如何使用 OpenCV 和 Python 在视频流中逐帧处理视频图像

Question

I am a beginner in OpenCV. I want to do some image processing on the frames of a video which is being uploaded to my server. I just want to read the available frames and write them in a directory. Then, wait for the other part of the video to be uploaded and write the frames to the directory. And , I should wait for each frame to be completely uploaded then write it to a file.

Can you tell me how can I do it with OpenCV (Python)?

Edit 1: I wrote this code for capturing the video from a file, while new data are being appended at the end of the file. In other words, the out.mp4 file is not a complete video and another program is writing new frames on it. What I'm going to do is, wait for the other program to write new frames then read them and display them.

Here is my code:

import cv2
cap = cv2.VideoCapture("./out.mp4")

while True:
    if cap.grab():
        flag, frame = cap.retrieve()
        if not flag:
            continue
        else:
            cv2.imshow('video', frame)
    if cv2.waitKey(10) == 27:
        break

So the problem is the cap.grab() call! When there is no frame, it will return False ! And it won't capture frames anymore, even if I wait for a long time.

Answer 1

After reading the documentation of VideoCapture . I figured out that you can tell VideoCapture , which frame to process next time we call VideoCapture.read() (or VideoCapture.grab() ).

The problem is that when you want to read() a frame which is not ready, the VideoCapture object stuck on that frame and never proceed. So you have to force it to start again from the previous frame.

Here is the code

import cv2

cap = cv2.VideoCapture("./out.mp4")
while not cap.isOpened():
    cap = cv2.VideoCapture("./out.mp4")
    cv2.waitKey(1000)
    print "Wait for the header"

pos_frame = cap.get(cv2.cv.CV_CAP_PROP_POS_FRAMES)
while True:
    flag, frame = cap.read()
    if flag:
        # The frame is ready and already captured
        cv2.imshow('video', frame)
        pos_frame = cap.get(cv2.cv.CV_CAP_PROP_POS_FRAMES)
        print str(pos_frame)+" frames"
    else:
        # The next frame is not ready, so we try to read it again
        cap.set(cv2.cv.CV_CAP_PROP_POS_FRAMES, pos_frame-1)
        print "frame is not ready"
        # It is better to wait for a while for the next frame to be ready
        cv2.waitKey(1000)

    if cv2.waitKey(10) == 27:
        break
    if cap.get(cv2.cv.CV_CAP_PROP_POS_FRAMES) == cap.get(cv2.cv.CV_CAP_PROP_FRAME_COUNT):
        # If the number of captured frames is equal to the total number of frames,
        # we stop
        break

Answer 2

Use this:

import cv2
cap = cv2.VideoCapture('path to video file')
count = 0
while cap.isOpened():
    ret,frame = cap.read()
    cv2.imshow('window-name', frame)
    cv2.imwrite("frame%d.jpg" % count, frame)
    count = count + 1
    if cv2.waitKey(10) & 0xFF == ord('q'):
        break

cap.release()
cv2.destroyAllWindows() # destroy all opened windows

Answer 3

According to the latest updates for OpenCV 3.0 and higher, you need to change the Property Identifiers as follows in the code by Mehran:

cv2.cv.CV_CAP_PROP_POS_FRAMES

to

cv2.CAP_PROP_POS_FRAMES

and same applies to cv2.CAP_PROP_POS_FRAME_COUNT .

Hope it helps.

Answer 4

In openCV's documentation there is an example for getting video frame by frame. It is written in c++ but it is very easy to port the example to python - you can search for each fumction documentation to see how to call them in python.

#include "opencv2/opencv.hpp"

using namespace cv;

int main(int, char**)
{
    VideoCapture cap(0); // open the default camera
    if(!cap.isOpened())  // check if we succeeded
        return -1;

    Mat edges;
    namedWindow("edges",1);
    for(;;)
    {
        Mat frame;
        cap >> frame; // get a new frame from camera
        cvtColor(frame, edges, CV_BGR2GRAY);
        GaussianBlur(edges, edges, Size(7,7), 1.5, 1.5);
        Canny(edges, edges, 0, 30, 3);
        imshow("edges", edges);
        if(waitKey(30) >= 0) break;
    }
    // the camera will be deinitialized automatically in VideoCapture destructor
    return 0;
}

Answer 5

This is how I would start to solve this:

1. Create a video writer:

    import cv2.cv as cv videowriter = cv.CreateVideoWriter( filename, fourcc, fps, frameSize)

   Check here for valid parameters

2. Loop to retrieve[1] and write the frames:

    cv.WriteFrame( videowriter, frame )

   WriteFrame doc

[1] zenpoy already pointed in the correct direction. You just need to know that you can retrieve images from a webcam or a file :-)

Hopefully I understood the requirements correct.

Answer 6

The only solution I have found is not to set the index to a previous frame and wait (then OpenCV stops reading frames, anyway), but to initialize the capture one more time. So, it looks like this:

cap = cv2.VideoCapture(camera_url)
while True:
    ret, frame = cap.read()

    if not ret:
        cap = cv.VideoCapture(camera_url)
        continue

    # do your processing here

And it works perfectly!

Answer 7

This is example where a.mp4 is the video file

import cv2
cap = cv2.VideoCapture('a.mp4')
count = 0
while cap.isOpened():
    ret,frame = cap.read()
    if not ret:
        continue
    cv2.imshow('window-name', frame)
    #cv2.imwrite("frame%d.jpg" % count, frame)
    count = count + 1
    if cv2.waitKey(10) & 0xFF == ord('q'):
        break

cap.release()
cv2.destroyAllWindows()