| rgb2hsv {grDevices} | R Documentation |
rgb2hsv transforms colors
from RGB space (red/green/blue)
into HSV space (hue/saturation/value).
rgb2hsv(r, g = NULL, b = NULL, maxColorValue = 255)
r |
vector of ‘red’ values in [0, M], (M
= |
g |
vector of ‘green’ values, or |
b |
vector of ‘blue’ values, or |
maxColorValue |
number giving the maximum of the RGB color values
range. The default |
Value (brightness) gives the amount of light in the color.
Hue describes the dominant wavelength.
Saturation is the amount of Hue mixed into the color.
An HSV colorspace is relative to an RGB colorspace, which in R is sRGB, which has an implicit gamma correction.
A matrix with a column for each color. The three rows of the matrix
indicate hue, saturation and value and are named "h",
"s", and "v" accordingly.
R interface by Wolfram Fischer wolfram@fischer-zim.ch;
C code mainly by Nicholas Lewin-Koh nikko@hailmail.net.
## These (saturated, bright ones) only differ by hue
(rc <- col2rgb(c("red", "yellow","green","cyan", "blue", "magenta")))
(hc <- rgb2hsv(rc))
6 * hc["h",] # the hues are equispaced
(rgb3 <- floor(256 * matrix(stats::runif(3*12), 3,12)))
(hsv3 <- rgb2hsv(rgb3))
## Consistency :
stopifnot(rgb3 == col2rgb(hsv(h=hsv3[1,], s=hsv3[2,], v=hsv3[3,])),
all.equal(hsv3, rgb2hsv(rgb3/255, maxColorValue = 1)))
## A (simplified) pure R version -- originally by Wolfram Fischer --
## showing the exact algorithm:
rgb2hsvR <- function(rgb, gamma = 1, maxColorValue = 255)
{
if(!is.numeric(rgb)) stop("rgb matrix must be numeric")
d <- dim(rgb)
if(d[1] != 3) stop("rgb matrix must have 3 rows")
n <- d[2]
if(n == 0) return(cbind(c(h=1,s=1,v=1))[,0])
rgb <- rgb/maxColorValue
if(gamma != 1) rgb <- rgb ^ (1/gamma)
## get the max and min
v <- apply( rgb, 2, max)
s <- apply( rgb, 2, min)
D <- v - s # range
## set hue to zero for undefined values (gray has no hue)
h <- numeric(n)
notgray <- ( s != v )
## blue hue
idx <- (v == rgb[3,] & notgray )
if (any (idx))
h[idx] <- 2/3 + 1/6 * (rgb[1,idx] - rgb[2,idx]) / D[idx]
## green hue
idx <- (v == rgb[2,] & notgray )
if (any (idx))
h[idx] <- 1/3 + 1/6 * (rgb[3,idx] - rgb[1,idx]) / D[idx]
## red hue
idx <- (v == rgb[1,] & notgray )
if (any (idx))
h[idx] <- 1/6 * (rgb[2,idx] - rgb[3,idx]) / D[idx]
## correct for negative red
idx <- (h < 0)
h[idx] <- 1+h[idx]
## set the saturation
s[! notgray] <- 0;
s[notgray] <- 1 - s[notgray] / v[notgray]
rbind( h=h, s=s, v=v )
}
## confirm the equivalence:
all.equal(rgb2hsv (rgb3),
rgb2hsvR(rgb3), tol=1e-14) # TRUE