Solution was something apparently studios/tech artists know.... and yet i find ZERO info on it‽
"Blend-Add" as it was called; uses the alpha to describe blending; lerping from additive when black (transparent) to traditional blend as you transition to white (opaque)
we are familiar with
Blend SrcAlpha OneMinusSrcAlpha // Alpha blending Blend One One // Additive Blend OneMinusDstColor One // Soft Additive Blend DstColor Zero // Multiplicative Blend DstColor SrcColor // 2x Multiplicative
I thought perhaps "Soft" additive was it, but that is the "screen" layering nomenclature as seen in Photoshop 1−(1−A)*(1−B)
aka (1-A) * B + A
I tried every PS layering technique. nada.
i'm sure this is a double .. possibly triple pass through CG, but as I said, I can't find ANY info.