How to process images of a video frame by frame in video streaming using OpenCV and Python

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

User · Answer

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  quot frame d jpg quot    count  frame      count   count   1     if cv2 waitKey 10   amp  0xFF    ord  q            break  cap release   cv2 destroyAllWindows     destroy all opened windows

User · Answer

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

User · Answer

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

User · Answer

This is how I would start to solve this    Create a video writer   import cv2 cv as cv videowriter   cv CreateVideoWriter  filename  fourcc  fps  frameSize    Check here for valid parameters 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

User · Answer

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  gt  gt  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   gt   0  break               the camera will be deinitialized automatically in VideoCapture destructor     return 0

User · Answer

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

[python] How to process images of a video, frame by frame, in video streaming using OpenCV and Python

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