Thinking about the sky mask, C.2.3 Mask for Curves for sky, my understanding is that the curves adjustment shouldn’t affect the non-selected area, so it wouldn’t matter what degree of transparency the mask had in that area, there should be no change to the image. And since over 95% of the selected area is white, the only area where the degree of transparency (the tone of grey) would be a factor is that small amount of grey within the selection. Why bother with adjusting the transparency for so little an effect, unless as afre says, the selection is a soft one, which I don’t at this time understand how that is done? I also don’t understand what he means by among the colour channels.
Based on your screenshot, it looks like the channels aren’t being affected evenly. I don’t remember the details of the tutorial, so my reply might be misleading.
Newbie error: I didn’t understand selections. From the manual:
“A GIMP selection is actually a full-fledged grayscale channel, covering the image, with pixel values ranging from 0 (unselected) to 255 (fully selected). The marching ants are drawn along a contour of half-selected pixels. Thus, what the marching ants show you as either inside or outside the boundary is really just a slice through a continuum.”
If you use the rectangle select tool, everything inside the selection will be 255. Selections by other methods can have pixel values less than that and can have RGB values affected by editing.
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