无法在 opencv 中保存视频

Question

我正在尝试使用 opencv 写入方法保存我的视频，但视频以 0 kb 保存。 我的代码有什么问题。

  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

问题是视频编解码器和视频容器格式不匹配。

执行代码时，我收到一条错误消息（在控制台窗口中）：

OpenCV: FFMPEG: tag 0x3234504d/'MP42' 不支持编解码器 ID 15 和格式 'mp4 / MP4 (MPEG-4 Part 14)'
[mp4 @ 00000155e95dcec0] 在流 #0 中找不到编解码器 msmpeg4v2 的标签，容器当前不支持编解码器

• 您正在使用fourcc = cv2.VideoWriter_fourcc(*'MP42') ，并且M420应用视频编解码器MPEG-4v2 。
• 视频输出文件名为'output.mp4' 。
  .mp4扩展名应用 MP4 容器格式。

显然.mp4视频文件不能包含使用MPEG-4v2编解码器编码的视频。

您可以更改编解码器或更改文件格式。

例子：

• 将输出文件名更改为'output.avi'或'output.wmv'有效。
• 将编解码器更改为MPEG-4 ： fourcc = cv2.VideoWriter_fourcc(*'mp4v') （并保留文件名'output.mp4' ）也有效。

---

还有一个问题：

在ret, frame = cap.read()之后添加以下代码：

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尺寸。 这可以通过将硬编码的 output 帧大小替换为从输入视频计算得出的帧大小来解决。

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()