on StackExchange we Wrote:

xenoid Wrote:I assume that after "Channel to selection" you bucket-filled the selection with I white on a black layer.

On Gimp 2.8 the behavior is as you expect(*). 2.8 works all over with 8-bit gamma-corrected values (so that there are more values to describe the dark tones).

In 2.10 things are different. Internally all computation is done in floating-point, using non-gamma-corrected values. When you copy the channel to a selection the plain channel data is copied, and since the selection is not really an image, there is no reason to gamma-correct it.

There is still a way to apply this gamma-correction, though. Assuming you have a selection from the Blue channel:

    Create a layer filled with black
    Add a new layer, and bucket fill the selection with white (you get the "too-light" result)
    Change the blend mode of the white layer from Normal to Normal (legacy) and you get your expected result (basically the "legacy" blend modes skip the gamma-correction).

(*) "Correct" is a bit strong here. There are plenty of ways to create monochrome images, all as a valid as the others.

I Wrote:Thanks. I did not create a white or black layer, I simply did Add Layer Mask and checked "Selection" but I'm sure it's the same.

joojaa Wrote:Are you sure its gamma corrected? And not inverse gamma corrected or profile corrected? Also its correctish assuming you manipulate images that are created with realworld physics and viewed with human eyes; Yes... But you might not

I Wrote:I did not apply a gamma correction, at least not knowingly. Is there a setting I may not be aware of?

xenoid Wrote:No that's just the way things work in 2.10, and why they kept the "legacy" blend modes for when you depend on the historical behavior (which itself is wrong in many occasions, they didn't change this without good reasons)