alphacol: Generate color of a given brightness / opacity

In plutils: Tools for plotting with style

Generate color of a given brightness / opacity

Description

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

Usage

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

Arguments

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.

Details

alphacol follows the concept of alpha compositing:
A number \alpha between 0 and 1 controls how opaque the colour is. When printing a colour A with alpha-level \alpha over a colour B, the result becomes \alphaA + (1-\alpha)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)

Value

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

Author(s)

Ute Hahn, [e-mail]

See Also

ploptions for setting package options.

Examples

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

plutils documentation built on Feb. 19, 2026, 3:01 a.m.