Layer masks do not seem to support full 0-255 grayscale range
GIMP version: 2.10.8
Operating System: Windows 10
Description of the bug
According to the manual, "every color channel, including the alpha channel, has a range of possible values from 0 to 255".
However, when trying to paste or paint on a layer's mask, which is one of the staples of manipulating alpha in Gimp, there is a serious degradation in quality noticeable right away.
Original layer:
Mask created with "grayscale copy of layer":
I tried painting on the mask in some of the in-between colours, and it just didn't work, reverting to the closest of the colours in the degraded fade that was already there.
Reproduction
Reproduction steps:
- Create a new image. (I used grayscale colour space to ensure no actual conversion should be needed to the grayscale mask, but as the images show, the same thing happens in RGB.)
- Use the Gradient Tool, with Linear shape, without dithering, using ctrl to make a 100% straight line, to make a fade from white to black that is exactly 256 pixels long.
- Add a layer mask using the "grayscale copy of layer" option.
- Disable the layer mask.
- Toggle "show layer mask" to see the difference between the mask and the original grayscale image.
Expected result:
The mask and the original grayscale image are identical, with the fade changing exactly 1 in brightness for each pixel along the fade.
Actual result:
The mask seems to be unable to use the full scope of 256 gray values, instead showing bands of multiple identical pixel values along the fade.
UPDATE:
It seems only the 0-127 range of values is affected by this. The upper range seems fine.