OpenCV videocapture read camera rtsp error

Question

I’m using opencv-python (version 4.4.0.46) python version 3.8.5 to capture my IP Camera stream, however, I cannot read the video frame. Here is my code:

import cv2
# cap = cv2.VideoCapture("rtsp://wowzaec2demo.streamlock.net/vod/mp4:BigBuckBunny_115k.mov")
cap = cv2.VideoCapture("rtsp://admin:xxx@xxx/Streaming/Channels/1")
print(cap.isOpened())

ret,frame = cap.read()
print(ret)

while ret:
    ret,frame = cap.read()
    print(ret)
    if cv2.waitKey(1) & 0xFF == ord('q'):
        break
cap.release()

In my code, when I use my IPC stream, I got "print(cap.isOpened())" as True, while "cap.read()" returns False. However, when it changes to "BigBuckBunny_115k" stream, everything is OK, and I can get the frame picture.

Somebody might say that your IPC should be damaged. But I've tried using ffmpeg to capture video, which can easily get the frame picture. My code:

ffmpeg -i "rtsp://admin:xxx@xxx/Streaming/Channels/1" -y -f image2 -r 1/1 img%03d.jpg

Using python to capture video, I got "False" like above, but there is not ant error log printed, making me difficult to solve the problem. So I changed to use java, and using opencv-4.4.0.so as the library, my code:

VideoCapture videoCapture = new VideoCapture();
// set the exception mode true so that it can print stack trace when error
videoCapture.setExceptionMode(true);
boolean openStatus = videoCapture.open("rtsp://admin:xxx@xxx/Streaming/Channels/1");
log.info("open rtsp status: {}", openStatus);
int i = 0;
status = true;
while (openStatus && status) {
    Mat mat = new Mat();
    videoCapture.read(mat);

    Imgcodecs.imwrite("../" + i + ".png", mat);
    i++;
}

by running this code, I got exception: org.opencv.core.CvException: cv::Exception: OpenCV(4.4.0) /xxx/opencv-4.4.0/modules/videoio/src/cap.cpp:177: error: (-2:Unspecified error) could not open 'rtsp://admin:xxx@xxx/Streaming/Channels/1' in function 'open'. The exception log above does not help to solve the problem, what does "-2:Unspecified error" means?

Forgot to mention that my runtime is CentOS 7.5 (kernel version 3.10.0-862.el7.x86_64) . Then I changed to another server with the same opencv-python version and CentOS version. It is strange that on this new server, using the same opencv code, I can get the video frame now.

The first question is, is there any relevant information that I can refer to? The second question is, how can I troubleshoot this problem. The third question is, why does it behave differently on different server, is there any system configuration on my first server error, like network configuration?

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Update on 2021.01.07

Thanks antoine for giving me information about OpenCV backend. I've tried to print the python-opencv build information using the code below.

import cv2
print(cv2.getBuildInformation())

And on the 2 servers I have (one of them can read the video stream and one of them not), I got console print totally the same. Here is the print:

General configuration for OpenCV 4.4.0 =====================================
  Version control:               4.4.0-dirty

  Platform:
    Timestamp:                   2020-11-03T00:52:03Z
    Host:                        Linux 4.15.0-1077-gcp x86_64
    CMake:                       3.18.2
    CMake generator:             Unix Makefiles
    CMake build tool:            /bin/gmake
    Configuration:               Release

  CPU/HW features:
    Baseline:                    SSE SSE2 SSE3
      requested:                 SSE3
    Dispatched code generation:  SSE4_1 SSE4_2 FP16 AVX AVX2 AVX512_SKX
      requested:                 SSE4_1 SSE4_2 AVX FP16 AVX2 AVX512_SKX
      SSE4_1 (15 files):         + SSSE3 SSE4_1
      SSE4_2 (1 files):          + SSSE3 SSE4_1 POPCNT SSE4_2
      FP16 (0 files):            + SSSE3 SSE4_1 POPCNT SSE4_2 FP16 AVX
      AVX (4 files):             + SSSE3 SSE4_1 POPCNT SSE4_2 AVX
      AVX2 (29 files):           + SSSE3 SSE4_1 POPCNT SSE4_2 FP16 FMA3 AVX AVX2
      AVX512_SKX (4 files):      + SSSE3 SSE4_1 POPCNT SSE4_2 FP16 FMA3 AVX AVX2 AVX_512F AVX512_COMMON AVX512_SKX

