Can't access webcam OpenCV 3.3 Python 3

Question

OpenCV on Python 2.7 has no problem accessing my webcam. For some reason it can't in Python 3. VideoCapture::read always returns none...

I even tried compiling from source.

This is my cmake (I don't think I did anything wrong):

cmake -D CMAKE_BUILD_TYPE=RELEASE \
    -D CMAKE_INSTALL_PREFIX=/usr/local \
    -D INSTALL_PYTHON_EXAMPLES=ON \
    -D INSTALL_C_EXAMPLES=OFF \
    -D WITH_V4L=ON \
    -D WITH_QT=ON \
    -D WITH_OPENGL=ON \
    -D OPENCV_EXTRA_MODULES_PATH=../../opencv_contrib/modules \
    -D PYTHON_EXECUTABLE3=/usr/bin/python3.6
    -D PYTHON_EXECUTABLE2=/usr/bin/python2.7
    -D PYTHON3_NUMPY_INCLUDE_DIRS=/usr/lib64/python3.6/site-packages/numpy/core/include \
    -D BUILD_EXAMPLES=ON ..

Has anything changed betwen implementations that could indicate whats causing my problems?

Edit:

As requested:

% python3.6 -c 'import cv2; print(cv2.getBuildInformation())'

General configuration for OpenCV 3.3.0 

=====================================
  Version control:               3.3.0

  Extra modules:
    Location (extra):            /io/opencv_contrib/modules
    Version control (extra):     3.3.0

  Platform:
    Timestamp:                   2017-09-03T11:00:53Z
    Host:                        Linux 4.4.0-83-generic x86_64
    CMake:                       3.9.0
    CMake generator:             Unix Makefiles
    CMake build tool:            /usr/bin/gmake
    Configuration:               Release

  CPU/HW features:
    Baseline:                    SSE SSE2 SSE3
      requested:                 SSE3
    Dispatched code generation:  SSE4_1 SSE4_2 FP16 AVX AVX2
      requested:                 SSE4_1 SSE4_2 AVX FP16 AVX2
      SSE4_1 (2 files):          + SSSE3 SSE4_1
      SSE4_2 (1 files):          + SSSE3 SSE4_1 POPCNT SSE4_2
      FP16 (1 files):            + SSSE3 SSE4_1 POPCNT SSE4_2 FP16 AVX
      AVX (5 files):             + SSSE3 SSE4_1 POPCNT SSE4_2 AVX
      AVX2 (7 files):            + SSSE3 SSE4_1 POPCNT SSE4_2 FP16 FMA3 AVX AVX2

  C/C++:
    Built as dynamic libs?:      NO
    C++ Compiler:                /opt/rh/devtoolset-2/root/usr/bin/c++  (ver 4.8.2)
    C++ flags (Release):         -fPIC   -fsigned-char -W -Wall -Werror=return-type -Werror=non-virtual-dtor -Werror=address -Werror=sequence-point -Wformat -Werror=format-security -Wmissing-declarations -Wundef -Winit-self -Wpointer-arith -Wshadow -Wsign-promo -Wuninitialized -Winit-self -Wno-narrowing -Wno-delete-non-virtual-dtor -Wno-comment -fdiagnostics-show-option -Wno-long-long -pthread -fomit-frame-pointer -ffunction-sections  -msse -msse2 -msse3 -fvisibility=hidden -fvisibility-inlines-hidden -O3 -DNDEBUG  -DNDEBUG
    C++ flags (Debug):           -fPIC   -fsigned-char -W -Wall -Werror=return-type -Werror=non-virtual-dtor -Werror=address -Werror=sequence-point -Wformat -Werror=format-security -Wmissing-declarations -Wundef -Winit-self -Wpointer-arith -Wshadow -Wsign-promo -Wuninitialized -Winit-self -Wno-narrowing -Wno-delete-non-virtual-dtor -Wno-comment -fdiagnostics-show-option -Wno-long-long -pthread -fomit-frame-pointer -ffunction-sections  -msse -msse2 -msse3 -fvisibility=hidden -fvisibility-inlines-hidden -g  -O0 -DDEBUG -D_DEBUG
    C Compiler:                  /opt/rh/devtoolset-2/root/usr/bin/cc
    C flags (Release):           -fPIC   -fsigned-char -W -Wall -Werror=return-type -Werror=non-virtual-dtor -Werror=address -Werror=sequence-point -Wformat -Werror=format-security -Wmissing-declarations -Wmissing-prototypes -Wstrict-prototypes -Wundef -Winit-self -Wpointer-arith -Wshadow -Wuninitialized -Winit-self -Wno-narrowing -Wno-comment -fdiagnostics-show-option -Wno-long-long -pthread -fomit-frame-pointer -ffunction-sections  -msse -msse2 -msse3 -fvisibility=hidden -O3 -DNDEBUG  -DNDEBUG
    C flags (Debug):             -fPIC   -fsigned-char -W -Wall -Werror=return-type -Werror=non-virtual-dtor -Werror=address -Werror=sequence-point -Wformat -Werror=format-security -Wmissing-declarations -Wmissing-prototypes -Wstrict-prototypes -Wundef -Winit-self -Wpointer-arith -Wshadow -Wuninitialized -Winit-self -Wno-narrowing -Wno-comment -fdiagnostics-show-option -Wno-long-long -pthread -fomit-frame-pointer -ffunction-sections  -msse -msse2 -msse3 -fvisibility=hidden -g  -O0 -DDEBUG -D_DEBUG
    Linker flags (Release):
    Linker flags (Debug):
    ccache:                      NO
    Precompiled headers:         YES
    Extra dependencies:          /opt/Qt4.8.7/lib/libQtGui.so /opt/Qt4.8.7/lib/libQtTest.so /opt/Qt4.8.7/lib/libQtCore.so /lib64/libz.so stdc++ dl m pthread rt
    3rdparty dependencies:       ittnotify libprotobuf libjpeg libwebp libpng libtiff libjasper IlmImf

