Problems with VideoCapture on Windows

Python
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1

Hello,

I’m trying to port on Windows an application I wrote on Linux using Python(Qt5 and OpenCV).

The problem is that, when I try to open a webcam using cv2.VideoCapture(camera) where camera is obtained as follows:

self.available_cameras = QCameraInfo.availableCameras()
#The user select the camera from a QComboBox(shouldn't be important)
camera = self.available_cameras[combo_box_index].deviceName()

I get the following error:

[ERROR:0@14.725] global D:\a\opencv-python\opencv-python\opencv\modules\videoio\src\cap.cpp (166) cv::VideoCapture::open VIDEOIO(CV_IMAGES): raised OpenCV exception:

OpenCV(4.6.0) D:\a\opencv-python\opencv-python\opencv\modules\videoio\src\cap_images.cpp:253: error: (-5:Bad argument) CAP_IMAGES: can't find starting number (in the name of file): @device:pnp:\\?\usb#vid_045e&pid_076d&mi_00#7&315bdc05&0&0000#{65e8773d-8f56-11d0-a3b9-00a0c9223196}\global in function 'cv::icvExtractPattern'

Anybody knows why?

2

cv::VideoCapture, for video devices, is supposed to get an index, not a “name”. if a “name” ever worked, some system-specific backend felt responsible and accepted that.

You can use Qt to read from your camera. you should. and if you need some sensible way to choose a camera, you must.

3

Yes, actually, on Linux, it worked. I was not aware of this issue. I’ll try to do as you suggested. Thanks for your help

4

CameraInfo::availableCameras() returns a list of available cameras and that list obviously has indices starting from zero, so a name is not a necessary representation. Of course that doesn’t mean those ids could be passed to VideoCapture…

5

or that OpenCV would assign indices the same way Qt did, or that OpenCV would see the same cameras that Qt does (various APIs)

6

Hello, I’m still struggling a bit with this problem(I have a related thread on the Qt forum too). The problem is that, it looks like that video capture with Qt is not supported on Windows. The question then is: is there any way to obtain a complete list of available devices(the device’s numbers, in particular) using OpenCV?

7

Qt may lack MSMF support but it can do directshow.

“viewfinder” is what you want.

https://doc.qt.io/qt-5/qtmultimedia-windows.html

if that really doesn’t suit you, I’m sure there are recipes for accessing MSMF/DShow directly.

you can’t use OpenCV for the level of flexibility that you require. OpenCV does not have a list of video devices.