R help - Mar 2006 - extracting RGB values from a colorspace class object

Greetings,

After pouring over the documentation for the 'colorspace' package, I
have not
been able to figure out how the plot() method converts colorspace coordinates 
to RGB values for display on the screen. I am convert between colorspaces 
with the various as() methods... but cannot seem to find a way to extract RGB 
(i.e. for displaying on a computer screen) triplets from color space 
coordinates.

Here is a sample of the data that I am working with:
	H		V	C		x	y	Y
     2.5Y        0.2         2      1.43      0.97   0.237
     2.5Y        0.4         2     0.729     0.588   0.467
     2.5Y        0.6         2     0.563     0.491   0.699
     2.5Y        0.6         4     0.995     0.734   0.699
     2.5Y        0.8         2     0.479     0.439   0.943
     2.5Y        0.8         4      0.82      0.64   0.943
     2.5Y          1         2    0.4362    0.4177   1.21

I have converted xyY coordinates to XYZ coordinates, based on the equations 
presented here:
http://www.brucelindbloom.com/index.html?ColorCalcHelp.html

#read in the soil colors: munsell + xyY
soil <- read.table("soil_colors", header=F, 
col.names=c("H","V","C","x","y","Y"))

#convert to XYZ
X <- (soil$x * soil$Y ) / soil$y
Y <- soil$Y
Z <- ( (1- soil$x - soil$y) * soil$Y ) / soil$y

#make an XYZ colorspace object:
require(colorspace)
soil_XYZ <- XYZ(X,Y,Z)

#visualize the new XYZ colorspace object in the L*U*V colorspace:
plot(as(soil_XYZ, "LUV"), cex=2)

here is an example of the output:
http://169.237.35.250/~dylan/temp/soil_colors-LUV_space.png

Somehow the plot() method in the colorspace package is converting color space 
coordinates to their RGB values... Does anyone have an idea as to how to 
access these RGB triplets?

Thanks in advance!




-- 
Dylan Beaudette
Soils and Biogeochemistry Graduate Group
University of California at Davis
530.754.7341

Dylan Beaudette wrote:
> Greetings,
> 
> After pouring over the documentation for the 'colorspace' package,
I have not
> been able to figure out how the plot() method converts colorspace
coordinates
> to RGB values for display on the screen. I am convert between colorspaces 
> with the various as() methods... but cannot seem to find a way to extract
RGB
> (i.e. for displaying on a computer screen) triplets from color space 
> coordinates.
I don't know how it's done (I get really confused when trying to 
understand color spaces), but you'll have to download the code for the 
colorspace package and study the c code since that's where the 
conversion is done.

To get the RGB and hex triplets, this small example should help:

## Generate white in XYZ space
 > white = XYZ(95.047, 100.000, 108.883)
 > as(white,"RGB")
             R         G        B
[1,] 1.000008 0.9999907 1.000081
 > hex(white)
[1] "#FFFFFF"
> 
> Here is a sample of the data that I am working with:
> 	H		V	C		x	y	Y
>      2.5Y        0.2         2      1.43      0.97   0.237
>      2.5Y        0.4         2     0.729     0.588   0.467
>      2.5Y        0.6         2     0.563     0.491   0.699
>      2.5Y        0.6         4     0.995     0.734   0.699
>      2.5Y        0.8         2     0.479     0.439   0.943
>      2.5Y        0.8         4      0.82      0.64   0.943
>      2.5Y          1         2    0.4362    0.4177   1.21
> 
> I have converted xyY coordinates to XYZ coordinates, based on the equations
> presented here:
> http://www.brucelindbloom.com/index.html?ColorCalcHelp.html
> 
> #read in the soil colors: munsell + xyY
> soil <- read.table("soil_colors", header=F, 
>
col.names=c("H","V","C","x","y","Y"))
> 
> #convert to XYZ
> X <- (soil$x * soil$Y ) / soil$y
> Y <- soil$Y
> Z <- ( (1- soil$x - soil$y) * soil$Y ) / soil$y
> 
> #make an XYZ colorspace object:
> require(colorspace)
> soil_XYZ <- XYZ(X,Y,Z)
> 
> #visualize the new XYZ colorspace object in the L*U*V colorspace:
> plot(as(soil_XYZ, "LUV"), cex=2)
> 
> here is an example of the output:
> http://169.237.35.250/~dylan/temp/soil_colors-LUV_space.png
> 
> Somehow the plot() method in the colorspace package is converting color
space
> coordinates to their RGB values... Does anyone have an idea as to how to 
> access these RGB triplets?
> 
> Thanks in advance!
> 
> 
> 
> 

-- 
Jeffrey Horner       Computer Systems Analyst         School of Medicine
615-322-8606         Department of Biostatistics   Vanderbilt University