cap = cv.VideoCapture(cv.samples.findFile("0116-sample4-edited-short-throw.mp4"))
ret, frame1 = cap.read()
prv_frame = cv.cvtColor(frame1, cv.COLOR_BGR2GRAY)
hsv = np.zeros_like(frame1)
hsv[..., 1] = 255
cv.namedWindow("flow image", cv.WINDOW_NORMAL)
cv.resizeWindow("flow image", 800, 600)
frame_no = 0
while 1:
ret, origin_img = cap.read()
if not ret:
print('No frames grabbed!')
break
next_frame = cv.cvtColor(origin_img, cv.COLOR_BGR2GRAY)
flow = cv.calcOpticalFlowFarneback(prv_frame, next_frame, None, 0.5, 3, 15, 3, 5, 1.2, 0)
mag, ang = cv.cartToPolar(flow[..., 0], flow[..., 1])
hsv[..., 0] = ang * 180 / np.pi / 2
hsv[..., 2] = cv.normalize(mag, None, 0, 255, cv.NORM_MINMAX)
flow_image = cv.cvtColor(hsv, cv.COLOR_HSV2BGR)
flow_image = put_frame_no(flow_image, frame_no)
origin_img = put_frame_no(origin_img, frame_no)
frame_no += 1
vis_frame = np.concatenate((origin_img, flow_image), axis=1)
cv.imshow('flow image', vis_frame)
# cv.imshow('origin', flow_image)
k = cv.waitKey(30) & 0xff
if k == 27:
break
elif k == ord('s'):
cv.imwrite('opticalfb.png', origin_img)
cv.imwrite('opticalhsv.png', flow_image)
prv_frame = next_frame
cv.destroyAllWindows()