Another day and....using kubuntu 24.04 / Gimp 3.2.4
A few tests and it is a Floating Point (FP) precision issue. As a comparison:
(1) is a 8bit image promoted to 32FP Image -> Encoding -> 32-bit floating point and a Black (full transparency) layer mask
(2) Painting into the layer mask with both white and out-of-gamut white
(3) Using 32 bit fixed
A quick fix is convert (Image -> Encoding) to 32 bit fixed and back to 32 floating.
A better fix is not use out-of-gamut white in the first place.
I think this has been reported before, but not sure. I can find some old posts concerning 32FP colours but nothing exact. Edit: It is the same with Gimp 2.10.38, so nothing new.
A few tests and it is a Floating Point (FP) precision issue. As a comparison:
(1) is a 8bit image promoted to 32FP Image -> Encoding -> 32-bit floating point and a Black (full transparency) layer mask
(2) Painting into the layer mask with both white and out-of-gamut white
(3) Using 32 bit fixed
A quick fix is convert (Image -> Encoding) to 32 bit fixed and back to 32 floating.
A better fix is not use out-of-gamut white in the first place.
I think this has been reported before, but not sure. I can find some old posts concerning 32FP colours but nothing exact. Edit: It is the same with Gimp 2.10.38, so nothing new.