  OpenCV modules:
    To be built:                 core flann imgproc ml objdetect phase_unwrapping photo plot reg surface_matching video xphoto bgsegm dnn face fuzzy img_hash imgcodecs shape videoio xobjdetect highgui superres bioinspired dpm features2d line_descriptor saliency text calib3d ccalib datasets rgbd stereo structured_light tracking videostab xfeatures2d ximgproc aruco optflow stitching python3
    Disabled:                    world contrib_world
    Disabled by dependency:      -
    Unavailable:                 cudaarithm cudabgsegm cudacodec cudafeatures2d cudafilters cudaimgproc cudalegacy cudaobjdetect cudaoptflow cudastereo cudawarping cudev java python2 ts viz cnn_3dobj cvv dnn_modern freetype hdf matlab sfm

  GUI: 
    QT 4.x:                      YES (ver 4.8.7 EDITION = OpenSource)
    QT OpenGL support:           NO
    OpenGL support:              NO
    VTK support:                 NO

  Media I/O: 
    ZLib:                        /lib64/libz.so (ver 1.2.3)
    JPEG:                        libjpeg (ver 90)
    WEBP:                        build (ver encoder: 0x020e)
    PNG:                         build (ver 1.6.24)
    TIFF:                        build (ver 42 - 4.0.2)
    JPEG 2000:                   build (ver 1.900.1)
    OpenEXR:                     build (ver 1.7.1)
    GDAL:                        NO
    GDCM:                        NO

  Video I/O:
    DC1394 1.x:                  NO
    DC1394 2.x:                  NO
    FFMPEG:                      NO
      avcodec:                   NO
      avformat:                  NO
      avutil:                    NO
      swscale:                   NO
      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/NO
    XIMEA:                       NO
    Xine:                        NO
    Intel Media SDK:             NO
    gPhoto2:                     NO

  Parallel framework:            pthreads

  Trace:                         YES (with Intel ITT)

  Other third-party libraries:
    Use Intel IPP:               NO
    Use Intel IPP IW:            NO
    Use Intel IPP Async:         NO
    Use VA:                      NO
    Use Intel VA-API/OpenCL:     NO
    Use Lapack:                  NO
    Use Eigen:                   NO
    Use Cuda:                    NO
    Use OpenCL:                  YES
    Use OpenVX:                  NO
    Use custom HAL:              NO

  OpenCL:                        <Dynamic loading of OpenCL library>
    Include path:                /io/opencv/3rdparty/include/opencl/1.2
    Use AMDFFT:                  NO
    Use AMDBLAS:                 NO

  Python 2:
    Interpreter:                 NO

  Python 3:
    Interpreter:                 python (ver 3.6.0)
    Libraries:                   
    numpy:                       /opt/python/cp36-cp36m/lib/python3.6/site-packages/numpy/core/include (ver 1.11.3)
    packages path:               /opt/python/cp36-cp36m/lib/python3.6/site-packages

  Python (for build):            python

  Java:
    ant:                         NO
    JNI:                         NO
    Java wrappers:               NO
    Java tests:                  NO

  Matlab:                        Matlab not found or implicitly disabled

  Tests and samples:
    Tests:                       NO
    Performance tests:           NO
    C/C++ Examples:              NO

  Install path:                  /usr/local

  cvconfig.h is in:              /io/opencv/build
-----------------------------------------------------------------

Updated the question (I was wondering how to retrieve that info) — ovg, Nov 01 '17 at 20:09
How odd, I'm pretty sure V4L2 and GStreamer were yes when I compiled... — ovg, Nov 01 '17 at 20:12
should I have to do anything more than just `make install` once I have it compiled? (for python). I'll try comipiling the whole thing from scratch again ;) — ovg, Nov 01 '17 at 20:23
Just take a look at `cmake`'s output, if you want to change anything use `ccmake`; otherwise `sudo make install` will do. — zindarod, Nov 01 '17 at 20:34
Yay it works, not sure exactly what the issue was but I tried the newer version of opencv 3.3.1 and followed https://www.scivision.co/anaconda-python-opencv3/ . I think there was a combination of things gone wrong such as python defaulting to 2.7 and having a version of opencv installed through pip (I thought that's where it was installing to!) — ovg, Nov 01 '17 at 21:08

score 0 · Accepted Answer · answered Nov 01 '17 at 21:11

Followed this guide: https://www.scivision.co/anaconda-python-opencv3/

Removed all failed attempts with pip before installing.

Make sure that the commands you execute in the guide above use the version of python you are targeting. Either alias python to python36 or replace it in the commands.

Can't access webcam OpenCV 3.3 Python 3

1 Answers1