  C/C++:
    Built as dynamic libs?:      NO
    C++ standard:                11
    C++ Compiler:                /usr/lib/ccache/compilers/c++  (ver 9.3.1)
    C++ flags (Release):         -Wl,-strip-all   -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 -Wsuggest-override -Wno-delete-non-virtual-dtor -Wno-comment -Wimplicit-fallthrough=3 -Wno-strict-overflow -fdiagnostics-show-option -Wno-long-long -pthread -fomit-frame-pointer -ffunction-sections -fdata-sections  -msse -msse2 -msse3 -fvisibility=hidden -fvisibility-inlines-hidden -O3 -DNDEBUG  -DNDEBUG
    C++ flags (Debug):           -Wl,-strip-all   -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 -Wsuggest-override -Wno-delete-non-virtual-dtor -Wno-comment -Wimplicit-fallthrough=3 -Wno-strict-overflow -fdiagnostics-show-option -Wno-long-long -pthread -fomit-frame-pointer -ffunction-sections -fdata-sections  -msse -msse2 -msse3 -fvisibility=hidden -fvisibility-inlines-hidden -g  -O0 -DDEBUG -D_DEBUG
    C Compiler:                  /usr/lib/ccache/compilers/cc
    C flags (Release):           -Wl,-strip-all   -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-comment -Wno-strict-overflow -fdiagnostics-show-option -Wno-long-long -pthread -fomit-frame-pointer -ffunction-sections -fdata-sections  -msse -msse2 -msse3 -fvisibility=hidden -O3 -DNDEBUG  -DNDEBUG
    C flags (Debug):             -Wl,-strip-all   -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-comment -Wno-strict-overflow -fdiagnostics-show-option -Wno-long-long -pthread -fomit-frame-pointer -ffunction-sections -fdata-sections  -msse -msse2 -msse3 -fvisibility=hidden -g  -O0 -DDEBUG -D_DEBUG
    Linker flags (Release):      -Wl,--exclude-libs,libippicv.a -Wl,--exclude-libs,libippiw.a -L/root/ffmpeg_build/lib  -Wl,--gc-sections -Wl,--as-needed  
    Linker flags (Debug):        -Wl,--exclude-libs,libippicv.a -Wl,--exclude-libs,libippiw.a -L/root/ffmpeg_build/lib  -Wl,--gc-sections -Wl,--as-needed  
    ccache:                      YES
    Precompiled headers:         NO
    Extra dependencies:          ade Qt5::Core Qt5::Gui Qt5::Widgets Qt5::Test Qt5::Concurrent /lib64/libpng.so /lib64/libz.so dl m pthread rt
    3rdparty dependencies:       ittnotify libprotobuf libjpeg-turbo libwebp libtiff libjasper IlmImf quirc ippiw ippicv

  OpenCV modules:
    To be built:                 calib3d core dnn features2d flann gapi highgui imgcodecs imgproc ml objdetect photo python3 stitching video videoio
    Disabled:                    world
    Disabled by dependency:      -
    Unavailable:                 java js python2 ts
    Applications:                -
    Documentation:               NO
    Non-free algorithms:         NO

  GUI: 
    QT:                          YES (ver 5.15.0)
      QT OpenGL support:         NO
    GTK+:                        NO
    VTK support:                 NO

