Creating an "opposite" image

[Krikor]

(03-30-2022, 05:13 PM)Ofnuts Wrote: A faint gap is normal and expected!

If your process is correct, the pixels that are anti-aliased black-to-transparent become anti-aliased transparent-to-black, or, in other words, they get the complementary opacity so are still only partially opaque. So when you overlap both images, you overlap partially opaque pixels with other partially opaque pixels and cannot get a full opacity, so you see a faint gap.

If there is no gap, the opposite image is too thick, and if there are fully transparent pixels in the gap, the opposite image is too thin.

It's the same as doing this:

• Create a layer filled with black and duplicate it.
  
• Create a circle selection
  
• On one layer, delete the selection
  
• On the other layer, Select > Invert and delete the selection
  
• Check the result over a white layer
  
• You can also (without a white background) use the pointer dialog to check the alpha of the pixels in the gap and check that all pixels are above 75% opacity(*)
  

(*) because the pixels with opacity X in one layer will have opacity 1-X in the other. When you apply this in the alpha-compositing formula the resulting opacity is expressed by 1-X+X² which in the [0.0, 1.0] range has a minimum of 0.75 for X=0.5

I must be doing something wrong at some point.
I couldn't reproduce the expected faint gap.

In any case, it explains why Denzjos doesn't have that faint gap. With the stroke path, a belt is created that covers this gap.

  FaintGap.xcf (Size: 10.05 KB / Downloads: 148)

---

(03-30-2022, 05:24 PM)PixLab Wrote: I found something else. just for fun  

A Black layer under original
Original in Color Erase mode (or erase or split)
New from visible  (or faster with merge down button)
Fuzzy the outside on the new from visible
select Grow 1 or more
delete

gap almost non existent (you really need to zoom in to see some trace of transparency)

PixLab, I think you've squeezed it to the limit!
In terms of result and simplicity this method seems to be a winner!

I took advantage of the previous Ofnuts post and examined the faint gap generated by your last method.

In the coordinate X=742, Y=422 I found the following values of Alpha:
18.8% on the top layer and 81.2% on the bottom layer.

If I use the Color Picker with the "Sample merged" option selected, I get a value of 100% for that same point.
However visually the faint gap is still visible.