Develop Reference

Writing and Reading video without saving in opencv

I want to read a video after I write it with cv2.VideoWriter without saving the video.
For example:
video = cv2.VideoWriter('using.mp4', cv2.VideoWriter_fourcc(*'MJPG'), 10, size)
Now, after writing this cv2.VideoWriter object, is it possible to read it likevideo.read(), but since read() is a function of cv2.VideoCapture and it will throw an error
Exception has occurred: AttributeError
'cv2.VideoWriter' object has no attribute 'read'
So, is there possible way of reading the cv2.VideoWriter?

An alternative to reading frames from the video writer, is to save the frames in a list instead of saving each frame in the the loop. when you finished, you can write them outside the loop and have the save affect as video.read()
video = cv2.VideoWriter('using.mp4', cv2.VideoWriter_fourcc(*'MJPG'), 10, size)
for frame in frames:
writer.write(frame)
for frame in frames:
# do other stuff here
detailed example (Notice i changed the fourcc - your example didnt work for me)
import cv2
def cam_test(port: int = 0) -> None:
frames = []
cap = cv2.VideoCapture(port)
if not cap.isOpened(): # Check if the web cam is opened correctly
print("failed to open cam")
else:
print('cam opened on port {}'.format(port))
for i in range(10 ** 10):
success, cv_frame = cap.read()
if not success:
print('failed to capture frame on iter {}'.format(i))
break
frames.append(cv_frame)
cv2.imshow('Input', cv_frame)
k = cv2.waitKey(1)
if k == ord('q'):
break
cap.release()
cv2.destroyAllWindows()
# Now you have the frames at hand
if len(frames) > 0:
# if you want to write them
size = (frames[0].shape[1], frames[0].shape[0])
video = cv2.VideoWriter(
filename='using.mp4',
fourcc=cv2.VideoWriter_fourcc(c1='m', c2='p', c3='4', c4='v'),
fps=10,
frameSize=size
)
for frame in frames:
video.write(frame)
# and to answer your question, you wanted to do video.read() which would have gave you frame by frame
for frame in frames:
pass # each iteration is like video.read() if video.read() was possible
return
if __name__ == '__main__':
cam_test()

Why is my program not recognizing the Checkerboard?

I am running a Camera Calibration program from Python using openCV. I am using my computer camera from an XPS 15 9575 in order to capture different frames of a classic black and white checkerboard that I printed. For some reason, it never registers in the program that there is a checkerboard.
I've run this program by itself and with already produced images and it works. It only doesn't work as I try to capture new ones and process them instantly.
This is the beginning of the code. It runs to check to see if it finds the corners and then moves onto the next step. When running, it never makes it past this.
cam = cv2.VideoCapture(0)
cv2.namedWindow("test")
img_counter = 0
imgNames = []
size = (5,5)
while True:
ret, frame = cam.read()
gray = cv2.cvtColor(frame, cv2.COLOR_BGR2GRAY)
cv2.imshow("test", gray)
if not ret:
break
k = cv2.waitKey(1)
if k%256 == 27:
break
elif k%256 == 32:
img_name = "{}.png".format(img_counter)
imgtemp = cv2.imread(img_name)
graytemp = cv2.cvtColor(imgtemp,cv2.COLOR_BGR2GRAY)
ret, corners = cv2.findChessboardCorners(graytemp, size,None)
print (ret)
if ret == True:
print ("good!")
imgNames.append(img_name)
cv2.imwrite(img_name, frame)
img_counter += 1
else:
print ("again")

In your code above, you are trying to read an image which actually doesn't exist. See these lines:
img_name = "{}.png".format(img_counter)
imgtemp = cv2.imread(img_name)
Here, img_name is just a string, which doesn't point to an image file yet. You can do one thing, capture a frame and save it here and give it a name img_name and then try to read it via cv2.imread function, like below:
img_name = "{}.png".format(img_counter)
cv2.imwrite(img_name, frame)
imgtemp = cv2.imread(img_name)
Alternatively, you can replace imgtemp = cv2.imread(img_name) with imgtemp = frame. In this case, you don't have to save and then process a frame. Here, once a spacebar is pressed, processing is done on the current captured video frame without saving it.
And don't forget to add below lines at the end of your code:
cam.release()
cv2.destroyAllWindows()

Exit function doesn't work in OpenCV with python

I just want to capture one frame, save it to test.png and then exit the code.
In this program exit() doesn't work, I have to use CTRL+C from the terminal every time.
import cv2
#cv2.namedWindow("window")
cap = cv2.VideoCapture(0)
if cap.isOpened(): # if we are able to capture the first frame.
val, frame = cap.read()
else:
val = False
while val:
#cv2.imshow("window", frame)
cv2.imwrite('test.png',frame)
val, frame = cap.read()
key = cv2.waitKey(20)
if key == 27: # exit on ESC
break
cap.release
cv2.destroyAllWindows()
exit()
Thanks in advance.

