如何在 Python 中使用 OpenCV 存储网络摄像头视频

Question

我有一个 Python 脚本，它可以读取我的网络摄像头并将其显示在一个窗口中。我现在想存储结果，所以按照this tutorial我写了以下代码：

import cv2
import imutils
camera = cv2.VideoCapture(0)

# Define the codec and create VideoWriter object to save the video
fourcc = cv2.VideoWriter_fourcc(*'XVID')
video_writer = cv2.VideoWriter('output.avi', fourcc, 20.0, (640, 480))

while True:
    try:
        (grabbed, frame) = camera.read()  # grab the current frame
        frame = imutils.resize(frame, width=640, height=480)
        cv2.imshow("Frame", frame)  # show the frame to our screen
        key = cv2.waitKey(1) & 0xFF  # I don't really have an idea what this does, but it works..
        video_writer.write(frame)  # Write the video to the file system
    except KeyboardInterrupt:
        break

# cleanup the camera and close any open windows
camera.release()
video_writer.release()
cv2.destroyAllWindows()
print "\n\nBye bye\n"

这完美地在新窗口中显示了来自我的网络摄像头的实时视频片段。但是写入视频文件似乎失败了。它确实创建了一个名为output.avi 的文件，但该文件为空（零字节），并且在命令行上我看到以下错误：

OpenCV: Frame size does not match video size.
OpenCV: Frame size does not match video size.
OpenCV: Frame size does not match video size.
etc.

我清楚地将帧的大小调整为我想要保存视频的大小 (640x480)，所以我不确定它为什么不匹配。

当我再次运行脚本时（所以在这种情况下，空的 output.avi 已经存在），它会显示以下错误：

2017-04-17 10:57:14.147 Python[86358:5848730] AVF: AVAssetWriter status: Cannot Save
2017-04-17 10:57:14.332 Python[86358:5848730] mMovieWriter.status: 3. Error: Cannot Save
2017-04-17 10:57:14.366 Python[86358:5848730] mMovieWriter.status: 3. Error: Cannot Save
2017-04-17 10:57:14.394 Python[86358:5848730] mMovieWriter.status: 3. Error: Cannot Save
etc.

在教程中，它说四位 FourCC 代码用于指定取决于平台的视频编解码器，可用代码列表可以在fourcc.org 中找到。我在 OSX 上，所以我尝试了一堆不同的编解码器代码：DIVX、XVID、MJPG、X264、WMV1、WMV2。但不幸的是，它们都不适合我。它们都给出了相同的错误，除了MJPG，它给了我以下错误：

OpenCV Error: Assertion failed (img.cols == width && img.rows == height && channels == 3) in write, file /tmp/opencv3-20170216-77040-y1hrk1/opencv-3.2.0/modules/videoio/src/cap_mjpeg_encoder.cpp, line 829
Traceback (most recent call last):
  File "store_video.py", line 15, in <module>
    video_writer.write(frame)  # Write the video to the file system
cv2.error: /tmp/opencv3-20170216-77040-y1hrk1/opencv-3.2.0/modules/videoio/src/cap_mjpeg_encoder.cpp:829: error: (-215) img.cols == width && img.rows == height && channels == 3 in function write

有人知道这里有什么问题吗？欢迎所有提示！

Answer 1

这可能是因为您使用 AVFoundation 构建了 OpenCV，它不支持 XVID 或其他编解码器。你可以试试mp4v 和m4v 扩展。

import cv2
camera = cv2.VideoCapture(0)

# Define the codec and create VideoWriter object to save the video
fourcc = cv2.VideoWriter_fourcc('m','p','4','v')
video_writer = cv2.VideoWriter('output.m4v', fourcc, 30.0, (640, 480))

while True:
        (grabbed, frame) = camera.read()  # grab the current frame
        frame = cv2.resize(frame, (640,480))
        cv2.imshow("Frame", frame)  # show the frame to our screen
        key = cv2.waitKey(33) & 0xFF  # I don't really have an idea what this does, but it works..
        video_writer.write(frame)  # Write the video to the file system
        if key==27:
            break;

# cleanup the camera and close any open windows
camera.release()
video_writer.release()
cv2.destroyAllWindows()
print("\n\nBye bye\n")

另一方面，错误

OpenCV Error: Assertion failed (img.cols == width && img.rows == height && channels == 3) in write, file /tmp/opencv3-20170216-77040-y1hrk1/opencv-3.2.0/modules/videoio/src/cap_mjpeg_encoder.cpp, line 829

表示你把维度搞砸了

frame = imutils.resize(frame, width=640, height=480)

你可以试试我在代码中使用的cv2.resize。当cv2 已经可以做到这一点时，就不需要使用另一个库了。