Develop Reference

OpenCV works with threads but not with processes

Hi everyone and thanks for the help.
I've got this function to save a video from frames taken by my webcam.
import cv2
import multiprocessing
import threading
def rec():
# Define the codec and create VideoWriter object
fourcc = cv2.VideoWriter_fourcc(*'XVID')
out = cv2.VideoWriter('output.avi',fourcc, 20.0, (640,480))
while(cap.isOpened()):
ret, frame = cap.read()
if ret==True:
out.write(frame)
else:
break
# Release everything if job is finished
cap.release()
out.release()
cv2.destroyAllWindows()
If i use it with threads, and so with this code, it works well:
s1 = threading.Thread(target=rec)
s1.start()
But if i want to start another process, using the following code, when i open the video it contains only black frames with some noise.
s1 = multiprocessing.Process(target=rec)
s1.start()
I searched all around but couldn't find any solution.
Also, i'm using Python 3.6

where is cap defined ? Try defining that in the function that you give to multiprocessing. If it is defined in the parent and is passed from the parent to the child, it is being pickled and that probably makes it unusable.

I solved the problem.
I was calling cap = cv2.VideoCapture(0) in my main and also in one of my imported modules, and that conflicted. I solved by calling it once.

Corrupt JPEG Data: 1273 extraneous bytes before marker. Opencv 3/python2.7

This is the code when i execute it:
You can see the frame opens but doesnt show anything
I want to use a usb camera with a raspberry pi 3 model b v1.2 using opencv 3.3 and python 2.7.
I work with opencv in an virtual enviroment.
import numpy as np
import cv2
cap = cv2.VideoCapture(0)
while(True):
ret, frame = cap.read() #Capture frame-by-frame
#Our operations on the frame come here
#gray = cv2.cvtColor(frame, cv2.COLOR_BGR2GRAY)
#Display resulting frame
cv2.imshow('frame',frame)
cv2.waitKey(10)
#if cv2.waitKey(1) & 0xFF == ord('q'):
# break
#when everything done, release the capture
cap.release()
cv2.destroyAllWindows()
I just have no idea how to get around this error. I already searched the error and i am getting helpless, anyone having an idea?
EDIT: i am currently playing around with the code and i can get frames but most of the time the screen stays grey. I use # to show how the code looks now

Ok, it now opens a window and shows the output of the camera
Because of this code:
import sys
sys.path.append('/home/pi/.virtualenvs/cv/lib/python2.7/site-
packages/usr/local/lib/python2.7/site-packages')
and i also use sudo python program.py in the terminal
But this Error :"NameError: name 'CV_CAP_PROP_FRAME_HEIGHT' is not defined" still persists...

cv2.videocapture doesn't works on Raspberry-pi

How can i make the cv2.VideoCapture(0) recognize the USB camera of raspberry-pi.
def OnRecord(self, evt):
capture = cv2.VideoCapture(0)
if (not capture.isOpened()):
print "Error"
# video recorder
fourcc = cv2.cv.CV_FOURCC(*'XVID') # cv2.VideoWriter_fourcc() does not exist
video_writer = cv2.VideoWriter.open("output.mp4", fourcc, 20, (640, 480), True)
# record video
while (capture.isOpened()):
ret, frame = capture.read()
if ret==True:
video_writer.write(frame)
cv2.imshow('Video', frame)
else:
break
def OnCancel(self, evt):
capture.release()
video_writer.release()
cv2.destroyAllWindows()
but it only prints Error.
So i guess capture is not opening. What might be the reason?
I tried this code from opencv documentation but doesn't worked out for me.
import numpy as np
import cv2
cap = cv2.VideoCapture(0)
# Define the codec and create VideoWriter object
fourcc = cv2.VideoWriter_fourcc(*'XVID')
out = cv2.VideoWriter('output.avi',fourcc, 20.0, (640,480))
while(cap.isOpened()):
ret, frame = cap.read()
if ret==True:
frame = cv2.flip(frame,0)
# write the flipped frame
out.write(frame)
cv2.imshow('frame',frame)
if cv2.waitKey(1) & 0xFF == ord('q'):
break
else:
break
# Release everything if job is finished
cap.release()
out.release()
cv2.destroyAllWindows()
Any help would be greatly appreciated.

