Opencv python recording from webcam in 720p is 5 seconds less of file video


I am using opencv python module to record videos from webcam to 30fps in 720p(1280x720) to a file avi. But I have some issue with that, beacuse when video is recorded from webcam I start th stopwatch and when video stops (stopwatcht in the same time). Then, the file has 20seconds, but in the stopwatcher is around 5 seconds more.

I 'm trying to change fps and resolution without success. When I use another webcam, in 640x480 25fps, but when stopwatcher is 50seconds, the file is 58seconds.

The sample code is very simple and I picked from a tutorial.

import numpy as np
import os
import cv2

filename = 'video.avi'
frames_per_second = 24.0
res = '720p'

# Set resolution for the video capture
# Function adapted from
def change_res(cap, width, height):
    cap.set(3, width)
    cap.set(4, height)

# Standard Video Dimensions Sizes
    "480p": (640, 480),
    "720p": (1280, 720),
    "1080p": (1920, 1080),
    "4k": (3840, 2160),

# grab resolution dimensions and set video capture to it.
def get_dims(cap, res='1080p'):
    width, height = STD_DIMENSIONS["480p"]
    if res in STD_DIMENSIONS:
        width,height = STD_DIMENSIONS[res]
    ## change the current caputre device
    ## to the resulting resolution
    change_res(cap, width, height)
    return width, height

# Video Encoding, might require additional installs
# Types of Codes:
    'avi': cv2.VideoWriter_fourcc(*'XVID'),
    #'mp4': cv2.VideoWriter_fourcc(*'H264'),
    'mp4': cv2.VideoWriter_fourcc(*'XVID'),

def get_video_type(filename):
    filename, ext = os.path.splitext(filename)
    if ext in VIDEO_TYPE:
      return  VIDEO_TYPE[ext]
    return VIDEO_TYPE['avi']

cap = cv2.VideoCapture(0)
out = cv2.VideoWriter(filename, get_video_type(filename), 25, get_dims(cap, res))

while True:
    ret, frame =
    if cv2.waitKey(1) & 0xFF == ord('q'):


I don’t know what it happens. If is the resolution, fps, camera, … I am new in opencv. I appreciate every tip or idea for making videos with the right time in file.

Thanks a lot

It does not work as you think.
You must grab 25 images per second if you want a video at 25fps.

try this [code] (Python-sans-detour/ at master · LaurentBerger/Python-sans-detour · GitHub) in french.

Yes, I guess. But this equation for calculate time in that code

tps_img_sui = time.time_ns() + 1 / fps * 1e9
while True:
    ret, img =
    if not ret:
        print("erreur ou fin de la lecture du flux")
    tps_attente = tps_img_sui - time.time_ns()
    if tps_attente > 0:
        time.sleep(tps_attente * 1e-9)
        print("FPS trop grand")
    tps_img_sui += 1 / fps * 1e9

are there any documentation in opencv in order to get the right time in file recorded.?