webcam.py

"""Raspberry Pi Face Recognition Treasure Box 
Webcam OpenCV Camera Capture Device
Copyright 2013 Tony DiCola 

Webcam device capture class using OpenCV.  This class allows you to capture a
single image from the webcam, as if it were a snapshot camera.  

This isn't used by the treasure box code out of the box, but is useful to have
if running the code on a PC where only a webcam is available.  The interface is
the same as the picam.py capture class so it can be used in the box.py code
without any changes.
"""
import threading
import time

import cv2

import config


# Rate at which the webcam will be polled for new images.
CAPTURE_HZ = 30.0


class OpenCVCapture(object):
	def __init__(self, device_id=0):
		"""Create an OpenCV capture object associated with the provided webcam
		device ID.
		"""
		# Open the camera.
		self._camera = cv2.VideoCapture(device_id)
		if not self._camera.isOpened():
			self._camera.open()
		self._camera.set(3, 320)
		self._camera.set(4, 240)
		# Start a thread to continuously capture frames.
		# This must be done because different layers of buffering in the webcam
		# and OS drivers will cause you to retrieve old frames if they aren't 
		# continuously read.
		self._capture_frame = None
		# Use a lock to prevent access concurrent access to the camera.
		self._capture_lock = threading.Lock()
		self._capture_thread = threading.Thread(target=self._grab_frames)
		self._capture_thread.daemon = True
		self._capture_thread.start()

	def _grab_frames(self):
		while True:
			retval, frame = self._camera.read()
			with self._capture_lock:
				self._capture_frame = None
				if retval:
					self._capture_frame = frame
			time.sleep(1.0/CAPTURE_HZ)

	def read(self):
		"""Read a single frame from the camera and return the data as an OpenCV
		image (which is a numpy array).
		"""
		frame = None
		with self._capture_lock:
			frame = self._capture_frame
		# If there are problems, keep retrying until an image can be read.
		while frame is None:
			time.sleep(0)
			with self._capture_lock:
				frame = self._capture_frame
		if config.DEBUG:
			# Save captured image for debugging.
			cv2.imwrite(config.DEBUG_IMAGE, frame)
		# Return the capture image data.
		return frame