Load the correct video for linux drivers.
sudo modprobe bcm2835-v4l2

In my experience with CV2 replacing a webcam source on linux isn't always easy. How OpenCV works is it automatically draws from the systems default video source, which is known (usually) as video0. Unplug your usb webcam and go into a terminal and typing ls /dev/video*
Remember the number it says. Now plug in your USB webcam and type in ls /dev/video* again and look for any new /video, this is your USB webcam. Now type mv /dev/videoX videoY while X is the number of your USB webcam and Y the original number. This will replace your pi's default camera.
This isn't permanent as you will need to do this every time your pi starts up, an alternative to this is creating a bash file that runs on start up. Create a text file and copy the following into it.
#!/bin/bash
mv /dev/videoX videoY
(replace the X and Y of course)
and place that in /etc/init.d directory of your pi. Don't forget you may need to use
chmod 755 /etc/init.d/FILENAME.sh
to give it permission to run

Go to terminal and type lsusb and check whether the USB camera is recognized or not. If it is recognized then try to give different device ID such as 1 or 2 or 3 rather than 0.

looks Like you might have issue with codec, try using 'MJPG' codec instead of XVID.
For more details have a look here

Make sure that the camera that you are using is UVC compatible, as openCV running on linux based systems (like a raspi) starts to do some silly things when it is working with non UVC cameras.

python.exe crashing when running basic OpenCV example

I'm just following the second example from this tutorial and using a different video. Can anyone help solve the problem of why it crashes? I thought it would be my cheap Windows 7 running on 2 GB RAM, but it's not even using up the RAM when it crashes.
import numpy as np
import cv2
cap = cv2.VideoCapture("F:\\OpenCV\\Cam1_Indoor.avi")
fgbg = cv2.createBackgroundSubtractorMOG2()
while(1):
ret, frame = cap.read()
fgmask = fgbg.apply(frame)
cv2.imshow('frame',fgmask)
k = cv2.waitKey(30) & 0xff
if k == 27:
break
cap.release()
cv2.destroyAllWindows()
UPDATE 1:
I still can't get this working, but I have tried to debug it some more. I found that regardless of the video file name I give it (even if it's totally bogus), the same crash happens.
UPDATE 2:
I found that the code below works on images, so my problem may have to do with video capture.
im_in = cv2.imread("F:\\OpenCV\\nickel.jpg", cv2.IMREAD_GRAYSCALE);
fgbg = cv2.createBackgroundSubtractorMOG2()
fgmask = fgbg.apply(im_in)
cv2.imshow('frame',fgmask)
cv2.waitKey(0)

Add this below import statements
cv2.ocl.setUseOpenCL(False)

Video Capture with Python

I'm trying to write some code for automatically capturing video from a webcam when activated through a batch script on Windows. I've managed to piece together a working script, but it does not appear to be saving the file. I know the code is (at least on a basic level) working because there are no errors and the activation light on the webcam lights up when the code is run. I've reproduced the code below if anyone has any suggestions on how to get it writing to a file, that would be great!
import numpy as np
import cv2
import msvcrt
cap = cv2.VideoCapture(0)
w=int(cap.get(cv2.CAP_PROP_FRAME_WIDTH ))
h=int(cap.get(cv2.CAP_PROP_FRAME_HEIGHT ))
sub=raw_input("Subject#: ")
#Define the codec and create VideoWriter object
#fourcc = cv2.VideoWriter_fourcc(*'DIVX')
fourcc = cv2.VideoWriter_fourcc(*'FMP4')
out = cv2.VideoWriter('C:\path\to\output_' + sub + '.mp4', fourcc, 30, (w,h))
while(cap.isOpened()):
ret, frame = cap.read()
if ret==True:
out.write(frame)
if msvcrt.kbhit():
if ord(msvcrt.getch()) != None:
break
else:
break
#Release everything if job is finished
cap.release()
out.release()
cv2.destroyAllWindows()

The codec FMP4 is just not being supported by your camera I think. Neither in mine. If I change in your code the encoding to 'MJPG' and write to an '.avi' extension, it works very well.
P.S. Wanted to make a comment but I'm still underage (< 50).