Re: [R] rgb and col2rgb color conversion/modification/shading

From: Earl F. Glynn <efg_at_stowers-institute.org>
Date: Wed 02 Aug 2006 - 05:21:42 EST

<ccarey@fhcrc.org> wrote in message
news:1154392412.44cea15c0c1fa@webmail.fhcrc.org...
>I want to get a lighter shade of a color...I have a lot of colored objects
>and
> want each one printed as a foreground against a slightly lighter
> background.
>
> I thought I could try something like changing the alpha channel by first
> converting it to rgb.

I'm not sure what you want to do with the alpha channel - it's sometimes used for transparency, especially on Macs, but is not used much on PCs (AFAIK). Let's say you want different shades of gold:

> colors()[142]

[1] "gold"

Instead of RGB color space perhaps you should consider HSV (Hue-Saturation-Value) color space.

Let's convert "gold" to rgb to hsv:

> col2rgb( colors()[142] )

      [,1]

red 255

green 215

blue 0

> rgb2hsv( col2rgb( colors()[142] ) )

       [,1]

h 0.1405229

s 1.0000000

v 1.0000000

The "hue" (h) is the color ranging from 0 to 1 around a color circle (with red= 0 or 1). Find h = 0.140 ("gold") in this color circle:

hue <- seq(0.0, 1.0, by=1/40)

pie(rep(1,40),

    labels=formatC(hue, digits=3, format="f"), cex=0.75,

    col=hsv(hue, 1.0, 1.0),

    radius=1.0,

    main="HSV (S=1, V=1)" )

Hues range from 0.0 to 1.0.

A color is saturated (s=1) when it is "far" from a shade of gray (ranging from black to white). Grays are unsaturated (no color) colors with s = 0. Saturation ranges from 0.0 to 1.0.

The value (v) is the brightness of the color. Low values appear quite dark but still have color. v=1 is as bright as possible. Values range from 0.0 to 1.0.

You can get different "shades" of the same color by varying changing the saturation and value for a given hue. The hsv function returns the RGB color in hex form.

Consider:

> hsv(0.1405, 1, 1)

[1] "#FFD700" Hex FF = decimal 255 = red

Hex D7 = decimal 215 = green

Hex 00 = decimal 0 = blue

Let's vary Saturation from 0.0 to 1.0 and Value from 0.0 to 1.0 in this plot:

MakeHSVRectangle <- function(saturation, value)

{

  GoldHue <- 0.140

  color <- hsv(GoldHue, saturation, value)

  rect(100*saturation, 100*value, 100*saturation+4, 100*value+4, col=color)

}

plot(0:110,0:110, type="n",

     xlab="Saturation[%]", ylab="Value[%]",

     main="Shades of Gold, H=0.140")

outer(seq(0.0, 1.0, 0.05), seq(0.0, 1.0, 0.05), MakeHSVRectangle)

With Value = 0, all colors are "black". With Saturation=0, the only "colors" along the y axis are the shades of gray. The original "gold" rectangle is at the upper right.

So, given a starting color, you have a number of "shades" (various saturations and values) with the same color hue.

I hope this helps.

efg

Earl F. Glynn

Scientific Programmer

Stowers Institute for Medical Research



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