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Using OpenCV for python to capture webcam get white screen grab

I'm using python 2.7 on DragonBoard 410c with Debian OS to capture webcam Logitech C920 by opencv but get a white sceen result.
Here is my whole code.
import cv2
cap = cv2.VideoCapture(0)
cap.set(3,1280)
cap.set(4,720)
while(cap.isOpened()):
ret, frame = cap.read()
gray = cv2.cvtColor(frame, cv2.COLOR_BGR2BGRA)
cv2.imshow('frame', gray)
if cv2.waitKey(1) & 0xFF == ord('q'):
break
cap.release()
cv2.destroyAllWindows()
It worked well without the code "cap.set(3,1280)" and "cap.set(4,720)".
I can get the frame with 640x480.
but if I added the code to change the resolution, the window would change but the frame is white.
https://imgur.com/flsKfTp
By the way, if I set the resolution to 800x600, the frame will flash between white sceen and camera view.
How can I solve this problem?

Trying to read an online video using OpenCV in python, Not seeing the output but the code is not throwing any error

Not sure what is the problem. Code is running fine and not showing any error. But Not seeing any output.
import numpy as np
import cv2
cap = cv2.VideoCapture("https://youtu.be/_3elg-_1m_c")
while(cap.isOpened()):
ret, frame = cap.read()
gray = cv2.cvtColor(frame, cv2.COLOR_BGR2GRAY)
cv2.imshow('frame',gray)
if cv2.waitKey(1) & 0xFF == ord('q'):
break
cap.release()
cv2.destroyAllWindows()
If, I am using video saved in my laptop, I can see the output, but for online vidoes, I am not.

playing video in Centos using opencv python

I am following this tutorial from openCV to play back a video on centos .
openCV tutorial
see the code below , it doesn't throw any errors but does not show any videos being played. If I use the image display program it works fine which tells me the X11 forwarding part is working fine .
[root#hadoop1 basic-motion-detection]# more demo1.py
import numpy as np
import cv2
cap = cv2.VideoCapture('/home/admin/example_02.mp4')
while(cap.isOpened()):
ret, frame = cap.read()
gray = cv2.cvtColor(frame, cv2.COLOR_BGR2GRAY)
cv2.startWindowThread()
cv2.namedWindow("preview")
cv2.imshow('preview',gray)
if cv2.waitKey(1) & 0xFF == ord('q'):
break
cap.release()
cv2.destroyAllWindows()

OpenCV: Face detection taking advantage of a command line

I run this (first one) example that launches the webcam of my latop so that I can see myself on the screen.
import numpy as np
import cv2
cap = cv2.VideoCapture(0)
while(True):
# Capture frame-by-frame
ret, frame = cap.read()
# Our operations on the frame come here
gray = cv2.cvtColor(frame, cv2.COLOR_BGR2GRAY)
# Display the resulting frame
cv2.imshow('frame',gray)
if cv2.waitKey(1) & 0xFF == ord('q'):
break
# When everything done, release the capture
cap.release()
cv2.destroyAllWindows()
I installed OpenBr on Ubuntu 14.04 LTS and I run successfully this command on a picture of myself:
br - gui -algorithm ShowFaceDetection -enrollAll -enroll /home/nakkini/Desktop/myself.png
The above command I run on the Terminal displays my picture and draws a square around my face (face detection), it also highlights my eyes in green.
My Dream:
I wonder if there is a way to combine this command with the short program above so that when the webcam is launched I can see my face surrounded by the green rectangle ?
Why do I need this ?
I found similar programs in pure OpenCV/Python for this purpos. However, for later needs, I need more things than the simple face detection and I judge by myself that OpenBR will save me lot of headache. That is why I am looking for a way to run the command line somewhere inside the code above as a first but big step.
Hints:
The frame in the code corresponds to myself.png of the command line. The solution to be found will try to pass frame in the place of myself.png to the command line within the program itself.
Thank you very much in advance.
EDIT:
After correcting the typos of #Xavier's solution I have no errors. However the program does not run as I want it:
First, the camer is launched and I see myself but my face is not detected with a green rectangle. Secondly, I press any key to exit but the program does not exit: it shows me a picture of myself with my face detected. A last key press exists the program. My goal is to see my face detected during the camera functionment.

