I was tempted to file this as a bug, but after investigating it I'm not so sure. This happens to me in GIMP 2.6.12, try it yourself:

1 - Create a new white image, paint a black stroke across it (with any standard brush). 2 - Configure your Fuzzy Select with antialiasing enabled, sensitivity threshold 210. 3 - Click on any white area of the image.

The result is that your fuzzy select appears to grab the entire image region, ignoring the black line which was outside any threshold below 250. Wow, what happened?

But it's not as simple as that: Selections are their own 8-bit layer internally, and the selection mask outline is based on a 50% threshold. So enable your QuickMask and look closer -- the clicked white areas (and indeed, anything within the threshold) are 100% selected, but the areas outside the sensitivity threshold are ~50% selected.

On the other hand, if you disable antialiasing then the Fuzzy Select works as expected - areas within the threshold are 100% selected and areas without are 0%.

Is this a bug in the 'antialiasing' option, and why does it only occur on very high threshold values?

-- Stratadrake strata_ranger@hotmail.com
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Numbers may not lie, but neither do they tell the whole truth.