More progress. The internal model is almost completely transitioned to linear light and associated alpha (premultiplied color). Here in this video you can see how you can build up light intensity beyond what the display can show (without adjusting exposure, still need to add that OCIO controller). Then you can add opaque “darkness” back to bring back the original lines (since they were there all the time just brighter than the white).
Painting with a “zero opacity” or zero alpha brush is not intuitive at first glance, so I’m looking at either adding an addition control (alpha) or possibly changing the default blend mode to Multiply, and make this current “normal” mode into a new option like “additive mode”.
From the OpenEXR technical introduction:
Calling the color channels “premultiplied” does not mean that the color values in an image have actually been multiplied by alpha at some point during the creation of the image, or that pixels with zero alpha and non-zero color channels are illegal. Non-zero color with zero alpha is legal; such a pixel represents an object that emits light even though it is completely transparent, for example, a candle flame or a lens flare.