When you convert to L*a*b* space, the conversion depends on a function that uses a cube root for pixels where the luminance-to-luminance_max is >0.9%, but reverts to a linear function if you go below that. The baking-in of this piecewise function is where the remarks come from that L*a*b* is not well suited for sutuations where this ratio exceeds 100:1. This is why the scene-referred blending modes in the latest darktable releases use instead the JzCzhz colour space, which is a perceptual space like L*a*b*, but one that doesn’t suffer from this same weakness.
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