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Lesson 2 of the Edge Detection unit. Three operators compute the same derivative and behave differently on a noisy photo. Worked by hand on a 3x3 patch: Sobel gives Gx = Gy = 1.80, a magnitude of 2.55 and an orientation of 45 degrees, while Prewitt gives 1.20 and 1.70 on identical pixels. Measured across a noise sweep, Prewitt beats Sobel at every level, and the reason is that its normalised weights have a smaller sum of squares.
