Predict Munsell hue / value / chroma from Vis-NIR reflectance spectra

Combines predict_xyz_from_spectra with the Munsell renotation interpolation in munsellinterpol (CRAN, GPL). Returns hue (e.g. "7.5YR"), value (0..10) and chroma (0..20) per sample, plus the soilKey fields

Usage

predict_munsell_from_spectra(spectra, wavelengths, round_chip = TRUE)

Arguments

spectra: Reflectance values, in 0..1 or 0..100. A numeric vector (one sample), a numeric matrix (rows = samples, cols = wavelengths) or a data.frame.
wavelengths: Numeric vector of the wavelengths (in nm) corresponding to the columns of spectra. Must cover at least 400-700 nm; values outside 380-780 are ignored.
round_chip: If TRUE (default), snaps the predicted HVC to the nearest standard Munsell chip grid via munsellinterpol::roundHVC(). FALSE returns continuous HVC (useful for further numeric work).

Value

A data.frame with columns munsell_hue_moist, munsell_value_moist, munsell_chroma_moist, munsell_string (e.g. "7.5YR 4/6"), X, Y, Z, one row per sample.

Details

The Munsell renotation is defined under Illuminant C, while the colorimetry here is computed under D65, so the conversion adapts D65 -> C. It calls munsellinterpol::XYZtoMunsell(XYZ, white =) (munsellinterpol >= 3.4-0), which performs that chromatic adaptation internally, and falls back to the numerically identical XYZ -> CIELAB(D65) -> LabToMunsell() route on older versions. Feeding D65 chromaticities straight to xyYtoMunsell() (with no white) would bias every colour toward green-yellow (a perfect neutral would return Chroma ~ 0.65 rather than 0); this routine avoids that. The D65 reference white is derived from the same bundled CIE table the colorimetry integrates against (so a constant-reflectance spectrum maps to an exact neutral, and a perfect reflecting diffuser to Munsell value 10), and the conversion is vectorised over all rows of spectra at once. At zero Chroma the Munsell hue is undefined, so a neutral is reported with hue "N" in both the rounded and the continuous (round_chip = FALSE) notation. munsell_hue_moist, munsell_value_moist, munsell_chroma_moist ready to write into a PedonRecord via the pedon's add_measurement method (see also fill_munsell_from_spectra).

This is the v0.9.47 unblock for the v0.9.35 Argissolo Vermelho / Amarelo / Vermelho-Amarelo color-confusion case: when a user has Vis-NIR spectra (which Embrapa's BDsolos / FEBR do not include but the OSSL does), the Munsell hue can be recovered physically without waiting for the surveyor's morphological description.

Examples

if (FALSE) { # \dontrun{
# White reflector across the visible: should map to a near-neutral
# high-value Munsell color.
wl <- seq(380, 780, by = 5)
R  <- rep(0.9, length(wl))
predict_munsell_from_spectra(R, wavelengths = wl)
} # }