nx_skinsample_lib.nx

nx_skinsample_lib.nx -- THE FIRST PHOTO RULER FOR THE TWIN CARD: skin colour MEASURED from a mirrored photograph (aesthetictwin AT23 rulers skin_L skin_a skin_b ita, 2026-09-16). Decodes the JPEG through the estate's own baseline and progressive decoder (nx_jpeg_ascii.nx composes nx_jpeg_decode and nx_jpeg_progressive), keeps the pixels a declared skin chroma rule admits, averages them in LINEAR light (never in gamma bytes), converts the mean to CIE L*a*b* with the transform constants nx_skin_ita already carries (one colour organ, never two), and takes the individual typology angle through the estate's fixed-point CORDIC arctangent (nx_fixq30_lib). Every number a card receives from this ruler is MEASURED: method measured, source the declared reference key, and the receipt prints the mask count beside the mean so a reader can see how much of the image the value rests on. WHAT IS CHOSEN, said plainly: the skin mask is the YCbCr chroma rule Cb in [77,127] and Cr in [133,173] (a published hand-tuned rule, not a model of this subject) with a luma gate that drops clipped shadows and highlights. It admits skin under most daylight and studio light and also admits skin-coloured backgrounds, sand and wood; the count and the coverage are therefore PRINTED and journaled with the value, and a region chart (a face or torso window) narrows the mask when the card names one. What is DERIVED: the sRGB transfer, the D65 matrix, the Lab transform and the ITA definition (Chardon 1991). LIB, no main. license_tier: ORIGINAL No hw writes (Rule 26).