  Media I/O: 
    ZLib:                        /lib64/libz.so (ver 1.2.7)
    JPEG:                        libjpeg-turbo (ver 2.0.5-62)
    WEBP:                        build (ver encoder: 0x020f)
    PNG:                         /lib64/libpng.so (ver 1.5.13)
    TIFF:                        build (ver 42 - 4.0.10)
    JPEG 2000:                   build Jasper (ver 1.900.1)
    OpenEXR:                     build (ver 2.3.0)
    HDR:                         YES
    SUNRASTER:                   YES
    PXM:                         YES
    PFM:                         YES

  Video I/O:
    DC1394:                      NO
    FFMPEG:                      YES
      avcodec:                   YES (58.109.100)
      avformat:                  YES (58.61.100)
      avutil:                    YES (56.60.100)
      swscale:                   YES (5.8.100)
      avresample:                NO
    GStreamer:                   NO
    v4l/v4l2:                    YES (linux/videodev2.h)

  Parallel framework:            pthreads

  Trace:                         YES (with Intel ITT)

  Other third-party libraries:
    Intel IPP:                   2020.0.0 Gold [2020.0.0]
           at:                   /tmp/pip-req-build-99ib2vsi/_skbuild/linux-x86_64-3.8/cmake-build/3rdparty/ippicv/ippicv_lnx/icv
    Intel IPP IW:                sources (2020.0.0)
              at:                /tmp/pip-req-build-99ib2vsi/_skbuild/linux-x86_64-3.8/cmake-build/3rdparty/ippicv/ippicv_lnx/iw
    Lapack:                      NO
    Eigen:                       NO
    Custom HAL:                  NO
    Protobuf:                    build (3.5.1)

  OpenCL:                        YES (no extra features)
    Include path:                /tmp/pip-req-build-99ib2vsi/opencv/3rdparty/include/opencl/1.2
    Link libraries:              Dynamic load

  Python 3:
    Interpreter:                 /opt/python/cp38-cp38/bin/python (ver 3.8.6)
    Libraries:                   libpython3.8.a (ver 3.8.6)
    numpy:                       /tmp/pip-build-env-qru8fff1/overlay/lib/python3.8/site-packages/numpy/core/include (ver 1.17.3)
    install path:                python

  Python (for build):            /bin/python2.7

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

  Install to:                    /tmp/pip-req-build-99ib2vsi/_skbuild/linux-x86_64-3.8/cmake-install
-----------------------------------------------------------------

In the print, I found the information:

  Video I/O:
    DC1394:                      NO
    FFMPEG:                      YES
      avcodec:                   YES (58.109.100)
      avformat:                  YES (58.61.100)
      avutil:                    YES (56.60.100)
      swscale:                   YES (5.8.100)
      avresample:                NO
    GStreamer:                   NO
    v4l/v4l2:                    YES (linux/videodev2.h)

Seems my two servers both have FFMPEG and don't have GStreamer. So, is there any other reason may cause the difference?

By the way, I've tried passing API preference to the constructor like below:

cap = cv2.VideoCapture("rtsp://admin:xxx@xxx/Streaming/Channels/1", cv2.CAP_FFMPEG)

However, passing the parameter didn't solve the problem, I cannot read video frame either.

Answer 1

OpenCV uses different backend to read/write/encode/decode video stream depending on what is installed on your machine and which one you build with OpenCV. That could explain why you got different result on different server even with the same OS and OpenCV version: you could have GStreamer on one and not on the other.

You could find some information about those backends on the OpenCV Documentation.

IIRC on Linux, OpenCV uses GStreamer as the default backend and you tested your stream with FFMPEG. OpenCV could also use FFMPEG, try to pass cv.CAP_FFMPEG as your API preference to cv.VideoCapture constructor:

cap = cv2.VideoCapture("rtsp://admin:xxx@xxx/Streaming/Channels/1", cv.CAP_FFMPEG)

And concerning the verbosity, OpenCV videoio module is not that verbose. But some backend are, for example you could increase GStreamer verbosity with the GST_DEBUG environment variable, see GStreamer documentation. For FFMPEG you could use FFREPORT environment variable or some of the AV_LOG* ones, see FFMPEG documentation.