Can't open webcan or video file with python3 and opencv

[patrickelectric]

System information (version)

• OpenCV => 3.2.0-1
• Python OpenCV => 3.2.0.6
• Python => 3.5.2
• Operating System / Platform => ArchLinux 64bits

Detailed description

Can't open VideoCapture don't work with files and /dev/video

Steps to reproduce

import cv2
video = cv2.VideoCapture(0)
if video.isOpened():
    while True:
        check, frame = video.read()
        if check:
            cv2.imshow('Color Frame', frame)
            key = cv2.waitKey(50)
            if key == ord('q'):
                break
        else:
            print('Frame not available')
            print(video.isOpened())

The code don't show webcan or video file
Works with python2 ..

[patrickelectric]

@alalek what is missing ?

[alalek]

Provided details is not enough to reproduce the issue.
Especially difference between Python2/3 looks strange.

What does "3.2.0-1" mean in OpenCV version? Is it Arch package version? AUR?

Please check correctness of loaded modules (cv2.__file__, cv2.__version__).
Also please provide OpenCV build configuration: cv2.getBuildInformation()

[mgruben]

Can't open videos or webcam:

System information (version)
opencv-python Version: 3.2.0.7
python Version: 3.6.1
Operating System / Platform: 4.10.11-1-ARCH Linux

Detailed description
VideoCapture doesn't work with files or with /dev/video

Steps to reproduce

# Fails with files, direct instantiation
import cv2
camera = cv2.VideoCapture("path/to/a/valid/video.mp4")
print(camera.isOpened()) # False
print(camera.read()) # (False, None)

# Fails with files, using .open() method
import cv2
camera = cv2.VideoCapture()
camera.open("path/to/a/valid/video.mp4") # False
print(camera.read()) # (False, None)

# Fails with webcam
import cv2
camera = cv2.VideoCapture(0)
print(camera.isOpened()) # False
print(camera.read()) # (False, None)

[dkloving]

Same issue with Python 3.6.0 under MacOS 10.12 Sierra.

[alalek]

This is still looks like an usage question. OpenCV requires 3rdparty libraries to work with cameras/videos. You need to install them properly before building OpenCV Python bindings.

opencv-python doesn't support them (almost all 3rdparty dependencies are turned off, because they are system dependent - system with minimal configuration is used during build).

For more information please refer to this question and others:

Q: Why I can't open video files on GNU/Linux distribution X or on macOS?

(https://pypi.python.org/pypi/opencv-python)

opencv-python binary package is not officially supported by OpenCV dev team, so we can't help here.

Usage questions should go to Users OpenCV Q/A forum: http://answers.opencv.org

[iiAnthony]

I have had a comparable issue on Ubuntu, using the pip install opencv-python process.
Solution used : I recompiled both ffmpeg and opencv 3.2 from sources on my station (following the install processes indicated on offical portals). Everything worked well, but I still was not able to open video from cv2.videocapture on python.

My solution has been to replace the cv2.so library provided by pypi with the one compiled from sources :
sudo cp opencv-3.2.0/build/lib/cv2.so /usr/local/lib/python2.7/dist-packages/cv2/

And I now can play videos with cv2, from my python scripts. Hope this will help those experiencing the same issue (I spent some time on trying to find a solution and never found this suggestion so ...).

[chutsu]

I did a similar thing to @iiAnthony where I compiled OpenCV 3.2.0 from source, double checking with CMake that it can find both python 2 and python 3

then I replaced the compiled .so dynamic library with the one that comes with opencv-python python3 package. The difference between my comment and @iiAnthony's is that his caters for Python2 users?

sudo cp <path to opencv source repo>/build/lib/python3/cv2.cpython-35m-x86_64-linux-gnu.so /usr/local/lib/python3.5/dist-packages/cv2/cv2.cpython-35m-x86_64-linux-gnu.so

Or better yet, just the ln command to symlink the built .so instead of copying, so that if you decide to compile OpenCV3 with different compilation flags you don't have to copy the library file over again.

Note: Your .so library file may have a different name to mine, but the idea is the same, compile your own opencv3 .so library file and replace the one that opencv-python comes with.

[alundiak]

Considering these 2 commands:

sudo cp build/lib/cv2.so /usr/local/lib/python2.7/dist-packages/cv2/

sudo cp build/lib/python3/cv2.cpython-35m-x86_64-linux-gnu.so /usr/local/lib/python3.5/dist-packages/cv2/cv2.cpython-35m-x86_64-linux-gnu.so

In my case opencv-3.3.1/build/lib doesn't have folder python nor python 2.7 nor python 3.6.
It contains lot of files with prefix libopencv_XYZ.so
And I'm on CentOS 7.0

UPD: ISSUE solved see my last comment here.

[chutsu]

That could be a number of things:

1. CMake failed to find your python version, double check it is listed in the cmake .. stage (like in the screenshot I took in the previous post).
2. You don't have the python development library (on debian based systems this corresponds to python-dev or python3-dev packages)
3. Try and checkout a specific version of OpenCV instead of building master (i.e. git checkout 3.3.1)
4. Maybe do a find . -name "cv2.*.so" to find if it is hidden somewhere? Though I am not convinced that building on CentOS would mean the .so file is at a different location during build.

