R help - Jul 2006 - Log color scale

Hi all,

In response to a previous post about plotting a numeric square matrix
as a colored matrix, I was referred to both image and the
color2D.matplot function in the plotrix package.  Both have worked for
me thanks!!

However I need to plot my data in a log transformed color scale.  Is
this possible?  I will be happy to explain further, but basically I
need to do this because there are large variations in the max and min
values of my raw data and I am trying to highlight the differences in
the values at the lower end of my raw data (while still displaying the
values at the high end of the spectrum for comparison) so if figured
the best way to represent this on a (RGB) color scale is to do it with
a log transform.  I do not want to use too many colors because that
make the figure too busy.

Any suggestions on how to achieve this.

Kartik

Kartik Pappu a ?crit :
> However I need to plot my data in a log transformed color scale.  Is
> this possible?  I will be happy to explain further, but basically I
> need to do this because there are large variations in the max and min
> values of my raw data and I am trying to highlight the differences in
> the values at the lower end of my raw data (while still displaying the
> values at the high end of the spectrum for comparison) so if figured
> the best way to represent this on a (RGB) color scale is to do it with
> a log transform.  I do not want to use too many colors because that
> make the figure too busy.
> Any suggestions on how to achieve this.
Don't use too much time to search a good palette, build it yourself.
build your own color palette, specific to your problem.
It's not very difficult, see ?rgb et al.

a small example (not a log scale) :
palblancbleu  = rgb(15:0, 15:0, 15, max=15);
palrougeblanc = rgb(15, 0:15, 0:15, max=15);
palblanc      = rep(rgb(1,1,1), 8);
palRBB        = c(palrougeblanc, palblanc, palblancbleu);

#palRBB = c(
#"#FF0000","#FF1111","#FF2222","#FF3333","#FF4444","#FF5555","#FF6666","#FF7777"
#,"#FF8888","#FF9999","#FFAAAA","#FFBBBB","#FFCCCC","#FFDDDD","#FFEEEE","#FFFFFF"
#,"#FFFFFF","#FFFFFF","#FFFFFF","#FFFFFF","#FFFFFF","#FFFFFF","#FFFFFF","#FFFFFF"
#,"#FFFFFF","#EEEEFF","#DDDDFF","#CCCCFF","#BBBBFF","#AAAAFF","#9999FF","#8888FF"
#,"#7777FF","#6666FF","#5555FF","#4444FF","#3333FF","#2222FF","#1111FF","#0000FF");

# to visualize it
# to visualize a palette

showpal = function(pal=palRBB)
{
n = 200;
fp = matrix(0,n,1);
for (i in 1:n) fp[i,] = i;

par(mar=c(0,0,0,0), mgp=c(0,0,0), ann=F);
image(fp, col=pal);
}

(images at http://7d4.com/r/pal.php)