R: RGB to HSV Conversion

rgb2hsv {grDevices} | R Documentation

## RGB to HSV Conversion

### Description

`rgb2hsv` transforms colors from RGB space (red/green/blue) into HSV space (hue/saturation/value).

### Usage

    rgb2hsv(r, g = NULL, b = NULL, maxColorValue = 255)

### Arguments

`r` | vector of ‘red’ values in [0, M], (M = `maxColorValue`) or 3-row rgb matrix.
`g` | vector of ‘green’ values, or `NULL` when `r` is a matrix.
`b` | vector of ‘blue’ values, or `NULL` when `r` is a matrix.
`maxColorValue` | number giving the maximum of the RGB color values range. The default `255` corresponds to the typical `0:255` RGB coding as in `col2rgb()`.

### Details

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.

### Value

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.

### Author(s)

R interface by Wolfram Fischer wolfram@fischer-zim.ch;
C code mainly by Nicholas Lewin-Koh nikko@hailmail.net.

### See Also

`hsv`, `col2rgb`, `rgb`.

### Examples

    ## 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), tolerance = 1e-14) # TRUE

[Package grDevices version 3.1.2 Index]