Addition layer and its treatment of bright colours

This image summarises my mathematical struggles with the seemingly simple rule of “Addition” layer modes in different graphics applications (tested with GIMP and Inkscape). Since the greyscale layers of RGB decomposed images are subject to the same non-additive behaviour, even this is not viable. Neither does linear light RGB mode (via Image → Precision → Linear light) nor Composite space → RGB (linear) (in the layers context menu) come to our rescue, there still is 0.5+0.5=0.688 always.

I erroneously thought that the simple, component-wise evaluated formula E=min((M+I),255) taken from the official documentation means: values from layer M and I are added (and clipped if necessary). But this is obviously not the case.

Is there a way to obtain plain stupid component-wise addition, without such probably well-intentioned but badly documented Gamma mess?

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    Hi. Welcome to GDSE. I think this is probably a question for GIMP's developers. The maths behind their implementation is not really a graphic design question, even though it may be quite interesting. – Billy Kerr Apr 3 '20 at 14:57
  • It works in Photoshop just as expected. – joojaa Apr 3 '20 at 15:02
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    That equation you linked to seems to relate to the Legacy Addition (I) Mode, and not the current GIMP 2.10 Default Addition mode. You can switch between Legacy and Default modes in the layers panel. It looks like that page in the user manual hasn't been updated yet for the new Default Addition mode. I have no idea where you can find it. Perhaps ask the GIMP devs. – Billy Kerr Apr 3 '20 at 15:42
  • @Billy Kerr: I opened GIMP 2.10.18 and navigated via Help → Gimp Online → User Manual Web Site to docs.gimp.org/2.10/en/gimp-concepts-layer-modes.html, that is word by word the same as in my link, just with hard-coded version number in the URL. If GIMP help is indeed so horribly outdated, then this is a real issue for GIMP. But still doesn't explain why Inkscape does it the same way. – H. Weirauch Apr 3 '20 at 16:26
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    @Moini - then that seems to makes perfect sense. The new default Addition mode in GIMP appears to be attempting the same kind of blend mode. – Billy Kerr Apr 3 '20 at 20:37