我需要在给定时间从视频中提取帧，我正在使用OpenCV。我正在使用Linux Mint 18.1，但它也必须在Ubuntu上运行。
此代码示例对我不起作用：
import cv2
print('making screenshot from {0}'.format('/tmp/big_buck_bunny_720p_5mb.mp4'))
cap = cv2.VideoCapture(video_path)
cap.set(cv2.CAP_PROP_POS_MSEC,1000)
ret,frame = cap.read()
print(ret) # This is false
cv2.imwrite("image.jpg", frame) # Creates an empty file
（我已检查过视频的路径是否正确，文件是否已损坏）。
我已经安装了Python OpenCV 3.3.0版本：
>>> import cv2
>>> cv2.__version__
'3.3.0'
我编译并安装了OpenCV的源代码（使用WITH_FFMPEG=ON选项）：
git clone https://github.com/Itseez/opencv.git /tmp/opencv-3
/tmp/opencv-3
mkdir -p release
cd release
cmake -D WITH_FFMPEG=ON -D CMAKE_BUILD_TYPE=RELEASE -D CMAKE_INSTALL_PREFIX=/usr/local ..
make
sudo make install
ffmpeg -version的输出如下：
ffmpeg version 2.8.11-0ubuntu0.16.04.1 Copyright (c) 2000-2017 the FFmpeg developers
built with gcc 5.4.0 (Ubuntu 5.4.0-6ubuntu1~16.04.4) 20160609
configuration: --prefix=/usr --extra-version=0ubuntu0.16.04.1 --build-suffix=-ffmpeg --toolchain=hardened --libdir=/usr/lib/x86_64-linux-gnu --incdir=/usr/include/x86_64-linux-gnu --cc=cc --cxx=g++ --enable-gpl --enable-shared --disable-stripping --disable-decoder=libopenjpeg --disable-decoder=libschroedinger --enable-avresample --enable-avisynth --enable-gnutls --enable-ladspa --enable-libass --enable-libbluray --enable-libbs2b --enable-libcaca --enable-libcdio --enable-libflite --enable-libfontconfig --enable-libfreetype --enable-libfribidi --enable-libgme --enable-libgsm --enable-libmodplug --enable-libmp3lame --enable-libopenjpeg --enable-libopus --enable-libpulse --enable-librtmp --enable-libschroedinger --enable-libshine --enable-libsnappy --enable-libsoxr --enable-libspeex --enable-libssh --enable-libtheora --enable-libtwolame --enable-libvorbis --enable-libvpx --enable-libwavpack --enable-libwebp --enable-libx265 --enable-libxvid --enable-libzvbi --enable-openal --enable-opengl --enable-x11grab --enable-libdc1394 --enable-libiec61883 --enable-libzmq --enable-frei0r --enable-libx264 --enable-libopencv
libavutil 54. 31.100 / 54. 31.100
libavcodec 56. 60.100 / 56. 60.100
libavformat 56. 40.101 / 56. 40.101
libavdevice 56. 4.100 / 56. 4.100
libavfilter 5. 40.101 / 5. 40.101
libavresample 2. 1. 0 / 2. 1. 0
libswscale 3. 1.101 / 3. 1.101
libswresample 1. 2.101 / 1. 2.101
libpostproc 53. 3.100 / 53. 3.100
我错过了什么？