There are many graphical functions offering the availability of the parameter
alpha which is usually used to specify semi-transparent colors, however, such kind of colors can only be displayed in certain devices, as stated in the help of
Semi-transparent colors (
0 < alpha < 1) are supported only on some devices: at the time of writing only on the
quartzdevices as well as several third-party devices such as those in packages Cairo, cairoDevice, JavaGD and RSvgDevice.
Here is an example illustrating semi-transparent colors in a
(tmp = file.path(tempdir(), "alpha.pdf")) pdf(tmp) # open a pdf device in the temp dir plot(rnorm(100), ylim = c(-3, 3), xlab = "x", ylab = "y", main = "Add a semi-transparent color image to a graph") image(x = seq(1, 100, length = 20), y = seq(-3, 3, length = 20), z = matrix(rnorm(400), 20), col = heat.colors(20, alpha = 0.5), add = T) # here we set alpha = 0.5 dev.off()
You may try other devices such as
bmp(), etc, and you will find
alpha is not supported there.