Make text stand out with outlines

Adding text directly to a plot is a useful way of annotating features of your graphic. But if the background of the chart is quite busy, as is often the case with image plots or dense scatterplots, the text can sometimes be difficult to read. Placing the text in a "calm" area of the plot can help, but this isn't an option if the graph is dynamically generated from simulated or live data.

To solve this problem, Greg Snow published a very handy function "shadowtext" to the r-help mailing list. It works just like the standard "text" function for adding annotations, but adds an optional extra argument to specify the color of an outline around the text for contrast. The default, bg="black", adds a black outline around each letter.

Here's the definition of the function:

shadowtext <- function(x, y=NULL, labels, col='white', bg='black',

theta= seq(pi/4, 2*pi, length.out=8), r=0.1, ... ) {

xy <- xy.coords(x,y)

xo <- r*strwidth('A')

yo <- r*strheight('A')

for (i in theta) {

text( xy$x + cos(i)*xo, xy$y + sin(i)*yo, 

  labels, col=bg, ... )

}

text(xy$x, xy$y, labels, col=col, ... )

}

It's quite clever: it actually plots 8 copies of the text in the background color, each slightly offset in a different direction, and then overlays the text in the foreground color on top. You can increase the weight of the outline with the r argument (the default of 0.1 makes the outline about 10% of the text size), and for very large text you can smooth the outline by specifying more than 8 angles (in radians) with the theta argument.

Here's an example of the function in action:

x <- 10*(1:nrow(volcano))

y <- 10*(1:ncol(volcano))

image(x, y, volcano, col = terrain.colors(100), axes = FALSE)

contour(x, y, volcano, levels = seq(90, 200, by = 5),

        add = TRUE, col = "peru")

axis(1, at = seq(100, 800, by = 100))

axis(2, at = seq(100, 600, by = 100))

box()

text(531,544, 'Maunga Whau Volcano', col='lightsteelblue')

shadowtext(206,257, 'Caldera Rim', col='lightsteelblue')

The text "Maunga Whau Volcano" is plotted with the ordinary text function; lacking an outline it's a bit hard to read on the background. The text "Caldera Rim" is plotted with an outline by the shadowtext function.

Download the Rcode for the function and graphic here: shadowtext.R (0.8K) 

Comments

[Barry Rowlingson]

Eek! We'll be doing drop shadow and WordArt curvy things next!

If your graphics device supports transparency, I'd put the text on a transparent box, using strwidth and strheight to get the location where the text is going. Here's a start:

boxText <- function(x,y,string){
width=strwidth(string)
width=width+strwidth("m")
height=strheight(string)
height=height*2

rect(x-width/2,y-height/2,x+width/2,y+height/2,col="#77777766",border=NA)
text(x,y,string)

return(res)
}

Then do:

boxText(206,257, 'Caldera Rim')

Adjust colours and transparency levels to taste...

Barry