A python script that generates a video with a "ghost" effect, like this one: https://www.youtube.com/watch?v=Op3OuOc-9CY

ghost_effect.py

import os.path

import cv2
import numpy as np
import pafy


class AddressableQueue(object):
    """
    Creates a data structure that will hold some number of records.
    A newly added record is always put on the position 0, the previusly
    first record will become second, second will become third, and so
    on. The structure only holds some defined number of records.
    """
    def __init__(self, max_records=20):
        self.max_records = max_records
        self.data = []

    def add(self, i):
        self.data.append(i)
        if len(self.data) > 2*self.max_records:
            del self.data[0:self.max_records]

    def get_at_or_first(self, idx):
        """
        Returns the idx-th record, or the first record if there is
        nothing on that index.
        """
        if idx >= len(self.data):
            idx = 0
        return self.data[-idx-1]



def main(url):
    video = pafy.new(url)
    best = video.getbest()
    fn = best.generate_filename()
    if not os.path.exists(fn):
        print("Downloading %s" % fn)
        best.download(filepath=fn)
    else:
        print("Using %s" % fn)

    camera = cv2.VideoCapture(fn)
    prev = None
    pic_queue = AddressableQueue(max_records=20)
    (grabbed, frame) = camera.read()
    prev = frame
    out_buf = np.zeros(shape=(frame.shape[0]*2, frame.shape[1]*2, frame.shape[2]),
                       dtype=np.uint8)
    BASE = 10
    FRAME_COUNT = 10
    while grabbed:
        gray0 = cv2.cvtColor(prev, cv2.COLOR_BGR2GRAY)
        gray1 = cv2.cvtColor(frame, cv2.COLOR_BGR2GRAY)

        diff = cv2.absdiff(gray1, gray0)
        diff = cv2.threshold(diff, 35, 0, cv2.THRESH_TOZERO)[1]
        pic_queue.add(diff)

        out = frame
        shadow = np.zeros_like(diff)
        for i in range(0, FRAME_COUNT):
            white = pic_queue.get_at_or_first(BASE + i)
            shadow = cv2.addWeighted(shadow, 1.0, white, 1.0/(1 + i), 0)
            white = cv2.cvtColor(white, cv2.COLOR_GRAY2RGB)
            out = cv2.addWeighted(out, 1.0, white, 1.0/(1 + i), 0)

        out_buf[0:frame.shape[0],0:frame.shape[1],:] = frame
        out_buf[frame.shape[0]:2*frame.shape[0],0:frame.shape[1],:] = out
        out_buf[0:frame.shape[0],frame.shape[1]:2*frame.shape[1],
                :] = cv2.cvtColor(diff, cv2.COLOR_GRAY2RGB)
        out_buf[frame.shape[0]:2*frame.shape[0],frame.shape[1]:2*frame.shape[1],
                :] = cv2.cvtColor(shadow, cv2.COLOR_GRAY2RGB)
        cv2.imshow("Frame", out_buf)

        key = cv2.waitKey(1) & 0xFF
        if key == ord("q"):  # quit
            break
        if key == ord("p"):  # pause
            cv2.waitKey(0)
        if key == ord("s"):  # pause
            cv2.imwrite("frame.jpg", diff)

        prev = frame
        (grabbed, frame) = camera.read()

    camera.release()

if "__main__" == __name__:
    main("https://www.youtube.com/watch?v=rBDiIYaFEF0")