Python OpenCV gives black screen on webcam capture

Question

System params: Windows 8.1, Python 2.7.13, OpenCV 3.2.0.7

This code works fine. I tried .avi and .mp4 videos:

import numpy as np
import cv2
import time

cap = cv2.VideoCapture('tmp.avi')

while(True):
    ret, frame = cap.read()

    cv2.imshow('frame',frame)
    if cv2.waitKey(1) & 0xFF == 27:
         break

cap.release()
cv2.destroyAllWindows()

But when I try to get picture from web camera I always see black screen though ret is True:

import numpy as np
import cv2
import time

cap = cv2.VideoCapture(0)

while(True):
    ret, frame = cap.read()

    if not ret: continue

    cv2.imshow('frame',frame)
    if cv2.waitKey(1) & 0xFF == 27:
         break

cap.release()
cv2.destroyAllWindows()

I've tried to use grab and retrieve methods instead of read method, so grab returns True, but retrieve returns ret=False:

import numpy as np
import cv2
import time

cap = cv2.VideoCapture(0)

while(True):
    if not cap.grab(): break
    ret, frame = cap.retrieve()
    if not ret: continue

    cv2.imshow('frame',frame)
    if cv2.waitKey(1) & 0xFF == 27:
         break

cap.release()
cv2.destroyAllWindows()

Last code works fine for videos.

Windows Camera soft works fine, so webcam is OK.

I've tried to reinstall OpenCV, it didn't help.

What is the problem? Why retrieve method returns False though read method returns True?

Answer 1

I also had this problem today on windows using gocv.io library.

I removed and reinstalled the webcam driver and the problem solved.