Layer Mode: Addition equation is working differently to official guide

[Ofnuts]

It's not "linear light" which is a concept, it is "gamma correction". Linear light is just the ratio of the channel value to the maximum possible value (perfect diffused reflection) and is really what you see. Quick experiment:

• In the image below:
  • The NW corner is 50% black pixels 50% white pixels (picked at random)
    
  • The SE corner is a dense checker board with also by design 50% black pixels 50% white pixels
    
  • The NE corner is a uniform #7F7F7F (half-range in gamma corrected values)
    
  • The SW corner is a uniform #BCBCBC (half-range in linear values)(in the Pointer dialog, you will see that its Pixel value is very close to 0.5
    

   

• Open the image in a new tab (to make sure it is not scaled(*)...), or download and open in an image viewer
  
• Stand back sufficiently far to not make out the pixels or even the "grain" in the NW corner
  
• Note
  • Despite different implementation, the two 50/50 corners look the same because they roughly emit the same light
    
  • The NE corner is much darker,
    
  • The SW corner is about the same  as the NW and SE ones (if your display is well setup, they could be identical)
    

Also, if you use Filter > Bur > Pixelize on the NW/SE corners to average them out, you will find that their average color is #BCBCBC.... and the pixelization doesn't change the image much:

   

If you try the same thing on Gimp 2.8 (that works directly on the gamma-corrected value) that same filter yields a #7F7F7F and the pixellized areas are much darker:

   

(*) Because scaling directly on the gamma values (as done by many apps when doing thumbnails/previews) introduces the same kind of erroneous behavior as Gimp 2.8 Pixellize