Very strange hidden paint data in a layer

I noticed it when trying to use the Global Remesh feature.
This is what it looks like normally:

perfectly fine. Then hit the remesh button, you can see dark paint artefacts:

So I thought of a possible fix to remedy the issue: just turn down the color slider inside the layers to 0 (which does get rid of the paint) and then just merge the layer down into an empty dummy layer, But this is what happens:

I tested the same thing on a different project, and it doesn’t have this problem.
Maybe I inadvertently enabled paint information without knowing as I was sculpting? But if so, how come it never showed up before?

For now the only way I know to fix the issue is to manually erase by hand with the paint tool, but maybe there is an underlying issue…

Is filling an object with full white color, %50 gray roughness and full black metalness the same as erasing paint data?

If you want to erase paint data you need to use DelLayer it’s the only way.
(Otherwise just set the sub layer paint slider to 0 to ignore painting on this layer)

There’s a hidden opacity channel associated with each layer, it’s used to resolve layer compositing (the order of layers matters for painting).

Back to the topic.
I can reproduce the bug when you merge a layer onto another layer while the top layer have the color slider at 0 (if it’s slightly above 0 it’s working fine).

Ok long story short, 2 issues:

  • the merging issue: this one is an annoying one because the only way for me is to have an opacity per channel (like substance) instead of a single opacity for color/roughness/metalness. It’s something that have been bothering me since ages so I’ll simply add an opacity per channel (not sure when, it might take some times as I’d like to do things smart and avoid extra memory, especially if I add more channel in the future like SSS or emissive).
    As a bonus point, it will also be possible to use the paint channel masking for layers as well.

  • black thing appearing after remesh: I have no idea, if there is a simple way to replicate it would be nice

Thanks for the clarification on paint data.

I can 1000% appreciate this!

I don’t mind at all any of the above mentioned fixes, as they are fairly painless band-aids to the problem.
I didn’t get that merging color information works like that…so I would just simply avoid doing that to begin with.

Alright, I figured out what was happening to my mesh:
tldr: user error

I did, in fact, accidentally enabled painting white in the middle of sculpting without realizing. Because it’s white, there’s no visible change in the viewport.

It happens because in the process of sculpting, I’ll sometimes switch to pbr and access the quick paint panel to select white and change roughness to get a preview of how the highlights hits the forms. I don’t actually fill the object, just use the real-time preview, and then switch back to matcap mode and carry on my business.

When you change the paint settings it automatically turns on painting for that brush.
In previous versions without the red slash icon there was no immediate way to see if it was enabled or not. But with the added slash icon you can catch the change.

At least mystery solved.