[R] [OT] two question about color space.

[baptiste auguie]

On 14 Mar 2009, at 13:08, Jinsong Zhao wrote:

>> Hi,
>>
>> For a good discussion of the link between colour and spectra I would
>> suggest,
>>
>> http://www.fourmilab.ch/documents/specrend/
>>
>> which provides an open-source C code to perform the conversion you  
>> ask
>> for. I asked for some advice on how to wrap a R function around this
>> code last week but sadly I didn't get anywhere. Do let me know if you
>> succeed. (alternatively, one could port the implementation in pure R
>> as the code is not too complicated or computationally demanding).
>>
>> Hope this helps,
>>
>> baptiste
>>
>
> I have visited the site before I posted my question. I don't know  
> what's
> the relationship between black body spectra and specific wavelength.
> And frankly, I don't understand C.

The black body radiation in that code is only provided as a test case,  
as far as i see it.

>
> I have translate a Fortran code
> (http://www.isc.tamu.edu/~astro/color/spectra.html) to R as given  
> below,
> however, I don't know how to modify it and make it could accept a
> vector, e.g., 380:780, as input.

if you don't mind about the efficiency of your program, just use ? 
sapply on a range of wavelengths. I'm not particularly keen on the  
implementation of the FORTRAN program you used compared to the one I  
suggested, it seems like a very crude convolution.


>
> And when plot a line, how to color the line using different color?


The easiest way I know of is using ggplot2, see the last example of  
this page:

http://had.co.nz/ggplot2/geom_path.html


In base graphics, you could use segments to join the observations and  
supply a vector of colours of appropriate length.

Hope this helps,

baptiste


>
> Thanks in advance!
> Jinsong
>
> wl2rgb <- function(wl, gamma) {
>    if (missing(gamma)) gamma <- 1
>    if (wl >= 380 & wl <= 440) {
>        r <- -1 * (wl - 440) / (440 - 380)
>        g <- 0
>        b <- 1
>    }
>    if (wl >= 440 & wl <= 490 ) {
>        r <- 0
>        g <- (wl - 440) / (490 - 440)
>        b <- 1
>    }
>    if (wl >= 490 & wl <= 510) {
>        r <- 0
>        g <- 1
>        b <- -1 * (wl - 510) / (510 - 490)
>    }
>    if (wl >= 510 & wl <= 580) {
>        r <- (wl - 510) / (580 - 510)
>        g <- 1
>        b <- 0
>    }
>    if (wl >= 580 & wl <= 645) {
>        r <- 1
>        g <- -1 * (wl - 645) / (645 - 580)
>        b <- 0
>    }
>    if (wl >= 645 & wl <= 780) {
>        r <- 1
>        g <- 0
>        b <- 0
>    }
>    s <- 1
>    if (wl > 700) {
>        s <- .3 + .7 * (780 - wl) / (780 - 700)
>    }
>    if ( wl < 420) {
>        s <- .3 + .7 * (wl - 380) / (420 - 380)
>    }
>    r <- as.integer(255 * (s * r)^gamma)
>    g <- as.integer(255 * (s * g)^gamma)
>    b <- as.integer(255 * (s * b)^gamma)
>    return(rgb(r, g, b, maxColorValue = 255))
> }
>
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_____________________________

Baptiste Auguié

School of Physics
University of Exeter
Stocker Road,
Exeter, Devon,
EX4 4QL, UK

Phone: +44 1392 264187

http://newton.ex.ac.uk/research/emag