alphacol: Generate color of a given brightness / opacity
In plutils: Tools for plotting with style

Description Usage Arguments Details Value Author(s) See Also Examples

Create colors with a given transparency value, or mimick transparency by mixing with the background colour.

1	alphacol(col = par("col"), alpha = 1, alphamixing = NULL)

`col`	a valid color specification
`alpha`	numeric, alpha level, between 0 and 1.
`alphamix`	logical, if `TRUE`, return rgb-alpha colour, see Details. Defaults to the value stored in the package options.

alphacol follows the concept of alpha compositing:
A number α between 0 and 1 controls how opaque the colour is. When printing a colour A with alpha-level α over a colour B, the result becomes αA + (1-α)B. Thus, alpha = 1 equals to a colour that overplots other colours, and for alpha = 0, the colour is invisible.

R allows to define RGB colours with true alpha transparency, however, not all devices support that. Therefore, the default behaviour of alphacol is to mix the input colour A with the background colour found by par("bg"). A colour with alpha < 1 thus actually also prints opaque. If you want to change this behaviour once, let alphamix = TRUE. To change it globally, call ploptions(alphamix = TRUE)

a colour in RGB format. If alpha < 1, the colour is lighter than the input colour, see the details.

Ute Hahn, ute@imf.au.dk

ploptions for setting package options.

alphacol("red", alpha = .5, alphamixing = FALSE)
alphacol("red", alpha = .5, alphamixing = TRUE)
ploptions(alphamix = FALSE)
alphacol("red", alpha = .5)