'**SUMMARY**
Prewitt operator for edge detection
**PARAMETERS**
None
**RETURNS**
Image with prewitt opeartor applied on it
**EXAMPLE**
>>> img = Image("lenna")
>>> p = img.prewitt()
>>> p.show()
**NOTES**
Read more at: http://en.wikipedia.org/wiki/Prewitt_operator'

def prewitt(self):

    img = self.copy()
    grayimg = img.grayscale()
    gx = [[1, 1, 1], [0, 0, 0], [(-1), (-1), (-1)]]
    gy = [[(-1), 0, 1], [(-1), 0, 1], [(-1), 0, 1]]
    grayx = grayimg.convolve(gx)
    grayy = grayimg.convolve(gy)
    grayxnp = np.uint64(grayx.getGrayNumpy())
    grayynp = np.uint64(grayy.getGrayNumpy())
    retVal = Image(np.sqrt(((grayxnp ** 2) + (grayynp ** 2))))
    return retVal
