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?