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Grokking The Gimp
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4.5.3.2 The Threshold Tool Versus the Magic Wand

The Magic Wand, presented in Section  3.1.1, is very similar in principle to Threshold but not nearly as effective. As already described, the Magic Wand works by choosing a seed pixel in the image and interactively setting a threshold that controls how many pixels around the seed are included in the selection. Thus, if the value of the pixel at the seed is S, and the value of the threshold is T, then the range of pixel values that are included in the selection is [S-T,S+T].

Now suppose that the range of pixel values that separates the subject from the background is [R1,R2]. To make the Magic Wand work on this image, the threshold must have the value T=(R2-R1)/2 and the seed must have the value S=(R1+R2)/2. The problem, then, is finding a pixel in the subject having the correct seed value that, when experimenting with threshold values, will produce an acceptable result. This is impractical for several reasons, the main difficulty being that there is no way to use the visual feedback from several tries of the Magic Wand to discover a more refined solution.

On the other hand, Threshold requires only that the end points of the range be specified, so it's much better adapted to experimentation. It is easy to try several contiguous value-regions, and the visual feedback from this is very useful for improving the search. In addition, the histogram in the Threshold dialog provides important clues as to which regions may be most useful.

Finally, the algorithm used by the Magic Wand is slow, because for each change in the threshold value, it must recursively grow the selected region around the seed. In comparison, the algorithm for Threshold is very fast, because it must only compare each pixel in the image with a threshold.

Grokking The Gimp
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  Published under the terms of the Open Publication License Design by Interspire