To keep the camera stream active across multiple pages in a Flask web application without converting the whole app into a Single Page Application (SPA), you can use a combination of techniques such as embedding the video stream in an iframe or utilizing JavaScript for persistent streaming. Here’s an approach that aligns with the requirements you described.

Approach 1: Using iframe to Persist the Camera Stream

By embedding the camera stream inside an iframe that remains constant across page navigations, you can maintain the stream without reinitializing it every time the user moves to a new page. The main idea is to have a dedicated route in Flask that serves the video stream, and then display it in an iframe that doesn't refresh across pages.

Step 1: Set up the Camera Stream Route in Flask

Use OpenCV to stream the camera feed and serve it as an MJPEG stream, which is a format that browsers can easily handle. Create a dedicated Flask route for this stream:

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This route `/camera_stream` provides a continuous MJPEG stream that you can embed on any page.

Step 2: Embed the Stream Using an iframe

On your HTML pages, embed the video stream inside an iframe that stays constant across pages. You can place the iframe in a persistent section, such as a fixed header or side bar, and keep it out of the regular navigation process:

In this setup, the iframe fetches the live camera stream from the `/camera_stream` route. Since the iframe stays constant across pages (you can even make it part of a fixed navigation), the camera stream won’t stop when the user navigates.

Step 3: JavaScript to Handle Partial Reloads

To make the transition between pages smoother without reloading the entire page (including the iframe), you could use JavaScript to load content dynamically. This avoids reloading the iframe but is still simpler than converting the whole application into a full SPA.

Here's how you could load only the page content dynamically via AJAX, without reloading the iframe:

This simple script intercepts clicks on links, fetches the new content via `fetch()`, and injects the new content into the `#content` div, without refreshing the entire page.

Approach 2: Using WebSockets (for live video streaming)

An alternative to iframe embedding is using WebSockets to stream the video feed continuously across pages. WebSockets provide a two-way communication protocol that allows you to stream the camera feed from the server to the client in real-time. However, setting up WebSockets in Flask may require using `Flask-SocketIO`.

Pros:

- Persistent stream: The camera feed stays active.

- Low latency: WebSockets are ideal for real-time communication.

Cons:

- More complex setup: WebSockets introduce additional complexity, especially for live video.

Approach 3: SPA (Single Page Application)

If you prefer an even smoother user experience, you could consider converting the site to an SPA using a framework like React or Vue.js. This would allow you to keep the camera stream running without reloading any part of the page, including the iframe, as all content would be dynamically rendered.

However, if you don't want to move the whole application to an SPA, using an iframe with some JavaScript partial page reloads should be sufficient for your needs.

Summary:

To keep the camera stream active across pages:

- iframe approach: Use an iframe to embed the camera stream, keeping the iframe constant across pages.

- JavaScript: Use JavaScript to dynamically load page content without reloading the iframe.

- WebSocket alternative: For a more real-time solution, consider WebSockets, though they add more complexity.

The iframe-based solution should meet your needs and is easier to implement without requiring a full SPA conversion. Let me know if you'd like more details on any part of this!