you do not need openbr for this at all.
just see opencv's python face-detect tutorial

something like this should work
import numpy as np
import cv2
import os
cap = cv2.VideoCapture(0)
while(True):
# Capture frame-by-frame
ret, frame = cap.read()
# Our operations on the frame come here
gray = cv2.cvtColor(frame, cv2.COLOR_BGR2GRAY)
# Display the resulting frame
cv2.imshow('frame',gray)
if cv2.waitKey(1) & 0xFF == ord('q'):
cv2.imwrite( "/home/nakkini/Desktop/myself.png", gray );
os.system('br - gui -algorithm -ShowFaceDetection -enrollAll -enroll /home/nakkini/Desktop/myself.png')
break
# When everything done, release the capture
cap.release()
cv2.destroyAllWindows()

Webcam + Open CV Python | Black screen

I am using the code below, but I get a black image. Could you please help me rectify the error?
import cv2
import numpy as np
c = cv2.VideoCapture(0)
while(1):
_,f = c.read()
cv2.imshow('e2',f)
if cv2.waitKey(5)==27:
break
cv2.destroyAllWindows()

Update: See github.com/opencv/opencv/pull/11880 and linked conversations, only few backends support -1 as index.
Although this is an old post, this answer can help people who are still facing the same problem. If you have a single webcam but it renders all black, use cv2.VideoCapture(-1). This will get you the working camera.

Just change cv2.waitKey(0) to cv2.waitKey(30) and this issue will be resolved.

I've faced with same problem. Updating neither opencv nor webcam driver works. I am using kaspersky as antivirus. When I disable the kaspersky, then black output problem solved.
BTW, I can see the running .py file in kaspersky console > reports > host intrusion prevention. It reports application privilege control rule triggered - application: myfile.py, result: blocked: access to video capturing devices

Try this:
import cv2
import numpy as np
cap = cv2.VideoCapture(0)
while(True):
ret, frame = cap.read()
cv2.imshow('frame',frame)
if cv2.waitKey(1) & 0xFF == ord('q'):
break
cap.release()
cv2.destroyAllWindows()

This worked for me:
I did a pip install imutils. Imutils is a library with series of convenience functions to make basic image processing functions such as translation, rotation, resizing, skeletonization, displaying Matplotlib images, sorting contours, detecting edges, and much more easier with OpenCV and both Python 2.7 and Python 3.
import cv2
import imutils
cap = cv2.VideoCapture(0) # video capture source camera (Here webcam of laptop)
ret, frame = cap.read() # return a single frame in variable `frame`
while (True):
# gray = cv2.cvtColor(frame, cv2.COLOR_BGR2GRAY)
(grabbed, frame) = cap.read()
frame = imutils.resize(frame, width=400)
cv2.imshow('img1', frame) # display the captured image
if cv2.waitKey(1) & 0xFF == ord('q'): # save on pressing 'y'
cv2.imwrite('capture.png', frame)
cv2.destroyAllWindows()
break
cap.release()

Try put -0 on the index and pause any antivirus running
import cv2
import numpy as np
cap = cv2.VideoCapture(-0)
cap.set(3,640)
cap.set(3,480)
while(True):
success, img = cap.read()
cv2.imshow('frame',img)
if cv2.waitKey(1) & 0xFF == ord('q'):
break
cap.release()
cv2.destroyAllWindows()

I faced the same issue after many calls with:
cap = cv2.VideoCapture(0)
and it solved when I changed the index to 1 :
cap = cv2.VideoCapture(1)

In my case just disabling Kaspersy has solved the problem.