How to disable Buffer in OpenCV Camera?

Question

I have this situation where I use OpenCV to detect faces in front of the camera and do some ML on those faces. The issue that I have is that once I do all the processing, an

Answer 1

I set cap value after reading each frame to None and my problem solved in this way:

import cv2
from PyQt5.QtCore import QThread

if __name__ == '__main__':
while True:
    cap = cv2.VideoCapture(0)
    ret, frame = cap.read()
    cv2.imshow('A', frame)
    cv2.waitKey(0)

    print('sleep!')
    QThread.sleep(5)
    print('wake up!')
    cap = None

Answer 2

Whether CAP_PROP_BUFFERSIZE is supported appears quite operating system and backend-specific. E.g., the 2.4 docs state it is "only supported by DC1394 [Firewire] v 2.x backend currently," and for backend V4L, according to the code, support was added only on 9 Mar 2018.

The easiest non-brittle way to disable the buffer is using a separate thread; for details, see my comments under Piotr Kurowski's answer. Here Python code that uses a separate thread to implement a bufferless VideoCapture: (I did not have a opencv4nodejs environment.)

import cv2, Queue, threading, time

# bufferless VideoCapture
class VideoCapture:

  def __init__(self, name):
    self.cap = cv2.VideoCapture(name)
    self.q = Queue.Queue()
    t = threading.Thread(target=self._reader)
    t.daemon = True
    t.start()

  # read frames as soon as they are available, keeping only most recent one
  def _reader(self):
    while True:
      ret, frame = self.cap.read()
      if not ret:
        break
      if not self.q.empty():
        try:
          self.q.get_nowait()   # discard previous (unprocessed) frame
        except Queue.Empty:
          pass
      self.q.put(frame)

  def read(self):
    return self.q.get()

cap = VideoCapture(0)
while True:
  frame = cap.read()
  time.sleep(.5)   # simulate long processing
  cv2.imshow("frame", frame)
  if chr(cv2.waitKey(1)&255) == 'q':
    break

The frame reader thread is encapsulated inside the custom VideoCapture class, and communication with the main thread is via a queue.

This answer suggests using cap.grab() in a reader thread, but the docs do not guarantee that grab() clears the buffer, so this may work in some cases but not in others.

Answer 3

I have the same problem but in C++. I didn't find proper solution in OpenCV but I found a workaround. This buffer accumulates a constant number of images, say n frames. So You can read n frames without analysis and read frame once more. That last frame will be live image from camera. Something like:

buffer_size = n;

for n+1
{
  // read frames to mat variable
}
// Do something on Mat with live image