Layer modes GIMP documentation does not mention gamma correction nor opacity component.
GIMP version: 2.10+, Online Documentation
Operating System: Any, Web
Description of the bug
Layer mode blending documentation does not mention that gamma correction is removed before blending layers, and re-applied after. In particular, the fact that GIMP uses the following conversion to a linear color space is not mentioned: http://entropymine.com/imageworsener/srgbformula/
The documentation also does not mention how opacity is incorporated in any blending mode.
Both of these lead to false descriptions of the blending algorithms that GIMP actually uses.
A suggested note is as follows:
Prior to blending, images have gamma correction removed and are converted from sRGB to linear via:
sRGB to Linear 0 ≤ S ≤ 0.04045 => L = S/12.92 0.04045 < S ≤ 1 => L = ((S+0.055)/1.055)^2.4
After blending, they are converted back from linear to sRGB via:
Linear to sRGB 0 ≤ L ≤ 0.0031308 => S = L * 12.92 0.0031308 < L ≤ 1 => S = 1.055 * L^(1/2.4) − 0.055
However. Before converting back to sRGB, opacity is applied.
E = E * O + I * (1.0 - O)
Where 'O' is the opacity of the blend mode, and 'E' & 'I' are as below.