05-13-2024, 11:50 PM
This is banding, caused by a "slow" gradient where an identical change of values occur at the same time on neighboring pixels and your brain interprets it as a line.
In your case this is worsened by round-off errors and the quantization that happens because you are in a low precision image. If you change the image to high-precision (Image > Precision) (typically 32-bit FP, the other HP modes are rarely useful) and repaint, the result should be a lot better as far as my tests go. But the banding can reappear later, when you will export the image to a low-precision format (JPEG or 8bpc PNG). You can mitigate it using Filters > Noise > Spread. This stirs the pixels a bit and the "identical change of values" no longer occurs on a line so our brains don't perceive it.
In your case this is worsened by round-off errors and the quantization that happens because you are in a low precision image. If you change the image to high-precision (Image > Precision) (typically 32-bit FP, the other HP modes are rarely useful) and repaint, the result should be a lot better as far as my tests go. But the banding can reappear later, when you will export the image to a low-precision format (JPEG or 8bpc PNG). You can mitigate it using Filters > Noise > Spread. This stirs the pixels a bit and the "identical change of values" no longer occurs on a line so our brains don't perceive it.