One might say that adjustcolor() performs affine color transformation. Argument
'offset' defines the shift in color space after the linear transformation
defined by argument 'transform'. In other words, 'offset' represents additive
color mixing (in the sense of adding lights, not paints) after linear
transformation.
> red <- rgb(1, 0, 0)
> green <- c(0, 0.9, 0, 1)
> adjustcolor(red, 1, 0.9, 0.9, 0.9, offset=green)
[1] "#E6E600FF"
("red + green = yellow")
However, the implementation of adjustcolor() is a bit inconsistent with this
description as it does not force 'offset' to be a color by using
col2rgb(offset, alpha=TRUE)/255. The help page already gives an example for
using 'offset' for the purpose of adding white light.
________________________________________
Von: R-devel <r-devel-boun...@r-project.org> im Auftrag von Martin Maechler
<maech...@stat.math.ethz.ch>
Gesendet: Mittwoch, 2. Dezember 2020 09:45:27
An: Ben Bolker
Cc: r-devel@r-project.org
Betreff: Re: [Rd] undocumented 'offset' argument in
src/library/grDevices/man/adjustcolor.Rd
>>>>> Ben Bolker
>>>>> on Mon, 30 Nov 2020 16:33:23 -0500 writes:
> The 'offset' argument description is blank ...
> maybe 'additive adjustment to each of the (red, green, blue, alpha)
> values defining the colors, after adjustment by the corresponding
> \code{.f} factor' ...?
Thank you, Ben. I'm using adjustcolor() very often, and I had
also wondered about the documentation "FIXME" there.
As I've not been a real expert in color space transformations, I
hadn't dared to describe it myself, but your description above
is clearly better than the emptyness ...
Maybe someone can add something, or contribute some example(s)
on how using offset != 0 makes sense ?
> This is the relevant code:
> x <- col2rgb(col, alpha = TRUE)/255
> x[] <- pmax(0, pmin(1,
> transform %*% x +
> matrix(offset, nrow = 4L, ncol = ncol(x))))
> rgb(x[1L,], x[2L,], x[3L,], x[4L,])
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