cv::waitKey only works if any openCV window is present and maybe active.
Since you commented #cv2.namedWindow("window") and #cv2.imshow("window", frame) out there is no waitKey time and no chance to fetch the key.
Does it work if you activate namedWindow and imshow?
To be sure additionally try
if key > 0: # exit on ESC
break
to cancel on ANY keypress (but you still need an active openCV window)
To capture a single frame try this:
import cv2
cap = cv2.VideoCapture(0)
val = False
maxTry = 100 # maximum number of tries to capture a frame from an opened device
cTry = 0
if cap.isOpened(): # if we are able to capture the first frame.
while (!val) and (cTry < maxTry)
val, frame = cap.read()
cTry = cTry + 1
else:
val = False
if val:
cv2.imwrite('test.png',frame)
else:
print "No image captured"
cap.release
exit()
I am not a python programmer so please forgive me any syntax errors (and give me a hint to correct them)

The method waitkey(t) waits t in milliseconds, so your code is waiting 20 ms for a key press for each loop. Considering that your
cv2.imwrite('test.png',frame)
might take some time to write to file, are you perhaps pressing your exiting key at the wrong moment?

Try adding & 0xFF to the wait key:
key = cv2.waitKey(20) & 0xFF
You also need an active window:
Note
The function only works if there is at least one HighGUI window created and the window is active. If there are several HighGUI windows, any of them can be active.

for python3
cv2.imshow('imafast', frame)
if cv2.waitKey(25) & 0xFF == ord('q'):
break

OpenCV - Save video segments based on certion condition

Aim : Detect the motion and save only the motion periods in files with names of the starting time.
Now I met the issue about how to save the video to the files with video starting time.
What I tested :
I tested my program part by part. It seems that each part works well except the saving part.
Running status: No error. But in the saving folder, there is no video. If I use a static saving path instead, the video will be saved successfully, but the video will be override by the next video. My codes are below:
import cv2
import numpy as np
import time
cap = cv2.VideoCapture( 0 )
bgst = cv2.createBackgroundSubtractorMOG2()
fourcc=cv2.VideoWriter_fourcc(*'DIVX')
size = (int(cap.get(cv2.CAP_PROP_FRAME_WIDTH)), int(cap.get(cv2.CAP_PROP_FRAME_HEIGHT)))
n = "start_time"
while True:
ret, frame = cap.read()
dst = bgst.apply(frame)
dst = np.array(dst, np.int8)
if np.count_nonzero(dst)>3000: # use this value to adjust the "Sensitivity“
print('something is moving %s' %(time.ctime()))
path = r'E:\OpenCV\Motion_Detection\%s.avi' %n
out = cv2.VideoWriter( path, fourcc, 50, size )
out.write(frame)
key = cv2.waitKey(3)
if key == 32:
break
else:
out.release()
n = time.ctime()
print("No motion Detected %s" %n)

What I meant is:
import cv2
import numpy as np
import time
cap = cv2.VideoCapture( 0 )
bgst = cv2.createBackgroundSubtractorMOG2()
fourcc=cv2.VideoWriter_fourcc(*'DIVX')
size = (int(cap.get(cv2.CAP_PROP_FRAME_WIDTH)),int(cap.get(cv2.CAP_PROP_FRAME_HEIGHT)))
path = r'E:\OpenCV\Motion_Detection\%s.avi' %(time.ctime())
out = cv2.VideoWriter( path, fourcc, 16, size )
while True:
ret, frame = cap.read()
dst = bgst.apply(frame)
dst = np.array(dst, np.int8)
for i in range(number of frames in the video):
if np.count_nonzero(dst)<3000: # use this value to adjust the "Sensitivity“
print("No Motion Detected")
out.release()
else:
print('something is moving %s' %(time.ctime()))
#label each frame you want to output here
out.write(frame(i))
key = cv2.waitKey(1)
if key == 32:
break
cap.release()
cv2.destroyAllWindows()
If you see the code there will be a for loop, within which the process of saving is done.
I do not know the exact syntax involving for loop with frames, but I hope you have the gist of it. You have to find the number of frames present in the video and set that as the range in the for loop.
Each frame gets saved uniquely (see the else condition.) As I said I do not know the syntax. Please refer and follow this procedure.
Cheers!

Detect media resolution change in video

I tried to search in SO but could not find.I want to iterate video file frame by frame and want to detect if there is any point when resolution of two consecutive frame changes. The desired functionality in pseudocode:
resolution1 = videoFile[0]->resolution
resolution2 = 0;
for frame in videoFile[1:]:
resolution2 = frame->resolution
if (resolution1 != resolution2):
print 'Changes occured'
else:
resolution1 = resolution2
Please give me the name of a library to implement them, I have tried and read documentation of OpenCV and PiCamera.
Thanks in advance

You should use OpenCV to iterate through each frame. This should print "Changes occured" whenever the video changes resolution:
import cv2
capture1 = cv2.VideoCapture('videoname') #Open the video
ret, frame = capture1.read() #Read the first frame
resolution1 = frame.shape #Get resolution
while capture1.isOpened():
ret, frame = capture1.read() #Read the next frame
if ret == False or frame == None:
break #Quit if video ends
resolution2 = frame.shape #Get the new resolution
if resolution1 != resolution2:
print 'Changes occured'
#Change resolution1 to resolution2 so that the new resolutions are
#compared with resolution2, not resolution1.
resolution1 = resolution2