'Return the binary bitmap which shows where edges are in the image.  The two
parameters determine how much change in the image determines an edge,
and how edges are linked together.  For more information refer to:
http://en.wikipedia.org/wiki/Canny_edge_detector
http://opencv.willowgarage.com/documentation/python/imgproc_feature_detection.html?highlight=canny#Canny'

def _getEdgeMap(self, t1=50, t2=100):

    if (self._edgeMap and (self._cannyparam[0] == t1) and (self._cannyparam[1] == t2)):
        return self._edgeMap
    self._edgeMap = self.getEmpty(1)
    cv.Canny(self._getGrayscaleBitmap(), self._edgeMap, t1, t2)
    self._cannyparam = (t1, t2)
    return self._edgeMap
