I am working on a school project which is related to Image Processing using OpenCV Python and Raspberry Pi 3.

The hardware of Raspberry Pi 3 cannot handle the video from camera consecutively, therefore I'm thinking about only capture a picture after every 5 seconds from the camera and use it to recognize what I need, then continue.

I did some research on the internet and found a function called time.sleep(5), however this function only pause the camera 5 seconds and then continue.

Can anyone help me to solve my problem. Thank you so much. Sorry for my bad English, here is my code so far. In my code, I use a video to test first, then when figure out the solution, I will apply on camera.

import cv2
import numpy as np
import time

headcc = cv2.CascadeClassifier('lib/heads_cascade.xml')
cap = cv2.VideoCapture('image/hockey_game.avi')

def video():
    ret, frame = cap.read()

    # gray = cv2.cvtColor(frame, cv2.COLOR_BGR2GRAY)

    head = headcc.detectMultiScale(frame, 1.2, 2, 0 , (20, 20), (40, 40))

    print type(head)
    print head
    print head.shape
    print "Number of heads detected: " + str(head.shape[0])


    if len(head) > 0:
        for (x, y, w, h) in head:
            cv2.rectangle(frame, (x,y), (x+w, y+h), (0, 255, 255), 1)

    cv2.rectangle(frame, ((0,frame.shape[0] -25)),(270, frame.shape[0]), (255,255,255), -1)
    cv2.putText(frame, "Number of head detected: " + str(head.shape[0]), (0,frame.shape[0] -10), cv2.FONT_HERSHEY_TRIPLEX, 0.5,  (0,0,0), 1)

    cv2.namedWindow('Camera',cv2.WINDOW_NORMAL)
    cv2.imshow('Camera',frame)


while(cap.isOpened()):
    video()
    time.sleep(5)
    if cv2.waitKey(1) & 0xFF == ord('q'):
        break

cap.release()
cv2.destroyAllWindows()

Update March 15th, 2018: Thanks to comment from Joe, I have found the way to skip frames. However, the program still cannot know whether the video ends, and also the fps code still appear 0.0. So if there is any help for these errors, I will really appreciate. Here is the code so far:

import cv2
import numpy as np
import time

headcc = cv2.CascadeClassifier('lib/heads_cascade.xml')
cap = cv2.VideoCapture('image/hockey_game.avi')
fps = cap.get(cv2.CAP_PROP_POS_FRAMES)

def video():
    ret, frame = cap.read()

    # gray = cv2.cvtColor(frame, cv2.COLOR_BGR2GRAY)

    head = headcc.detectMultiScale(frame, 1.2, 2, 0 , (20, 20), (40, 40))

    # print type(head)
    # print head
    # print head.shape
    print "Number of heads detected: " + str(head.shape[0])

    if len(head) > 0:
        for (x, y, w, h) in head:
            cv2.rectangle(frame, (x,y), (x+w, y+h), (0, 255, 255), 1)

    # cv2.rectangle(frame, ((0,frame.shape[0] -25)),(270, frame.shape[0]), (255,255,255), -1)
    # cv2.putText(frame, "Number of head detected: " + str(head.shape[0]), (0,frame.shape[0] -10), cv2.FONT_HERSHEY_TRIPLEX, 0.5,  (0,0,0), 1)

    cv2.namedWindow('Camera',cv2.WINDOW_NORMAL)
    cv2.imshow('Camera',frame)


while(cap.isOpened()):
    video()
    cf = cap.get(cv2.CAP_PROP_POS_FRAMES) - 1
    cap.set(cv2.CAP_PROP_POS_FRAMES, cf+50)
    # cv2.setTrackbarPos("pos_trackbar", "Frame Grabber", 
int(cap.get(cv2.CAP_PROP_FPS)))
    time.sleep(2)
    if (cv2.waitKey(1) & 0xFF == ord('q')):
        break

print fps
cap.release()
cv2.destroyAllWindows()

I have a lot of long video samples (1-2 minutes) I've been using to test some computer vision code on, but the real-world use case would be taking in live image data over a couple seconds. Is there a way to take only a few seconds of video from the middle of one of the long videos? I can't just take frames from the start of the video since the phenomena I'm trying to analyze doesn't start till a while into the video.

I probably could brute force it by just going through the first section of frames without doing any analysis on them but I'd prefer a more elegant solution if there is one.

Edit: I should specify I'm using Python

I'm reading a frame from a usb webcam, but I need the frame to be guaranteed to be taken by the camera after I request it. Unfortunately, I get the last buffed frame. What I'm doing now is calling read twice, once to flush the buffer, and once to wait for and get another image. This works pretty well but seems to fail 2% of the time or so, and cuts my frame rate in half (under ideal circumstances it would be slower but not by that much if I'm thinking clearly). I just don't understand what it's actually doing. How do I make sure to get the next available frame that wasn't taken before I called cv.VideoCapture.read ?