Streaming video in memory with OpenCV VideoWriter and Python BytesIO

Question

I was wondering if it is possible to 'stream' data using the OpenCV VideoWriter class in Python?

Normally for handling data in memory that would otherwise go to disk I use BytesIO (or StringIO).

My attempt to use BytesIO fails though:

import cv2
from io import BytesIO

stream = cv2.VideoCapture(0)
fourcc = cv2.VideoWriter_fourcc('x264')

data = BytesIO()

# added these to try to make data appear more like a string
data.name = 'stream.{}'.format('av1')
data.__str__ = lambda x: x.name

try:
    video = cv2.VideoWriter(data, fourcc=fourcc, fps=30., frameSize=(640, 480))
    start = data.tell()

        # Check if camera opened successfully
        if (stream.isOpened() == False): 
            print("Unable to read camera feed", file=sys.stderr)
            exit(1)

        # record loop
        while True:
            _, frame = stream.read()
            video.write(frame)
            data.seek(start)
            # do stuff with frame bytes
            # ...

            data.seek(start)

    finally:
        try:
            video.release()
        except:
            pass
finally:
    stream.release()

However instead of writing the BytesIO object I end up with the following message:

Traceback (most recent call last):
  File "video_server.py", line 54, in talk_to_client
    video = cv2.VideoWriter(data, fourcc=fourcc, fps=fps, frameSize=(width, height))
TypeError: Required argument 'apiPreference' (pos 2) not found

... So when I modify the VideoWriter call to be cv2.VideoWriter(data, apiPreference=0, fourcc=fourcc, fps=30., frameSize=(640, 480)) (I read that 0 means auto, but I also tried cv2.CAP_FFMPEG ), I instead get the following error:

Traceback (most recent call last):
  File "video_server.py", line 54, in talk_to_client
    video = cv2.VideoWriter(data, apiPreference=0, fourcc=fourcc, fps=fps, frameSize=(width, height))
TypeError: bad argument type for built-in operation

So my question is, is it possible to write encoded video using the cv2.VideoWriter class in memory and if so how is it done?

At this point I'm fresh out of ideas, so any help would be most welcome :-)

Answer 1

Unfortunately, OpenCV doesn't support encoding to (or decoding from) memory. You must write to (or read from) disk for VideoWriter (or VideoCapture) to work.

Answer 2

如果你使用 Linux，你可以创建一个 ramdisk 并写入它。

mount -t tmpfs -o size=512m tmpfs /mnt/ramdisk

Answer 3

If you want some convenience to load a video that is already in memory, use a temporary file: https://docs.python.org/3/library/tempfile.html#tempfile.NamedTemporaryFile

import tempfile
import cv2

my_video_bytes = download_video_in_memory()

with tempfile.NamedTemporaryFile() as temp:
   temp.write(my_video_bytes)

   video_stream = cv2.VideoCapture(temp.name)

   # do your stuff.

This will still create a file on disk unfortunately . But hey, at least you don't have to manage that yourself. There is a SpooledTemporaryFile() implementation that will stay in memory, but, unfortunately, it won't create a file system name that OpenCV can reference.