Changing FPS in OpenCV

I have an application that needs to capture only a few frames per second from a webcam. Setting videowriter in the below code to 3 frames per second results in the webcam's normal framerate of approximately 30 fps being saved. What are the options to save only the recorded 3 frames per second, and let the other 27 or so go? Thanks in advance.

import cv2
import numpy as np
import time
import datetime
import pathlib
import imutils

cap = cv2.VideoCapture(0)

if (cap.isOpened() == False): 
  print("Unable to read camera feed")

capture_duration = 15
frame_per_sec = 3
frame_width = 80
frame_height = 60

out = cv2.VideoWriter('C:\\Users\\student\\Desktop\\videoFile.avi',cv2.VideoWriter_fourcc('m','j','p','g'),frame_per_sec, (frame_width,frame_height))

start_time = time.time()
while( int(time.time() - start_time) < capture_duration ):
    ret, frame = cap.read()
    if ret==True:
        frame = imutils.resize(frame, width=frame_width)
        out.write(frame)
        cv2.imshow('frame',frame) 
        if cv2.waitKey(1) & 0xFF == ord('q'):
          break
    else:
        break

cap.release()
out.release()
 
cv2.destroyAllWindows()

Solution

This method programmatically sets frames per second. A 6.1mb file was created when frame rate was set for 30fps, and a 0.9mb file when set for 3fps.

#!/usr/bin/env python3

import cv2
import numpy as np
import time
import datetime
import pathlib
import imutils

cap = cv2.VideoCapture(0)

if (cap.isOpened() == False): 
  print("Unable to read camera feed")

capture_duration = 15
frame_per_sec = 30
prev = 0
frame_width = int(cap.get(3))
frame_height = int(cap.get(4))
out = cv2.VideoWriter('C:\\videoPy\\LZ\\'outpout.avi',cv2.VideoWriter_fourcc('m','j','p','g'),frame_per_sec, (frame_width,frame_height))
start_time = time.time()
while( int(time.time() - start_time) < capture_duration ): 
#start fps
  time_elapsed = time.time() - prev
  while(time_elapsed > 1./frame_per_sec):                   
        ret, frame = cap.read()
        if not ret:
            break
        if time_elapsed > 1./frame_per_sec:
          prev = time.time()
#end fps
        if ret==True:
          frame = imutils.resize(frame, width=frame_width)
          out.write(frame)
          cv2.imshow('frame',frame) 
        if cv2.waitKey(1) & 0xFF == ord('q'):
          break
        else:
           break

cap.release()
out.release()
 
cv2.destroyAllWindows()