log a debug message if a capture backend is generally available but isn't capabable of a capture mode#22700

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asmorkalov merged 1 commit intoopencv:4.xfrom

kallaballa:capture_backened_debug_msg

May 4, 2023

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kallaballa commented Oct 26, 2022

The pertaining issue: #22699

Pull Request Readiness Checklist

See details at https://github.com/opencv/opencv/wiki/How_to_contribute#making-a-good-pull-request

I agree to contribute to the project under Apache 2 License.
To the best of my knowledge, the proposed patch is not based on a code under GPL or another license that is incompatible with OpenCV
The PR is proposed to the proper branch
There is a reference to the original bug report and related work
There is accuracy test, performance test and test data in opencv_extra repository, if applicable
Patch to opencv_extra has the same branch name.
The feature is well documented and sample code can be built with the project CMake

asmorkalov requested a review from alalek

October 27, 2022 09:21

asmorkalov added the category: videoio label

alalek reviewed

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modules/videoio/src/cap.cpp Outdated

+                      if(!foundFilenameBackend && cv::videoio_registry::hasBackend(static_cast<VideoCaptureAPIs>(apiPreference)))
+                      {
+                          CV_CAPTURE_LOG_DEBUG(NULL,

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alalek Oct 28, 2022

I believe CV_LOG_WARNING could be used in this case directly (without preconditions).

Contributor Author

kallaballa Oct 28, 2022

Alright

modules/videoio/src/cap.cpp Outdated

Comment on lines +123 to +129

+                      //Check if the backend generally is available but can't be used to capture by filename
+                      bool foundFilenameBackend = false;
+                      for (size_t i = 0; i < backends.size(); i++)
+                      {
+                          if((foundFilenameBackend = (apiPreference == backends[i].id)))
+                              break;
+                      }

Member

alalek Oct 28, 2022

Instead of scanning in multiple places we could add internal call:
const VideoBackendInfo& cv::videoio_registry::getBackendInfo(VideoCaptureAPIs api)
(similar to cv::videoio_registry::getBackendName() but non-public).

And then use VideoBackendInfo::mode field directly.

Contributor Author

kallaballa Oct 28, 2022

Sounds like a plan

Contributor Author

kallaballa Oct 30, 2022

I am not sure what internal in that case means. cv::videoio_registry::internal?

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alalek Nov 1, 2022

"internal" means that it should not be a part of OpenCV public API.

declaration should go to src/videoio_registry.hpp
definition/implementation src/videoio_registry.cpp

You could add ::internal nested namespace as your wish.

Contributor Author

kallaballa Nov 2, 2022

thx!

Contributor Author

kallaballa Nov 5, 2022 •

edited

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alalek reviewed

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alalek left a comment

Thank you for update!

modules/videoio/src/videoio_registry.cpp Outdated

Comment on lines +336 to +338

+              namespace videoio_registry { namespace internal {
+                  const VideoBackendInfo& getBackendInfo(VideoCaptureAPIs api)
+                  {

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alalek Nov 5, 2022

Please avoid indentation in namespaces

Contributor Author

kallaballa Nov 5, 2022

alright!

modules/videoio/src/videoio_registry.cpp Outdated

Comment on lines +339 to +340

		if (api == CAP_ANY)
		CV_Error(Error::StsError, "Wrong backend ID: CAP_ANY");

Member

alalek Nov 5, 2022

CV_Assert(api != CAP_ANY); is preferable (this check is an internal assumption)

Contributor Author

kallaballa Nov 5, 2022

ic

modules/videoio/src/videoio_registry.cpp Outdated

+                      }
+                      CV_Error(Error::StsError, "Unknown backend ID");
+                      return builtin_backends[0]; // unreachable but necessary to silence warnings

Member

alalek Nov 5, 2022

silence warnings

Which build configuration provides warning?

builtin_backends[0]

This array may be empty. So even a zero/first element is invalid.

Contributor Author

kallaballa Nov 5, 2022 •

edited

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silence warnings

Which build configuration provides warning?

I can't tell you exactly because it was the CI that was failing with a warning because of returning a temporary. Changing the code to this suppressed the warning.

builtin_backends[0]

This array may be empty. So even a zero/first element is invalid.

Of course, but it's (supposed) to be unreachable. How would you like me to handle that case?

modules/videoio/src/videoio_registry.cpp Outdated

+                      if (api == CAP_ANY)
+                          CV_Error(Error::StsError, "Wrong backend ID: CAP_ANY");
+                      const int N = sizeof(builtin_backends)/sizeof(builtin_backends[0]);

Member

alalek Nov 5, 2022

builtin_backends

Not correct.
We should use VideoBackendRegistry::enabledBackends instead.
We need to add getter for this "protected" field:

const std::vector<VideoBackendInfo>& getEnabledBackends() const
{
    return enabledBackends;
}

Contributor Author

kallaballa Nov 5, 2022

alright

modules/videoio/src/videoio_registry.cpp Outdated

+                              return info;
+                      }
+                      CV_Error(Error::StsError, "Unknown backend ID");

Member

alalek Nov 5, 2022

This exception changes existed behavior of .open() method of VideoCapture for non-existed/disabled backends.

