I am using GIMP 2.10.12 on Windows 10. I worked through the Pat David article Luminosity Masks at https://www.gimp.org/tutorials/Luminosity_Masks/ in January. Because I wanted to use the method on a recent photo, I looked at it again and worked though the creation of the masks. However, it seemed that GIMP was applying the wrong channel to a layer.
I followed the article to create a Lights channel and a Darks channel. To examine each mask, I applied them to copies of the base image and used Show Layer Mask. The resulting image was backwards to what I got before and backwards compared to the result in the article: the dark mask appeared like the light mask and vice versa.
An image of the Lights mask from my work:
From the article, “what the three different channels of Darks look like:”
The article images were produced by a much earlier version of GIMP, which is the reason for the difference in appearance.
An image of the Darks mask from my work:
The Lights masks from the article:
Perhaps I am not using the right way to display the masks, but I have checked multiple times that I am applying the correct channel for my layers, and I always get this result. I have also reinstalled GIMP twice.