(03-07-2024, 08:50 AM)Ofnuts Wrote: No, because Gimp 2.10 is working in "Linear light". 

The 0 .. 255 values are gamma-corrected. In other words,they don't represent directly the amount  of light in the color channel, but some encoding which is convenient to pack as much as possible of the visual range in 255 values, taking in consideration that our eyes are more sensitives to differences in low values.

If you image is in some high precision linear mode, you can check the linear value with the Pixel representation in the Pointer and Sample points dialog

In theory this would mean:

Linear = ( RGB255 / 255 ) ^ 2.2

and

RGB255 = 255 * ( Linear ^ ( 1 / 2.2 ) )

In practice the conversion is done using the image color profile but if the color profile is sRGB (which is the case most of the time), the formulas above give a good approximation.

For instance your top color:

So Addition mode  is really "Convert to Linear, add the values, convert back to RGB255", and using the formulas you are not so far off:

Note that you can ask Gimp to work directly with the RGB255 values, by using the "Legacy" layer modes:



In which case the result is probably what you expect:



However, if this makes mathematical sense, this has no relation to the physical reality.

• 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)