cv2.VideoCapture.open() always returns FALSE

问题:

I am trying to access a Logitech c310 webcam on my beaglebone. It always returns false for any device ID, I am not sure why.

I use the following code.

>>> import cv2, numpy as np
>>> cam = cv2.VideoCapture(0)
>>> cam.open(0)
False

The camera does show up as video0 in dev/ and also in root@arm:~#lsusb, like below,

root@arm:~# lsusb
Bus 001 Device 002: ID 046d:081b Logitech, Inc. Webcam C310
Bus 001 Device 001: ID 1d6b:0002 Linux Foundation 2.0 root hub
Bus 002 Device 001: ID 1d6b:0002 Linux Foundation 2.0 root hub

I could also access the camera with v4l2-ctl. Note that I am very new to OpenCV, so this may sound silly and I apologize for that in advance.

回答1:

video related functionality is not supported (not compiled with FFmpeg), if cv2 was installed from pypi wheel:

pip install opencv-python

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

回答2:

HIGHGUI ERROR: V4L: index 1 is not correct!
False
OpenCV Error: Bad flag (parameter or structure field) (Unrecognized or unsupported array  type) in cvGetMat, file /home/kaushik/Desktop/OpenCV-2.4.1/modules/core/src/array.cpp, line 2482
Traceback (most recent call last):
File "x2.py", line 8, in <module>
cv2.imshow('frame', frame)
cv2.error: /home/kaushik/Desktop/OpenCV-2.4.1/modules/core/src/array.cpp:2482: error: (-206) Unrecognized or unsupported array type in function cvGetMat

if you are getting this kind of an error then probably something is wrong with the indexing.

instead of cv2.VideoCapture(0) add:

cv2.VideoCapture(-1)

this will get you the first working camera. And if anything goes wrong, just post the stack trace here and i'll see if i can help you :)

回答3:

If you didn't solve it in many ways,you can try to
find and install "opencv3.2.0-dev",I use it to solve
the problem twice.
just pip install opencv-python is not enough.(hmm,
at least sometimes)

回答4:

It depends on the argument being passed to the cv2.VideoCapture().

Normally, it is 0 for making the primary webcam of your pc work. Similarly, if you want to get access to a second camera installed on you system pass the argument as 2.

But if you have only 1 camera and indexing '0' is not helping, then try passing the index as -1 instead.

回答5:

First If you are trying to capture video, then cam.open() is not the right way to do it. open() method initializes the camera object(in this case the "cam").

Secondly Here the cam.open() returns false because cv is not able to initialize the cam object. The VideoCapture() method already initializes your camera object. You can check this by adding

cam.isOpened()

right after

cam.VideoCapture(0)

that will return "True".

Thirdly if you want to vied camera feed, this is the conventional way to do it

import cv2

cam = cv2.VideoCapture(0)
print cam.isOpened()

while(True):
        ret, frame = cam.read()
        cv2.imshow('frame', frame)
        if cv2.waitKey(1) & 0xFF == ord('q'):
            break

cam.release()
cv2.destroyAllWindows()

回答6:

I found something in documentation which might just help.

cap.read() returns a bool (True/False). If frame is read correctly, it will be True. Sometimes, cap may not have initialized the capture. In that case, the code shows error. You can check whether it is initialized or not by the method cap.isOpened(). If it is True, OK. Otherwise open it using cap.open().

Source: http://opencv-python-tutroals.readthedocs.io/en/latest/py_tutorials/py_gui/py_video_display/py_video_display.html#display-video

回答7:

It is a matter of missing packages. I had installed a bunch of packages after installing OpenCV. I ran the cmake again and it worked.