[R] using a custom color sequence for image()

[Alberto Monteiro]

Rajarshi Guha asked:
>
> Hi, I'd like to use a custom color sequence (black - low values, 
> green - high values) in am image() plot. While I can specify colors 
> (say a  sequence of grays) to the col argument, the ordering is 
> getting messed up. I have two questions:
> 
> 1. How can I get a sequence of say 256 colors starting from black 
> and ending in green?
> 2. How is this specified to image() such that it uses the colors in the
> proper ordering?
> 
Does

image(x, y, z)

(whatever are the x, y and z) work correctly?

If so, probably this is what you want:

image(x, y, z, nlevels=256, col=rgb(0, seq(0, 1, length=256), 0))

Explanation:

rgb(0, seq(0, 1, length=256), 0)

creates a vector of colours that begin with "black" = "#00000" and
ends up with "green" = "#00FF00".

Parameter nlevels = 256 forces the image to use all colours.

Alberto Monteiro