wl2rgb: Wavelengths to Colours

In emmanuelparadis/sentinel: Sentinel

Wavelengths to Colours

Description

Conversion from wavelengths to colours.

Usage

wl2col(x, gamma = 0.8, RGB = FALSE)
spectrum2col(spec, RGB = FALSE, no.warn = TRUE, color.system = 3)
BlackBodySpectrum(x, Temp = 300)

Arguments

x	a numeric vector with wavelengths in nanometers (nm).
gamma	parameter for correcting the transitions. If gamma = 1, then the transitions are linear (see examples).
RGB	a logical value. By default, colours (in HTML code) are returned. If RGB = TRUE, a matrix with three columns is returned.
spec	a numeric vector with 81 values giving the (relative) intensity of the different wave lengths between 380 nm and 780 nm (see examples).
no.warn	a logical value. If TRUE and some approximate calculations were performed in the C routine, a warning message is issued.
color.system	a single integer between 1 and 6 specifying the colour system (see details).
Temp	temperature in Kelvins (K) of the black body.

Details

Computations are mainly performed by C and Fortran codes (see References).

The argument spec gives the (relative) intensity of visible light between 380 nm and 780 nm in intervals with a bandwith of 5 nm (i.e., [380–385], [385–390], ..., [775-780]). The returned value is the perceived colour of the given spectrum. It could happen that some calculations were approximate which is done silently unless no.warn = FALSE.

The six colour systems are: (1) NTSC, (2) EBU (PAL/SECAM), (3) SMPTE, (4) HDTV, (5) CIE, and (6) CIE REC 709.

BlackBodySpectrum calculates the emittance at specified wavelength(s) of a black body of temperature Temp using Planck's law.

Value

wl2col and spectrum2col return by default a vector of mode character with colours in HTML code. If RGB = TRUE, they return a matrix with the values (between 0 and 1) of red, green, and blue arranged in a three-column matrix. If the input x has names, these are used in the returned object (as names or rownames).

BlackBodySpectrum returns a numeric vector.

Author(s)

Emmanuel Paradis, John Walker, Dan Bruton

References

Bruton, D. (1996) Approximate RGB values for visible wavelengths. http://www.physics.sfasu.edu/astro/color/spectra.html

Planck, M. (1901) Ueber das Gesetz der Energieverteilung im Normalspectrum. Annalen der Physik, 309, 553–563. (English translation: http://web.ihep.su/dbserv/compas/src/planck01/eng.pdf)

Walker, J. (1996) Color rendering of spectra. https://www.fourmilab.ch/documents/specrend/

Examples

wl <- 370:790
COLS <- c("red", "green", "blue")
if (interactive()) layout(matrix(1:3, 3))
matplot(wl, wl2col(wl, , TRUE), "l", col = COLS, lty = 1, lwd = 3)
title("gamma = 0.8 (default)")
matplot(wl, wl2col(wl, 1, TRUE), "l", col = COLS, lty = 1, lwd = 3)
title("gamma = 1")
matplot(wl, wl2col(wl, 1/3, TRUE), "l", col = COLS, lty = 1, lwd = 3)
title("gamma = 1/3")
layout(1)

spec <- numeric(81)
spec[2] <- 1
names(spec) <- seq(380, 780, 5)
sapply(1:6, function(i) spectrum2col(spec, TRUE, color.system = i))

WL <- 380:780
xlab <- "Wavelength (nm)"
ylab <- expression("Emittance (W."*m^{-2}*")")
plot(WL, BlackBodySpectrum(WL, 306), type = "l", xlab = xlab,
     ylab =ylab, log = "")
lines(WL, BlackBodySpectrum(WL, 303), lty = 2)
legend("topleft", legend = paste(c(306, 303) - 273, "degrees C"), lty = 1:2)


## vector of wavelengths:
wl <- seq(382.5, by = 5, length.out = 81)
spectrum2col(BlackBodySpectrum(wl, 310), TRUE)
spectrum2col(BlackBodySpectrum(wl, 3100), TRUE)
spectrum2col(BlackBodySpectrum(wl, 31000), TRUE)

wl <- 10:1e5
col <- wl2col(wl)
plot(wl, BlackBodySpectrum(wl, 6000), "n", log = "xy", ylim = c(1, 1e14),
     xaxs = "i", xlab = xlab, ylab = ylab)
s <- col != "#000000" # do not show the black lines
abline(v = wl[s], col = col[s])
lines(wl, BlackBodySpectrum(wl, 300))
lines(wl, BlackBodySpectrum(wl, 3000))
lines(wl, BlackBodySpectrum(wl, 6000))
text(c(3000, 240, 200), c(5e6, 6e10, 5e13), paste(c(300, 3000, 6000), "K"))

emmanuelparadis/sentinel documentation built on May 2, 2024, 4:52 a.m.