Perhaps we need to return pointer (const VideoBackendInfo*) and check for NULL in caller.

Contributor Author

kallaballa Nov 28, 2022

Should I implement it like that?

Contributor

opencv-alalek Apr 11, 2023

This is internal API, so it could be adopted later if you have better solution.

Anyway implementation should not raise exception through CV_Error() (as high-level API of VideoCapture doesn't do that in case of failure of opening process).
Changing that behavior would break a lot of existed user code. This is a compatibility question.

asmorkalov requested a review from opencv-alalek

April 6, 2023 16:18

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asmorkalov commented Apr 11, 2023

@opencv-alalek Could do you have updates?

Contributor

opencv-alalek commented Apr 11, 2023

@asmorkalov There is nothing to add from my side. No idea which updates are expected here.

Contributor Author

kallaballa commented Apr 11, 2023

@asmorkalov There is nothing to add from my side. No idea which updates are expected here.

This exception changes existed behavior of .open() method of VideoCapture for non-existed/disabled backends.

Perhaps we need to return pointer (const VideoBackendInfo*) and check for NULL in caller.

The open question is if it should be implemented like that.


          Log a debug message if a capture backend is generally available but i…

a2be9e9

…sn't capabable of a capture mode.

asmorkalov force-pushed the capture_backened_debug_msg branch from 8a71dda to a2be9e9 Compare

May 4, 2023 16:20

Contributor

asmorkalov commented May 4, 2023

@kallaballa I simplified your solution a bit and squashed commits. It works as expected now:

$ python3 
Python 3.6.9 (default, Mar 10 2023, 16:46:00) 
[GCC 8.4.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import cv2
>>> c=cv2.VideoCapture(0, cv2.CAP_FFMPEG)
[DEBUG:0@13.436] global videoio_registry.cpp:206 VideoBackendRegistry VIDEOIO: Builtin backends(9): FFMPEG(1000); GSTREAMER(990); INTEL_MFX(980); V4L2(970); CV_IMAGES(960); CV_MJPEG(950); FIREWIRE(940); UEYE(930); OBSENSOR(920)
[DEBUG:0@13.436] global videoio_registry.cpp:230 VideoBackendRegistry VIDEOIO: Available backends(9): FFMPEG(1000); GSTREAMER(990); INTEL_MFX(980); V4L2(970); CV_IMAGES(960); CV_MJPEG(950); FIREWIRE(940); UEYE(930); OBSENSOR(920)
[ INFO:0@13.436] global videoio_registry.cpp:232 VideoBackendRegistry VIDEOIO: Enabled backends(9, sorted by priority): FFMPEG(1000); GSTREAMER(990); INTEL_MFX(980); V4L2(970); CV_IMAGES(960); CV_MJPEG(950); FIREWIRE(940); UEYE(930); OBSENSOR(920)
[ WARN:0@13.436] global cap.cpp:331 open VIDEOIO(FFMPEG): backend is generally available but can't be used to capture by index
>>> quit()

asmorkalov requested review from asmorkalov and removed request for opencv-alalek

May 4, 2023 17:32

asmorkalov approved these changes

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asmorkalov added this to the 4.8.0 milestone

asmorkalov merged commit dabf960 into opencv:4.x

Contributor Author

kallaballa commented May 5, 2023

thx!

asmorkalov mentioned this pull request

(5.x) Merge 4.x #23718

Merged

james-ro-williams commented Jul 26, 2023

I'm receiving this warning when attempting to capture from a UDP stream using FFMPEG as a backend in a Docker container: [ WARN:0@30.144] global cap.cpp:204 open VIDEOIO(FFMPEG): backend is generally available but can't be used to capture by name.

I think this warning could do with a bit of an evaluation as it took an awful lot of digging to get to here to figure out what was going on, and I'm still not entirely sure why my VideoCapture is failing - as it works outside the container and fails inside.

This warning implies that FFMPEG is installed correctly and OpenCV is too, unless I'm completely mistaken.

Contributor Author

kallaballa commented Jul 27, 2023 •

edited

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I'm receiving this warning when attempting to capture from a UDP stream using FFMPEG as a backend in a Docker container: [ WARN:0@30.144] global cap.cpp:204 open VIDEOIO(FFMPEG): backend is generally available but can't be used to capture by name.

I think this warning could do with a bit of an evaluation as it took an awful lot of digging to get to here to figure out what was going on, and I'm still not entirely sure why my VideoCapture is failing - as it works outside the container and fails inside.

This warning implies that FFMPEG is installed correctly and OpenCV is too, unless I'm completely mistaken.

Hi!
Would you please post the result of opencv_version -v for both environments?
Also could you make a minimal example that fails?

rursprung mentioned this pull request

videoio: capture by index/name check is wrong on failure with API preference set #26519

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category: videoio