can't read mp4 with opencv

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I've ran into a problem with my opencv installation, it is unable to open an mp4 video. My system is ubuntu 16.04, 64bit, opencv3.2 used from python 3.5.

VideoCapture.read returns False and None.

There are other questions with this problem, but they target different platforms or different opencv versions.

Apparently, I'm missing the proper codec. So I ran make uninstall from my build directory, purged opencv* with apt and built from source again. This time making sure that ffmpeg was installed before the compilation.

Here are my steps:

• clone opencv and opencv_contrib
• cd opencv/
• mkdir build
• cd build
• cmake -D CMAKE_BUILD_TYPE=RELEASE -D CMAKE_INSTALL_PREFIX=/usr/local -D INSTALL_PYTHON_EXAMPLES=ON -D INSTALL_C_EXAMPLES=OFF -D OPENCV_EXTRA_MODULES_PATH=../../opencv_contrib/modules -D BUILD_EXAMPLES=ON ..
• make -j 8
• sudo make install

I checked the output of cmake, ffmpeg is there:

Video I/O:
--     DC1394 1.x:                  NO
--     DC1394 2.x:                  NO
--     FFMPEG:                      YES
--       avcodec:                   YES (ver 56.60.100)
--       avformat:                  YES (ver 56.40.101)
--       avutil:                    YES (ver 54.31.100)
--       swscale:                   YES (ver 3.1.101)
--       avresample:                NO
--     GStreamer:                   NO
--     OpenNI:                      NO
--     OpenNI PrimeSensor Modules:  NO
--     OpenNI2:                     NO
--     PvAPI:                       NO
--     GigEVisionSDK:               NO
--     Aravis SDK:                  NO
--     UniCap:                      NO
--     UniCap ucil:                 NO
--     V4L/V4L2:                    NO/YES
--     XIMEA:                       NO
--     Xine:                        NO
--     gPhoto2:                     NO

But the problem persists. How can I fix this ?