R: To use a standard color with an alpha transparency chanel

col2alpha {seqinr} | R Documentation

## To use a standard color with an alpha transparency chanel

### Description

Takes as input a standard R color and an alpha value to return its rgb coding.

### Usage

    col2alpha(color, alpha = 0.5)

### Arguments

`color` | A standard R color as in `colors`.
`alpha` | An alpha transparency value in the interval [0,1].

### Value

same as in `rgb`.

### Author(s)

J.R. Lobry

### See Also

`colors`, `col2rgb`, `rgb`.

### Examples

    #
    # Need alpha transparency channel
    #
    par(mar = c(0, 0, 2, 2)+0.1, oma = c(0, 0, 2, 0), mfrow = c(3,2))
    for(testcol in c("blue", "red", "green", "yellow", "purple", "darkgreen")){
      plot(0,0, type="n", xlim=0:1, ylim = 0:1, axes = FALSE, xlab = "", ylab = "", main = testcol)
      n <- 11
      for(i in seq(0, 1, length = n)){
        col <- col2alpha(testcol, i)
        rect(i, 0, i + 1/n, 1, col = col, border = "black", xpd = NA)
        text(i+0.5/n, 0.5, round(i,2), xpd = NA)
      }
    }
    mtext("Effect of alpha on some colors\nNote: need alpha transparency channel",
     side = 3, outer = TRUE)
    #
    # The substractive color scheme:
    #
    par(mar = c(0,0,3,0))
    plot.new()
    plot.window(xlim = c(-1.5, 1.5), ylim = c(-1,1.75), asp = 1)
    n <- 10
    alpha <- 1/n
    for(i in 1:(2*n)){
      circle(x = -0.5, y = 0, col = col2alpha("yellow", alpha))
      circle(x = 0.5, y = 0, col = col2alpha("cyan", alpha))
      circle(x = 0, y = 3/4, col = col2alpha("magenta", alpha))
    }
    title("Substractive color scheme\nNote: need alpha transparency channel")

[Package seqinr version 3.4-5 Index]