How to capture this accidental effect

[nickbatz]

If you look closely at the area circled in white, you'll see that the little green blotches have thin outlines in the top picture. The bottom one has the same outlines, but they're not quite as pronounced.

It's doing that when the brush approaches, and the brush is much smaller than the area that's getting the lines.

What's going on and can you capture the effect?

[thomaso]

What brush did you use? Any divide/subtract blend mode involved or a layers blend range curve?

You can try to create such an effect for instance with the Detect Edges or Solarise filters.

[nickbatz]

Thanks very much.

I think I'm using Negation.