native_apps_blog_canny_plpython

native_apps_blog_canny_plpython.sql

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-- Gautam Muralidhar, Srivatsan Ramanujam, Oct-2014
-- PL/Python UDF, which calls the native Canny edge detection application
-- Input: the raw image byte stream encoded as a comma-separated string  and stored in a column in a HAWQ table
-- Output: a composite type as defined in Figure 5.
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CREATE OR REPLACE FUNCTION canny_edge_detect(img varchar)
   RETURNS  canny_output_type
AS
$$

    import ctypes
    from array import array
    from numpy.ctypeslib import ndpointer
    libfile = '/usr/local/lib/ds/canny_edge_detection.so'
    # Load the shared object of the native Canny edge detection application
    # GD is a global dictionary supplied by PL/Python that is available to each user session
    if(not GD.has_key('canny_edge_detection')):  
        GD['canny_edge_detection'] = ctypes.cdll.LoadLibrary(libfile)   
    dl  =  GD['canny_edge_detection']
    # Prepare to call the native getImgSizeFromByteStream function to determine the image size
    getImgSize = dl.getImgSizeFromByteStream
    # Specify the input argument types of the native function using ctypes mapping
    getImgSize.argtypes = [ctypes.c_char_p, ctypes.c_uint]
    # Specify the result type of the native function as an ndpointer from numpy.ctypeslib
    getImgSize.restype = ndpointer(dtype=ctypes.c_uint,shape=(2,))
    # Map the comma-separated  string representing the image byte stream to a character array
    buf = array('b', map(int, img.split(','))).tostring()
    # Call the native getImgSizeFromByteStream function
    imgSize = getImgSize(ctypes.c_char_p(buf),ctypes.c_uint(len(buf)) )
    # Repeat above steps to call the  native edgeDetectionFromByteStream function 
    edgeDetect = dl.edgeDetectionFromByteStream
    edgeDetect.argtypes = [ctypes.c_char_p,ctypes.c_uint]
    edgeDetect.restype = ndpointer(dtype=ctypes.c_uint,shape=(imgSize[0]*imgSize[1],))
    result = edgeDetect(ctypes.c_char_p(buf),ctypes.c_uint(len(buf)))
    # Return the composite type
    return [imgSize[0], imgSize[1], result]  
$$ LANGUAGE PLPYTHONU;