Add a stub OpenCV-based camera

This allows the server to run on a laptop, to some extent. I've not tested if this will make the whole
web app work properly.

As StreamingPiCamera2 is hardcoded in various places, this certainly doesn't make cameras totally
interchangeable - but it is a step towards that.

pyproject.toml

@@ -17,9 +17,9 @@ classifiers = [
 dependencies = [
   "labthings-fastapi[server]",
   "labthings-sangaboard",
-  "labthings-picamera2",
   "camera-stage-mapping ~= 0.1.6",
   "numpy ~= 1.20",
+  "piexif",
   "scipy ~= 1.6",
   "opencv-python ~= 4.7.0",
   "anyio ~= 4.0",
@@ -29,6 +29,10 @@ dependencies = [
 dev = [
   "labthings-fastapi[dev]",
 ]
+pi = [
+  "labthings-picamera2",
+]
+
 
 [project.urls]
 "Homepage" = "https://openflexure.org/"

src/openflexure_microscope_server/things/opencv_camera.py

@@ -0,0 +1,147 @@
+"""OpenFlexure Microscope OpenCV Camera
+
+This module defines a Thing that is responsible for using the stage and
+camera together to perform an autofocus routine.
+
+See repository root for licensing information.
+"""
+from __future__ import annotations
+import io
+import json
+import logging
+from typing import Literal, Optional
+from threading import Thread
+
+import cv2
+import piexif
+
+from labthings_fastapi.thing import Thing
+from labthings_fastapi.utilities import get_blocking_portal
+from labthings_fastapi.decorators import thing_action, thing_property
+from labthings_fastapi.dependencies.metadata import GetThingStates
+from labthings_fastapi.dependencies.blocking_portal import BlockingPortal
+from labthings_fastapi.outputs.mjpeg_stream import MJPEGStreamDescriptor
+from labthings_fastapi.outputs.blob import BlobOutput
+from labthings_fastapi.types.numpy import NDArray
+
+
+class JPEGBlob(BlobOutput):
+    media_type = "image/jpeg"
+
+
+class OpenCVCamera(Thing):
+    """A Thing representing an OpenCV camera"""
+    def __init__(self, camera_index: int=0):
+        self.camera_index = camera_index
+        self._capture_thread: Optional[Thread] = None
+        self._capture_enabled = False
+
+    def __enter__(self):
+        self.cap = cv2.VideoCapture(self.camera_index)
+        self._capture_enabled = True
+        self._capture_thread = Thread(target=self._capture_frames)
+        self._capture_thread.start()
+        return self
+    
+    def __exit__(self, _exc_type, _exc_value, _traceback):
+        self.cap.release()
+
+    @thing_property
+    def stream_active(self) -> bool:
+        "Whether the MJPEG stream is active"
+        if self._capture_enabled and self._capture_thread:
+            return self._capture_thread.is_alive()
+        return False
+    mjpeg_stream = MJPEGStreamDescriptor()
+    lores_mjpeg_stream = MJPEGStreamDescriptor()
+
+    def _capture_frames(self):
+        portal = get_blocking_portal(self)
+        while self._capture_enabled:
+            ret, frame = self.cap.read()
+            if not ret:
+                logging.error(f"Failed to capture frame from camera {self.camera_index}")
+                break
+            jpeg = cv2.imencode(".jpg", frame)[1].tobytes()
+            self.mjpeg_stream.add_frame(jpeg, portal)
+            jpeg_lores = cv2.imencode(".jpg", cv2.resize(frame, (320, 240)))[1].tobytes()
+            self.lores_mjpeg_stream.add_frame(jpeg_lores, portal)
+
+    @thing_action
+    def snap_image(self) -> NDArray:
+        """Acquire one image from the camera.
+
+        This action cannot run if the camera is in use by a background thread, for
+        example if a preview stream is running.
+        """
+        return self.capture_array()
+
+    @thing_action
+    def capture_array(self) -> NDArray:
+        """Acquire one image from the camera and return as an array
+
+        This function will produce a nested list containing an uncompressed RGB image.
+        It's likely to be highly inefficient - raw and/or uncompressed captures using
+        binary image formats will be added in due course.
+        """
+        ret, frame = self.cap.read()
+        if not ret:
+            raise RuntimeError(f"Failed to capture frame from camera {self.camera_index}")
+        return frame
+    
+    @thing_action
+    def capture_jpeg(
+        self,
+        metadata_getter: GetThingStates,
+    ) -> JPEGBlob:
+        """Acquire one image from the camera and return as a JPEG blob
+
+        This function will produce a JPEG image.
+        """
+        frame = self.capture_array()
+        jpeg = cv2.imencode(".jpg", frame)[1].tobytes()
+        exif_dict = {
+            "Exif": {
+                piexif.ExifIFD.UserComment: json.dumps(metadata_getter()).encode("utf-8")
+            },
+            "GPS": {},
+            "Interop": {},
+            "1st": {},
+            "thumbnail": None,
+        }
+        output = io.BytesIO()
+        piexif.insert(piexif.dump(exif_dict), jpeg, output)
+        return JPEGBlob.from_bytes(output.getvalue())
+
+    @thing_action
+    def grab_jpeg(
+        self,
+        portal: BlockingPortal,
+        stream_name: Literal["main", "lores"] = "main",
+    ) -> JPEGBlob:
+        """Acquire one image from the preview stream and return as an array
+
+        This differs from `capture_jpeg` in that it does not pause the MJPEG
+        preview stream. Instead, we simply return the next frame from that
+        stream (either "main" for the preview stream, or "lores" for the low
+        resolution preview). No metadata is returned.
+        """
+        logging.info(f"StreamingPiCamera2.grab_jpeg(stream_name={stream_name}) starting")
+        stream = (
+            self.lores_mjpeg_stream if stream_name == "lores" else self.mjpeg_stream
+        )
+        frame = portal.call(stream.grab_frame)
+        logging.info(f"StreamingPiCamera2.grab_jpeg(stream_name={stream_name}) got frame")
+        return JPEGBlob.from_bytes(frame)
+
+    @thing_action
+    def grab_jpeg_size(
+        self,
+        portal: BlockingPortal,
+        stream_name: Literal["main", "lores"] = "main",
+    ) -> int:
+        """Acquire one image from the preview stream and return its size"""
+        stream = (
+            self.lores_mjpeg_stream if stream_name == "lores" else self.mjpeg_stream
+        )
+        return portal.call(stream.next_frame_size)