RPi 2B python opencv camera demo example

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 *                     RPi 2B python opencv camera demo example
 * 声明：
 *     本文主要记录RPi 2B 使用python opencv来获取图片的方式。
 *
 *                                                   2016-2-24 深圳 南山平山村 曾剑锋
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一、参考文档：
    1. OpenCV with Raspberry Pi Camera Face Detection Tutorial - Robotics with Python Raspberry Pi and GoPiGo p.7
        https://pythonprogramming.net/raspberry-pi-camera-opencv-face-detection-tutorial/
    2. Raspberry Pi OpenCV，在树莓派下使用opencv
        http://www.educity.cn/wenda/565761.html

二、opencv install:
　　sudo apt-get update sudo apt-get install libopencv-dev
　　sudo apt-get install python-opencv

三、example code
    #!/usr/bin/python

    import cv2 as cv
    import numpy
    import io
    import picamera

    # Create a memory stream so photos doesn‘t need to be saved in a file
    stream = io.BytesIO()

    # Get the picture (low resolution, so it should be quite fast)
    # Here you can also specify other parameters (e.g.:rotate the image)
    with picamera.PiCamera() as camera:
        camera.resolution = (320, 240)
        camera.capture(stream, format=‘jpeg‘)

    # Convert the picture into a numpy array
    buff = numpy.fromstring(stream.getvalue(), dtype=numpy.uint8)

    # Now creates an OpenCV image
    image = cv.imdecode(buff, 1)

    # Save the result image
    cv.imwrite(‘result.jpg‘, image)