2.4.2 Edge Filters
Edge filters are like a spatial 1st derivative, and are used for edge detection.
Thus edge filters are susceptible to noise, so noise filtering should be done first.
Sobel Edge Detectors are direction and size sensitive, so are Prewitt Edge
Detectors which are similar, except the 2's are replace by 1's (of same sign) - after Adjust Auto Levels:
| Original |
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Note how the sign of the result also affects the appearance of the Sobel Edge output.
It would be better if an alternative taking the absolute value for each pixel calculated
would be available. It would be better still if there was a means to find edges which
are not direction sensitive. In fact there is, which by experience I know that using a
3x3 window taking the max-min of the 9 pixels, after first removing noise with a 3x3
median filter, is better. The same effect can be achieved by subtracting the 3x3 min of
an image from the 3x3 max of the image. Here is the result (after contrast and brightness
adjustment). Note how all direction sensitivity has been successfully eliminated.
Other Edge filters include the: Canny Edge Detector, Compass Edge Detector
and Zero Crossing Detector.
