How to capature still images?

[yozachar]

Here's part of my code:

...
capture_img = None
class OpenCVVideoProcessor(VideoProcessorBase):
   # Inherited class
    def recv(self, frame: av.VideoFrame) -> av.VideoFrame:
        # Overridden function
        global capture_img
        capture_img = frame.to_ndarray(format='bgr24')
        return av.VideoFrame.from_ndarray(capture_img, format="bgr24")


col1, col2 = st.beta_columns(spec=2)
with col1:
    # 1. Input
    st.header(body='Input')
    webrtc_streamer(
        key='Input', video_processor_factory=OpenCVVideoProcessor)

...

I hoped that the frame converted to ndarray would be stored in capture_img since I've already started the camera.

But even though capture_img = frame.to_ndarray(format='bgr24') gets executed, capture_img stays None. Because when a button is pressed capture_img is directly assigned to some and value it raises something like AttributeError: 'NoneType' object has no attribute 'shape'.

Why is it so? Is there another way to capture a frame/image?

Full code
Streamlit deployment is broken due to dependency issue, everything works on local setup.

[whitphx]

Please see https://discuss.streamlit.io/t/new-component-streamlit-webrtc-a-new-way-to-deal-with-real-time-media-streams/8669/23?u=whitphx.
This post may be an answer while transform() was used instead of recv() because the post was originally written when the version of streamlit-webrtc was less than v0.20.

The point is that you cannot use global for that purpose and should pass values between the global scope and recv() through instance attributes of a VideoProcessor class in a thread-safe way.

This is because

• recv() is running in another thread
• The capture_img variable you defined in the global scope changes over executions though the reference of global capture_img does not.
  • https://blog.streamlit.io/how-to-build-the-streamlit-webrtc-component/#the-basics-of-streamlit-s-execution-model may be nice to read to know about Streamlit's execution model where the script is run many times.