rgb2hsv              package:grDevices              R Documentation

_R_G_B _t_o _H_S_V _C_o_n_v_e_r_s_i_o_n

_D_e_s_c_r_i_p_t_i_o_n:

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

_U_s_a_g_e:

     rgb2hsv(r, g = NULL, b = NULL, gamma = 1, maxColorValue = 255)

_A_r_g_u_m_e_n_t_s:

       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.

   gamma: a "gamma correction" (supposedly applied to the r,g,b values
          previously), see 'hsv(...., gamma)'.

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()'.

_D_e_t_a_i_l_s:

     Value (brightness) gives the amount of light in the color.
      Hue describes the dominant wavelegth.
      Saturation is the amount of Hue mixed into the color.

_V_a_l_u_e:

     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.

_A_u_t_h_o_r(_s):

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

_S_e_e _A_l_s_o:

     'hsv', 'col2rgb', 'rgb'.

_E_x_a_m_p_l_e_s:

     ## 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(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, maxC = 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