I just downloaded and built the zipped OpenCV 3.3.1 in Ubuntu 16.04 and have found the cv2.so in the build/lib/python3 folder so you'll have to double check your environment.

[alundiak]

@chutsu thanks.
I tried master and simple download of 3.3.1, both behaves similarly, But for sure, smth wrong with Python. Researching again....

From $ opencv-3.3.1/build cmake ..

-- 
--   Python 2:
--     Interpreter:                 /usr/bin/python2.7 (ver 2.7.5)
-- 
--   Python 3:
--     Interpreter:                 /usr/bin/python3.4 (ver 3.4.5)
-- 
--   Python (for build):            /usr/bin/python2.7

I also see, that CmakeCache.txt file has predefined key/values, and I don't know how to tell cmake to know about my Python version.

I used this command:

which python3 => /usr/local/bin/python3

 build] $ cmake -D CMAKE_BUILD_TYPE=RELEASE -D CMAKE_INSTALL_PREFIX=/usr/local -DPYTHON_EXECUTABLE:FILEPATH=/usr/local/bin/python3 ..

Regarding python-dev it's odd, because I installed Installing OpenCV from source https://opencv-python-tutroals.readthedocs.io/en/latest/py_tutorials/py_setup/py_setup_in_fedora/py_setup_in_fedora.html

In fact
yum install python-devel installed 2.7 version, and yum install python3-devel is not available, and ym install python34-devel is not the version I need. I use Python 3.6.

Doing double checking of all...

I dropped build folder and tried again. I tried:

build]$ cmake -D CMAKE_BUILD_TYPE=RELEASE -D CMAKE_INSTALL_PREFIX=/usr/local -DPYTHON_EXECUTABLE:FILEPATH=/usr/local/bin/python3 -D PYTHON_EXECUTABLE:FILEPATH=/usr/local/bin/python3  PYTHON_EXECUTABLE:FILEPATH=/usr/local/bin/python3 ..

And now I have now folder build/lib/python3 but it's empty.

btw,

I have this

ls -la /usr/local/lib/python3.6/site-packages/cv2/
total 50196
drwxr-xr-x   4 root root      205 Dec  6 15:15 .
drwxr-xr-x. 53 poc  root     4096 Dec  6 12:21 ..
-rwxr-xr-x   1 root root 25676848 Dec  6 12:21 cv2.cpython-36m-x86_64-linux-gnu.so
-rwxr-xr-x   1 root root 25676848 Dec  6 15:15 cv2.cpython-36m-x86_64-linux-gnu.so_backup_dec_06
-rw-r--r--   1 root root      411 Dec  6 12:21 __init__.py
drwxr-xr-x   2 root root      140 Dec  6 12:21 .libs
-rw-r--r--   1 root root    29170 Dec  6 12:21 LICENSE-3RD-PARTY.txt
-rw-r--r--   1 root root     1097 Dec  6 12:21 LICENSE.txt
drwxr-xr-x   2 root root       37 Dec  6 12:21 __pycache__

and

ls /usr/lib/python
python2.7/ python3.4/ 

So should I create manually /usr/lib/python3.6 and copy *.so file to /usr/lib/python3.6/cv2.cpython-36m-x86_64-linux-gnu.so or /usr/lib/python3.6/cv2.so ?

or

sudo cp /usr/local/lib/python3.6/site-packages/cv2/cv2.cpython-36m-x86_64-linux-gnu.so /usr/lib/python3.6/site-packages/cv2/cv2.cpython-36m-x86_64-linux-gnu.so

?

[alundiak]

When I git checkout 3.3.1 && rm -rf build && mkdir build && cd build
Odd. Or I just don't know proper syntax of external params:

cmake -D CMAKE_BUILD_TYPE=RELEASE -D CMAKE_INSTALL_PREFIX=/usr/local -DPYTHON_EXECUTABLE:FILEPATH=/usr/local/bin/python3 -D PYTHON_EXECUTABLE:FILEPATH=/usr/local/bin/python3  PYTHON_EXECUTABLE:FILEPATH=/usr/local/bin/python3 ..

In output I see trace of some version logic and error:

-- Found PythonInterp: /usr/local/bin/python3 (found suitable version "3.6.3", minimum required is "2.7") 
-- Found PythonInterp: /usr/bin/python3.4 (found suitable version "3.4.5", minimum required is "3.4") 
-- Found PythonLibs: /lib64/libpython3.4m.so (found suitable exact version "3.4.5") 
Traceback (most recent call last):
  File "/usr/local/lib/python3.6/site-packages/numpy/core/__init__.py", line 16, in <module>
    from . import multiarray
ImportError: cannot import name 'multiarray'

[alalek]

ImportError: cannot import name 'multiarray'

There is build/installation issue of numpy package on your machine.
Check issues about this message - for example, numpy/numpy#9047