Mix Munsell Colors via Spectral Library
Simulate mixing of colors in Munsell notation, similar to the way in which mixtures of pigments operate.
mixMunsell(
x,
w = rep(1, times = length(x))/length(x),
mixingMethod = c("spectra", "estimate", "adaptive"),
n = 1,
keepMixedSpec = FALSE,
distThreshold = 0.025
)x |
vector of colors in Munsell notation |
w |
vector of proportions, can sum to any number |
mixingMethod |
approach used to simulate a mixture:
|
n |
number of closest matching color chips ( |
keepMixedSpec |
keep weighted geometric mean spectra, final result is a |
distThreshold |
spectral distance used to compute |
An accurate simulation of pigment mixtures ("subtractive" color mixtures) is incredibly complex due to factors that aren't easily measured or controlled: pigment solubility, pigment particle size distribution, water content, substrate composition, and physical obstruction to name a few. That said, it is possible to simulate reasonable, subtractive color mixtures given a reference spectra library (350-800nm) and some assumptions about pigment qualities and lighting. For the purposes of estimating a mixture of soil colors (these are pigments after all) we can relax these assumptions and assume a standard light source. The only missing piece is the spectral library for all Munsell chips in our color books.
Thankfully, Scott Burns has outlined the entire process, and Paul Centore has provided a Munsell color chip reflectance spectra library. The estimation of a subtractive mixture of soil colors can proceed as follows:
look up the associated spectra for each color
computed the weighted (area proportion) geometric mean of the spectra
search for the closest matching spectra in the reference library
suggest that Munsell chip as the best candidate for the simulated mixture
Key assumptions include:
similar particle size distribution
similar mineralogy (i.e. pigmentation qualities)
similar water content.
For the purposes of estimating (for example) a "mixed soil color within the top 18cm of soil" these assumptions are usually valid. Again, these are estimates that are ultimately "snapped" to the nearest chip and not do not need to approach the accuracy of paint-matching systems.
A message is printed when scaledDistance is larger than 1.
A data.frame with the closest matching Munsell color(s):
munsell: Munsell notation of the n-closest spectra
distance: spectral (Gower) distance to the n-closest spectra
scaledDistance: spectral distance scaled by distThreshold
mixingMethod: method used for each mixture
When keepMixedSpec = TRUE then a list:
mixed: a data.drame containing the same elements as above
spec: spectra for the 1st closest match
D.E. Beaudette
inspiration / calculations based on the work of Scott Burns: https://arxiv.org/ftp/arxiv/papers/1710/1710.06364.pdf
related discussion on Stack Overflow: https://stackoverflow.com/questions/10254022/implementing-kubelka-munk-like-krita-to-mix-colours-color-like-paint/29967630#29967630
spectral library source: https://www.munsellcolourscienceforpainters.com/MunsellResources/SpectralReflectancesOf2007MunsellBookOfColorGlossy.txt
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