OpenCV replay saved stream and process it - opencv

Question

My code processes a frame and it takes a couple of seconds to execute. If I'm streaming from a camera, I will naturally drop frames and get a frame every couple of seconds, right? I want to simulate the same thing in replaying a video file.
Normally, when you call vidcap.read(), you get the next frame in the video. This essentially slows down the video and does not miss a frame. This is not like processing a live camera stream. Is there a way to process the video file and drop frames during processing like when processing the camera stream?
The solution that comes to my mind is to keep track of time myself and call vidcap.set(cv2.CAP_PROP_POS_MSEC, currentTime) before each vidcap.read(). Is this how I should do it, or is there a better way?

Answer 1

One approach is to keep track of the processing time and skip that amount of frames:
import cv2, time, math
# Capture saved video that is used as a stand-in for webcam
cap = cv2.VideoCapture('/home/stephen/Desktop/source_vids/ss(6,6)_id_146.MP4')
# Get the frames per second of the source video
fps = 120
# Iterate through video
while True:
# Record your start time for the frame
start = time.time()
# Read the frame
_, img = cap.read()
# Show the image
cv2.imshow('img', img)
# What ever processing that is going to slow things down should go here
k = cv2.waitKey(0)
if k == 27: break
# Calculate the time it took to process this frame
total = time.time() - start
# Print out how many frames to skip
print(total*fps)
# Skip the frames
for skip_frame in range(int(total*fps)): _, _ = cap.read()
cv2.destroyAllWindows()
This is probably better than nothing, but it does not correctly simulate the way that frames will be dropped. It appears that during processing, the webcam data is written to a buffer (until the buffer fills up). A better approach is to capture the video with a dummy process. This processor intensive dummy process will cause frames to be dropped:
import cv2, time, math
# Capture webcam
cap = cv2.VideoCapture(0)
# Create video writer
vid_writer = cv2.VideoWriter('/home/stephen/Desktop/drop_frames.avi',cv2.VideoWriter_fourcc('M','J','P','G'),30, (640,480))
# Iterate through video
while True:
# Read the frame
_, img = cap.read()
# Show the image
cv2.imshow('img', img)
k = cv2.waitKey(1)
if k == 27: break
# Do some processing to simulate your program
for x in range(400):
for y in range(40):
for i in range(2):
dummy = math.sqrt(i+img[x,y][0])
# Write the video frame
vid_writer.write(img)
cap.release()
cv2.destroyAllWindows()