Convert Munsell colors to computer-friendly RGB triplets

Submitted by dylan on Fri, 2006-03-03 00:08.
Soil color conversion: Munsell in LUV colorspace
Figure 1: Munsell color chips.
Soil color conversion: LUV colorspace
Figure 2: Common soil colors.
Soil color conversion: RGB colorspace
Figure 3: Commom soil colors in RGB.
Soil color conversion: soil color matrix
Figure 4: Soil colors in multiple color spaces
Soil color conversion: Soil Profile in RGB colorspace
Figure 5: Soil profile colors.

The Munsell color system was designed as a series of discrete color chips which closely approximation to the color sensitivity of the human eye. The description of color via three variables tied to perceptible properties (hue, value, and chroma) under a standardized illuminant (sunlight on a clear day) makes the Munsell system a good choice for recording and interpreting soil color data. However, numerical analysis of colors encoded in the Munsell system is difficult because they are from a discrete set of color chips and referenced by values that include both letters and numbers. Rossel et. al. (2006) give a good background on various color space models and their relative usefulness in the realm of soil science. The conversion of Munsell soil colors to RGB triplets, suitable for displaying on a computer screen or printing, is made complicated by the numerous operations involved in converting between color spaces. Figure 1 shows all possible (both real and unreal) Munsell color chips in the L*U*V color space. Figure 2 shows some of the common soil color chips in the same color space. Figures 2 through 5 depict common soil colors in the RGB color space, visualized both in R and POVRAY. Example R code on the conversion is given below.

 
Munsell color data can be downloaded here.
 
Color conversion equations here.

 
References:

  1. Rossel, R.A.V.; Minasny, B.; Roudier, P. & McBratney, A.B. Colour space models for soil science Geoderma, 2006, 133, 320-337.

Manual Conversion in R

 
Setup environment and load lookup table data

## load some libs
library(plotrix)
library(colorspace)

## munsell data comes with a lookup table in xyY colorspace
## url: http://www.cis.rit.edu/mcsl/online/munsell.php

## note:
## Munsell chroma, CIE x, y, and Y. The chromaticity coordinates were calculated using illuminant C and the CIE 1931 2 degree observer.
all <- read.table("munsell-all.dat", header=T)

 
Convert xyY to XYZ [Equation Reference]

## x and y are approx (0,1)
## Y is approx (0,100)

## need manually rescale Y to (0,1)
all$Y <- all$Y/100.0

## do the conversion
X <- (all$x * all$Y ) / all$y
Y <- all$Y
Z <- ( (1- all$x - all$y) * all$Y )  / all$y

## combine to form matrix for simple manipulation
mun_XYZ_C <- matrix(c(X,Y,Z), ncol=3)

## test for y == 0
## X,Y,Z should then be set to 0
mun_XYZ_C[which(all$y==0),] <- c(0,0,0)

 
Perform Chromatic Adaption Functions in the colorspace package, and sRGB profiles assume a D65 illuminant [Reference]

## conversion matrix, from reference above
## this has been revised as of Jan, 2008
M_adapt_C_to_D65 <- matrix(c(0.990448, -0.012371, -0.003564, -0.007168, 1.015594, 0.006770, -0.011615, -0.002928, 0.918157), ncol=3, byrow=TRUE)


## perform the chromatic adaption: convert from C -> D65 using Bradford method
mun_XYZ_D65 <- mun_XYZ_C %*% M_adapt_C_to_D65


## how different are the two?
summary( (mun_XYZ_D65 - mun_XYZ_C)  )

 
Convert XYZ (D65) to sRGB (D65), step 1 this assumes that XYZ is scaled to (0,1) [Reference Primaries for sRGB]

## first get the reference primaries transformation matrix from above
##
## sRGB profile transformation:
M_XYZ_to_sRGB_D65 <- matrix(c(3.24071, -0.969258, 0.0556352, -1.53726, 1.87599, -0.203996, -0.498571, 0.0415557, 1.05707), ncol=3, byrow=TRUE)

## apply the conversion matrix
mun_sRGB_D65 <- mun_XYZ_D65 %*% M_XYZ_to_sRGB_D65

 
Convert XYZ (D65) to sRGB (D65), step 2 (sRGB, gamma = 2.4) [Conversion Function to sRGB]

## define the transformation functions:
## these are applied on a conditional basis:
fun1 <- function(col_comp) { 1.055 * ( col_comp ^ ( 1 / 2.4 ) ) - 0.055 }
fun2 <- function(col_comp) { 12.92 * col_comp }

## the specific function is contingent on the absolute value of r,g,b components
R <- ifelse(mun_sRGB_D65[,1] > 0.0031308, fun1(mun_sRGB_D65[,1]), fun2(mun_sRGB_D65[,1]))  
G <- ifelse(mun_sRGB_D65[,2] > 0.0031308, fun1(mun_sRGB_D65[,2]), fun2(mun_sRGB_D65[,2]))  
B <- ifelse(mun_sRGB_D65[,3] > 0.0031308, fun1(mun_sRGB_D65[,3]), fun2(mun_sRGB_D65[,3]))  


##clip values to range {0,1}
R_clip <- ifelse(R < 0, 0, R)  
G_clip <- ifelse(G < 0, 0, G)  
B_clip <- ifelse(B < 0, 0, B)  

R_clip <- ifelse(R > 1, 1, R_clip)  
G_clip <- ifelse(G > 1, 1, G_clip)  
B_clip <- ifelse(B > 1, 1, B_clip)


## add these back to the original table:
all$R <- R_clip
all$G <- G_clip
all$B <- B_clip

## done with the conversion

## the manually converted data
plot( as(RGB(R_clip,G_clip,B_clip), 'LUV'), cex=0.5)

AttachmentSize
soil_colors.pdf70.9 KB

Chromatic Adaption isn't correct

It seems there is an error in then matrix used for chromatic adaption. lindbloom revised his matrices on 14 of january 2008, maybe that's the reason why there's a little difference in the matrices.

Updates

Thanks for the heads-up. I have updated the appropriate equations to match the Lindbloom page. The resulting colors look pretty good!

## sample code extending the above examples to make a colorbook-like set of chips:
xyplot(V ~ C | factor(H, levels=c('2.5Y', '10YR', '7.5YR', '5YR', '2.5YR', '10R')),
main="UnCommon Soil Colors",
data=all, subset=H %in% c('2.5Y', '10YR', '7.5YR', '5YR', '2.5YR', '10R') & V > 1,
as.table=TRUE, subscripts=TRUE, xlab='Chroma', ylab='Value',
panel=function(x, y, subscripts, ...)
{
panel.xyplot(x, y, pch=15, cex=1, col=plot_cols[subscripts])
}
)

Example Color Book Page: generated within RExample Color Book Page: generated within R