在 rgb() 中使用命名的 r 颜色，以便我可以添加 alpha 值

Question

To make it easier to test colors in a complex plot, I'd like to use named colors but specify alpha. My first thought was to use col2rgb() . Something like this

rgb( col2rgb( "red" ), alpha=125, maxColorValue=255 )

But the output of col2rgb() is a matrix. I've tried transposing it, converting it to character, etc., but can't get it in a form that rgb() can use. Which is strange, since it's called col2rgb .

The situation is more complex than this, and I have to specify the alpha when giving the color.

Answer 1

We can t ranspose to create a 3 column matrix as the col2rgb is a single column matrix with row.names ie

col2rgb( "red" )
#       [,1]
#red    255
#green    0
#blue     0
t(col2rgb( "red" ))
#     red green blue
#[1,] 255     0    0

and according to ?rgb

The colors may be specified by passing a matrix or data frame as argument red, and leaving blue and green missing. In this case the first three columns of red are taken to be the red, green and blue values.

So, we can use a 3 column matrix

rgb(t(col2rgb( "red" )), alpha=125, maxColorValue=255 )
#[1] "#FF00007D"

Or as a data.frame

rgb(as.data.frame(t(col2rgb( "red" ))), alpha=125, maxColorValue=255 )
#[1] "#FF00007D"

which is the same as

rgb(red = 255, green = 0, blue = 0, alpha=125, maxColorValue=255 )
#[1] "#FF00007D"