I noticed, that gaussian blur or similar convolution matrix works somehow unevenly at contrast edges, between light and dark areas. When filter applies… literally, light pushes darkness a bit. Best way to see it in gimp - create copy of layer to be blured and invert (I used non-linear inversion). Blur both, then reverse copy back. Difference should be visible when toggling second layer. Comparing each other, both have disbalance, but in second (which was reversed), darkness wins (sic).
It seems to be problem of any approach, where additive/subtractive only approach is used. For example, if not this issue, sharpening, applied after blur so, that it exactly negates blur, should restore almost original image. Instead - when I did such experiment with white-on-black text (rendered grayscale), text became slightly bolder after sharpening. For most grotesque effect (on gimp) sharpening could be applied to negative while blur is not (or first negated than normal).
Now about blur solution. I got almost even blur by blending second layer (blured while negated) with 50% transparency. However - despite of visually correct margins, I’m not sure is it same about resulting blur size. More what I’m exactly sure in - this approach seems mismatch for sharpening.
I wonder, are there better approaches, probably differing by blending or each blured pixel?
At the moment of writing this - I forgot another test: blur could be applied after reversions pair (1 - linear, 2 - non-linear), which is also repeated after blur, but in reverse order (linear is second). At this moment my system undergoes full upgrade (I use gentoo-based distro), and I just noticed, that gimp doesn’t show gui (could be due to immense system load though), so I have no even illustrations yet.