Created attachment 180044 [details]
Buil up issue in 8bit document

SUMMARY

Tested with Krita 5.2.9 from Krita.org, 5.3 prealpha and 6.0 prealpha (Windows 10) - no pen, only touchpad of the notebook.

krita-artists.org thread:
https://krita-artists.org/t/brush-build-up-mode-issue-at-low-opacity-values/120242

Brush (pixel engine) is set to “Build up” in the settings (all pressure settings are off). As I understand it, in this mode the “Flow” settings get ignored (they don’t have an effect) and “Opacity” acts like the “Flow” parameter.

This mode builds up color in a single stroke if I paint over the same area multiple times (like with a real pencil on paper where you move it forth and back without lifting it).

What I expexted:
Regardless of what “Opacity” value I set for the brush, if paint over the same area often enough I will get 100% color.

But as shown in the screen shot, if the Opacity is at a low level (1% for example) this does not work. I paint endlessly over the same area but don’t get to 100%.

Additional information from Freyalupen posted in the above linked thread:

"Checking the color numbers, 16bit integer isn’t perfect either. With pure green on black I go from 65535 green to 65486. On 8bit it’s 255 to 206. Interestingly the difference in both cases is 49, suggesting they have the same problem. On 16bit it’s a visually indistinguishable .07% difference, but on 8bit it’s huge 19% difference."


STEPS TO REPRODUCE
1. Make a new Krita document (in 8bit mode the issue is visually obvious, in 16bit integer it is there as well but not visually perceptible)
2. Set a pixel engine brush to use build up mode
3. Set brush opacity to 1%
4. Paint over the same area multiple times without lifting the brush

OBSERVED RESULT
Color does not get to 100%

EXPECTED RESULT
Color gets to 100%

SOFTWARE/OS VERSIONS
Windows: 10 but happens with other versions as well.