python - cv2.videocapture.read() does not return a numpy array

Question

I have this code trying to capture a frame from my webcam on raspberry pi, and saving it as an image. I use opencv 2, but I get strange errors when I run the code..

import time
import sys
from subprocess import call
import cv2

cam = cv2.VideoCapture()

while True:
        cam.open(-1)
        image = cam.read()
        cv2.imwrite("current.jpeg",image)
        time.sleep(10);

This is what the program returns:

VIDIOC_QUERYMENU: Invalid argument
VIDIOC_QUERYMENU: Invalid argument
VIDIOC_QUERYMENU: Invalid argument
VIDIOC_QUERYMENU: Invalid argument
VIDIOC_QUERYMENU: Invalid argument
VIDIOC_QUERYMENU: Invalid argument
VIDIOC_QUERYMENU: Invalid argument

Traceback (most recent call last):
  File "kvamskogen.py", line 18, in <module>
    cv2.imwrite("current.jpeg",image)
TypeError: <unknown> is not a numpy array

What is wrong here?

Answer 1

Reading (cam.read()) from a VideoCapture returns a tuple (return value, image). With the first item you check wether the reading was successful, and if it was then you proceed to use the returned image.

This is documented at https://opencv-tutorial.readthedocs.io/en/latest/intro/intro.html#capture-live-video