The opencv works fine when doing other things. It can open images and show images. But it can't open a video.

The code I'm using to open a video is as below

import cv2

cap = cv2.VideoCapture("MOV_0006.mp4")

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

    cv2.imshow('video', frame)
    if cv2.waitKey(1) & 0xff == ord('q'):
        break

cap.release()
cv2.destroyAllWindows()

But when executing, it outputs error messages like below

[h264 @ 0x1053ba0] AVC: nal size 554779904
[h264 @ 0x1053ba0] AVC: nal size 554779904
[h264 @ 0x1053ba0] no frame!

My vlc and mplayer can play this video, but the opencv can't.

I have installed x264 and libx264-142 codec package. (using sudo apt-get install)

My version of ubuntu is 14.04 trusty.

I'm not sure is it a codec problem or not?

I have rebuilt opencv either with WITH_UNICAP=ON or with WITH_UNICAP=OFF, but it doesn't affect the problem at all. The error messages never change.

It's a codec problem

I converted that mp4 file to an avi file with ffmpeg. Then the above opencv code can play that avi file well.

Therefore I am sure that this is a codec problem.

(I then converted that mp4 file to another mp4 file using ffmpeg, thinking maybe ffmpeg would help turning that original unreadable .mp4 codec into a readable .mp4 codec, but the resulting .mp4 file ended up broken. This fact may or may not relate to this problem, just mentioning, in case anybody needs this information.)

The answer to it - Rebuild FFmpeg then Rebuild Opencv

Despite knowing this is a codec problem, I tried many other ways but still couldn't solve it. At last I tried rebuilding ffmpeg and opencv, then the problem was solved!

Following is my detailed rebuilding procedure.

(1) Build ffmpeg

1. Download ffmpeg-2.7.1.tar.bz2

   FFmpeg website: https://www.ffmpeg.org/download.html

   ffmpeg-2.7.1.tar.bz2 link: http://ffmpeg.org/releases/ffmpeg-2.7.1.tar.bz2

2. tar -xvf ffmpeg-2.7.1.tar.bz2
3. cd ffmpeg-2.7.1
4. ./configure --enable-pic --extra-ldexeflags=-pie

   From http://www.ffmpeg.org/platform.html#Advanced-linking-configuration

   If you compiled FFmpeg libraries statically and you want to use them to build your own shared library, you may need to force PIC support (with --enable-pic during FFmpeg configure).

   If your target platform requires position independent binaries, you should pass the correct linking flag (e.g. -pie) to --extra-ldexeflags.

   ---

   If you encounter error: yasm/nasm not found or too old. Use --disable-yasm for a crippled build.

   Just sudo apt-get install yasm

   ---

   Further building options: https://trac.ffmpeg.org/wiki/CompilationGuide/Ubuntu

   e.g. Adding option --enable-libmp3lame enables png encoder. (Before ./configure you need to sudo apt-get install libmp3lame-dev with version ≥ 3.98.3)

5. make -j5 (under ffmpeg folder)
6. sudo make install

(2) Build Opencv

1. wget http://downloads.sourceforge.net/project/opencvlibrary/opencv-unix/2.4.9/opencv-2.4.9.zip
2. unzip opencv-2.4.9.zip
3. cd opencv-2.4.9
4. mkdir build
5. cd build
6. cmake -D CMAKE_BUILD_TYPE=RELEASE -D CMAKE_INSTALL_PREFIX=/usr/local -D BUILD_NEW_PYTHON_SUPPORT=ON -D WITH_QT=OFF -D WITH_V4L=ON -D CMAKE_SHARED_LINKER_FLAGS=-Wl,-Bsymbolic ..

   You can change those options depend on your needs. Only the last one -D CMAKE_SHARED_LINKER_FLAGS=-Wl,-Bsymbolic is the key option. If you omit this one then the make will jump out errors.

   This is also from http://www.ffmpeg.org/platform.html#Advanced-linking-configuration (the same link of step 4 above)

   If you compiled FFmpeg libraries statically and you want to use them to build your own shared library, you may need to ... and add the following option to your project LDFLAGS: -Wl,-Bsymbolic

7. make -j5
8. sudo make install
9. sudo sh -c 'echo "/usr/local/lib" > /etc/ld.so.conf.d/opencv.conf'
10. sudo ldconfig

Now the opencv code should play a mp4 file well!

Methods I tried but didn't work

1. Try add WITH_UNICAP=ON WITH_V4L=ON when cmake opencv. But didn't work at all.
2. Try changing codec inside the python opencv code. But in vain.

   cap = cv2.VideoCapture("MOV_0006.mp4")

   print cap.get(cv2.cv.CV_CAP_PROP_FOURCC)

   I tested this in two environment. In the first environment the opencv works, and in the other the opencv fails to play a video. But both printed out same codec 828601953.0.

   I tried to change their codec by cap.set(cv2.cv.CV_CAP_PROP_FOURCC, cv2.cv.CV_FOURCC(*'H264')) but didn't work at all.

3. Try changing the libraries under opencv-2.4.8/3rdparty/lib/ into libraries in my workable environment. But couldn't even successfully build.

   I grep AVC: nal size and find the libraries contain this error message are opencv-2.4.8/3rdparty/lib/libavcodec.a etc. That's why I tried to replace them. But it turns out that this is a bad idea.

4. sudo apt-get -y install libopencv-dev build-essential cmake git libgtk2.0-dev pkg-config python-dev python-numpy libdc1394-22 libdc1394-22-dev libjpeg-dev libpng12-dev libtiff4-dev libjasper-dev libavcodec-dev libavformat-dev libswscale-dev libxine-dev libgstreamer0.10-dev libgstreamer-plugins-base0.10-dev libv4l-dev libtbb-dev libqt4-dev libfaac-dev libmp3lame-dev libopencore-amrnb-dev libopencore-amrwb-dev libtheora-dev libvorbis-dev libxvidcore-dev x264 v4l-utils unzip

   Try to install some that thing and some this thing. But it was totally useless.

Problems Related

I've searched the web and there are many similar problems, but NONE of them had a solution!

Below are problems I think the same as mine.

• OpenCV/ffmpeg does not play my mp4 video. — from opencv-users.nabble.com

• VideoCapture is not working in OpenCV 2.4.2 — from answers.opencv.org

  This one mentions rebuilding ffmpeg! But the arguments weren't enough for me.

• Mp4 reading problem - I installed OpenCV 2.4.1 and python 2.7 and made a short program that reads the avi file successfully. However it fails to read mp4 file. — from answers.opencv.org

• Cannot open “.mp4” video files using OpenCV 2.4.3, Python 2.7 in _Windows 7 machine — from Stack Overflow

• OpenCV 2.4 VideoCapture not working on Windows — from Stack Overflow