adjust_transparency {colorspace}

Adjust or Extract Transparency of Colors

Description

Adjust (i.e., add, remove, or modify) or extract alpha transparency of a vector of colors.

Usage

adjust_transparency(col, alpha = TRUE)

extract_transparency(col, mode = "numeric", default = 1)

Arguments

col

vector of R colors. Can be any of the three kinds of R colors, i.e., either a color name (an element of colors), a hexadecimal (hex) string of the form "#rrggbb" or "#rrggbbaa" (see rgb), or an integer i meaning palette()[i]. Additionally, col can be a formal color-class object or a matrix with three rows containing R/G/B (0-255) values.

alpha

either a new alpha transparency value or logical (to add/remove alpha) or NULL. See details.

mode

character specifying the output mode for the alpha transparency, can be "numeric", "integer", "character" or "hexmode". See details.

default

vector of length 1 specifying the default alpha transparency that should be returned for colors that do not specify any explicitly (defaulting to fully opaque). Can either be numeric, integer, character, or hexmode.

Details

Alpha transparency is useful for making colors semi-transparent, e.g., for overlaying different elements in graphics. An alpha value of 0 (or 00 in hex strings) corresponds to fully transparent and an alpha value of 1 (or FF in hex strings) corresponds to fully opaque. If a color hex string in R does not provide an explicit alpha transparency, the color is assumed to be fully opaque.

The adjust_transparency function can be used to adjust the alpha transparency of a set of colors. It always returns a hex color specification. This hex color can have the alpha transparency added/removed/modified depending on the specification of alpha:

• alpha = NULL: Returns a hex vector with alpha transparency only if needed. Thus, it keeps the alpha transparency for the colors (if any) but only if different from opaque.

• alpha = TRUE: Returns a hex vector with alpha transparency for all colors, using opaque (FF) as the default if missing.

• alpha = FALSE: Returns a hex vector without alpha transparency for all colors (even if the original colors had non-opaque alpha).

• alpha numeric: Returns a hex vector with alpha transparency for all colors set to the alpha argument (recycled if necessary).

The extract_transparency function can be used to extract the alpha transparency from a set of colors. It allows to specify the default value - that should be used for colors without an explicit alpha transparency (defaulting to fully opaque) - and mode of the return value. This can either be numeric (in [0, 1]), integer (0L, 1L, ..., 255L), character (“00”, “01”, ..., “FF”), or an object of class hexmode (internally represented as integer with printing as character). The default can use any of these modes as well (independent of the output mode) or be NA.

Value

For adjust_transparency character vector with hexadecimal color strings with alpha transparency corresponding to alpha argument. For extract_transparency a vector of alpha transparency values with the indicated mode.

References

Zeileis A, Fisher JC, Hornik K, Ihaka R, McWhite CD, Murrell P, Stauffer R, Wilke CO (2020). “colorspace: A Toolbox for Manipulating and Assessing Colors and Palettes.” Journal of Statistical Software, 96(1), 1–49. doi:10.18637/jss.v096.i01

See Also

rgb, desaturate, lighten

Examples

## modify transparency of a color (in different formats)
adjust_transparency("black",   alpha = c(0, 0.5, 1)) ## name
adjust_transparency("#000000", alpha = c(0, 0.5, 1)) ## hex string
adjust_transparency(1,         alpha = c(0, 0.5, 1)) ## palette() integer

## three shades of gray (in different formats:
## name/opaque, hex/opaque, hex/semi-transparent)
x <- c("gray", "#BEBEBE", "#BEBEBE80")

## adjust transparency
adjust_transparency(x, alpha = NULL)  ## only if necessary
adjust_transparency(x, alpha = TRUE)  ## add
adjust_transparency(x, alpha = FALSE) ## remove
adjust_transparency(x, alpha = 0.8)   ## modify

## extract transparency in different formats
extract_transparency(x, mode = "numeric") ## default
extract_transparency(x, mode = "integer")
extract_transparency(x, mode = "character")
extract_transparency(x, mode = "hexmode")

## extract transparency with different default values
extract_transparency(x, default = NA)
extract_transparency(x, default = 0.5)
extract_transparency(x, default = 128L)
extract_transparency(x, default = "80", mode = "integer")