R: add alpha-value to png-image

Question

Is there a way to make a rasterGrob-object partly transparent, so to add an alpha-factor to it? I\'m using a logo as a watermark within a ggplot2 plot by insert

Answer 1

Straight from the ggplot2 blog by @baptiste. You can adjust alpha when you create w.

 library(png)
 library(gridExtra)
 m <- readPNG(system.file("img", "Rlogo.png", package="png"), FALSE)
 w <- matrix(rgb(m[,,1],m[,,2],m[,,3], m[,,4] * 0.2), nrow=dim(m)[1]) #0.2 is alpha


 qplot(1:10, rnorm(10), geom = "blank") +
      annotation_custom(xmin=-Inf, ymin=-Inf, xmax=Inf, ymax=Inf, 
         rpatternGrob(motif=w, motif.width = unit(1, "cm"))) +
      geom_point()

Or if you want to have a single image:

qplot(1:10, rnorm(10), geom = "blank") +
  annotation_custom(xmin=-Inf, ymin=-Inf, xmax=Inf, ymax=Inf, 
    rasterGrob(w)) +
  geom_point()