[R] How to draw a transparent polygon

David L Carlson dcarlson at tamu.edu
Wed May 21 17:47:34 CEST 2014


The standard colors in R are opaque, but you can add an alpha value to make them semi-transparent. In this example we set alpha halfway between 0 and 255 to define a semi-transparent red:

> set.seed(42)
> x <- runif(10)*10
> y <- runif(10)*10
> plot(x, y, pch=16)
> col2rgb("red", alpha=TRUE)
      [,1]
red    255
green    0
blue     0
alpha  255
> redtrans <- rgb(255, 0, 0, 127, maxColorValue=255)
> polygon(c(2, 5, 8), c(2, 10, 2), col=redtrans)

-------------------------------------
David L Carlson
Department of Anthropology
Texas A&M University
College Station, TX 77840-4352





-----Original Message-----
From: r-help-bounces at r-project.org [mailto:r-help-bounces at r-project.org] On Behalf Of Jun Shen
Sent: Wednesday, May 21, 2014 10:32 AM
To: R-help
Subject: [R] How to draw a transparent polygon

Hi everyone,

How do I draw a transparent ploygon overlaying with a scatter plot?

Let's say, we call plot() to have a scatter plot, then call polygon() to
add a polygon. I was hoping the polygon can be transparent so the scatter
plot is still visible. I can't find any argument in polygon() for such a
feature. Is there another way to do it? Thanks.

Jun Shen

	[[alternative HTML version deleted]]

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