无法在 opencv 中保存视频

Question

I am trying to save my video using opencv write method but the video is saved with 0 kb. what's wrong in my code.

  import cv2

  cap = cv2.VideoCapture("k1.mp4")
  fgbg = cv2.bgsegm.createBackgroundSubtractorMOG()
  fourcc = cv2.VideoWriter_fourcc(*'MP42')
  out = cv2.VideoWriter('output.mp4', fourcc, 20.0, (640,480))

  while cap.isOpened():
     ret, frame = cap.read()
     gray = cv2.cvtColor(frame, cv2.COLOR_BGR2GRAY)
     fgmask = fgbg.apply(gray)
     thresh = 2
     maxValue = 255
     ret, th1 = cv2.threshold(fgmask, thresh, maxValue, cv2.THRESH_BINARY)

     color_space = cv2.applyColorMap(th1, cv2.COLORMAP_JET)
     result_vid = cv2.addWeighted(frame, 0.7, color_space, 0.7, 0)
     cv2.imshow("vid", result_vid)
     out.write(result_vid)
     if cv2.waitKey(20) == ord('q'):
         break

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

Answer 1

 import cv2
 cap = cv2.VideoCapture(0)

 # Automatically grab width and height from video feed
 # (returns float which we need to convert to integer for later on!)
 width = int(cap.get(cv2.CAP_PROP_FRAME_WIDTH))
 height = int(cap.get(cv2.CAP_PROP_FRAME_HEIGHT))


 # MACOS AND LINUX: *'XVID' (MacOS users may want to try VIDX as well just in case)
 # WINDOWS *'VIDX'
 writer = cv2.VideoWriter('local_capture.mp4', cv2.VideoWriter_fourcc(*'VIDX'),25, (width, height))


 # This loop keeps recording until you hit Q or escape the window
 # You may want to instead use some sort of timer, like from time import sleep and then just record for 5 seconds.
while True:

     # Capture frame-by-frame
     ret, frame = cap.read()


     # Write the video
     writer.write(frame)

     # Display the resulting frame
     cv2.imshow('frame',frame)

     # This command let's us quit with the "q" button on a keyboard.
     # Simply pressing X on the window won't work!
     if cv2.waitKey(1) & 0xFF == ord('q'):
         break
cap.release()
writer.release()
cv2.destroyAllWindows()

Answer 2

The problem is that the video codec and the video container format do not match.

When executing your code, I am getting an error message (in the console windows):

OpenCV: FFMPEG: tag 0x3234504d/'MP42' is not supported with codec id 15 and format 'mp4 / MP4 (MPEG-4 Part 14)'
[mp4 @ 00000155e95dcec0] Could not find tag for codec msmpeg4v2 in stream #0, codec not currently supported in container

• You are using fourcc = cv2.VideoWriter_fourcc(*'MP42') , and M420 applies video codec MPEG-4v2 .
• Video output file name is 'output.mp4' .
  The .mp4 extension applies MP4 container format.

Apparently .mp4 video file cannot contain video encoded with MPEG-4v2 codec.

You may either change codec, or change file format.

Example:

• Changing output file name to 'output.avi' or 'output.wmv' works.
• Changing the codec to MPEG-4 : fourcc = cv2.VideoWriter_fourcc(*'mp4v') (and keeping file name 'output.mp4' ) also works.

---

One more issue:

Add the following code after ret, frame = cap.read() :

if not ret:
    break;

Answer 3

Although your solution was to match the video codec and the video container format correctly, I wanted to add that another common reason the output file size is 0 kb when writing videos with OpenCV is a discrepancy between the captured video frame size and the output video frame size. This can be solved by replacing the hard coded output frame size with one calculated from the input video.

cap = cv2.VideoCapture('input.mp4')
w = int(cap.get(3))
h = int(cap.get(4))
frameSize = (w, h)
fps = 20
fourCC = cv2.VideoWriter_fourcc(*'MP4V')
out = cv2.VideoWriter('output.mp4', fourCC, fps, frameSize)

while(True):
    ret, frame = cap.read()
    ...
    result_vid = ...
    ...
    out.write(result_vid)
    ...

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