[Gimp-user] Layer addition - bug, feature, or user misunderstanding?

[Elle Stone <ellestone ninedegreesbelow com>]

Using GIMP 2.9 updated yesterday, two different ways of adding layers 
produce different results. But it seems to me that the two ways should 
produce the same results. Here is a screenshot:
http://ninedegreesbelow.com/bug-reports/gimp29/layer-addition/addition-results-vary.png,

Are the different results a bug, a feature, or am I making an obvious 
mistake or just not understanding something?

Looking at the screenshot, the "channel" layers were produced by making 
a solid white layer and dragging the Red, Blue, and Green channels over 
to the layer stack. So of course each channel layer also has R=G=B=1.0.

The channel layers are added using layer percent opacities of 22.2 for 
the Red channel layer, 71.7% opacity for the Green channel layer, and 
6.1% opacity for the Blue channel layer. The percentages are the correct 
percentages for producing a Luminance conversion to black and white by 
adding the Red, Green, and Blue channels together as layers.

The result of adding the three layers should be white, R=G=B=1.0, which 
is what happens with the second way of adding the layers. But the first 
way, using the more obvious "add each layer to the layers below", 
produces R=G=B=0.793257.

Here's a download link for the actual XCF file:
http://ninedegreesbelow.com/bug-reports/gimp29/layer-addition/white-gimpdefault.xcf 


You'll need to reset *all* of the layer opacities to the values given 
above, because for some reason saving to disk and reopening causes layer 
opacities to shift slightly (for example, the Blue layer opacities shift 
to 5.9% instead of staying at 6.1%).

The image is an sRGB image and the precision is 32-bit floating point 
(linear) in order to get the layers to properly add up to R=G=B=1.0, 
which they should anyway for solid white, but results would be wrong for 
colors other than solid white or solid black.

Elle, puzzled
--
http://ninedegreesbelow.com
Color management and free/libre photography