Importing Bitmaps into R as Vector Graphics
by Paul Murrell

One way to obtain a vector version of (at least simple) bitmap images is to draw them in an R window and then "trace" around the edges with mouse clicks.

Here is an R function, trace(), which allows you to click in an R window; each click location gets a point drawn and a line drawn from the previous click. Right click to end. The grid argument allows you to control the resolution of valid click locations.

The following code uses this function to trace around the R in the R logo (PNG snapshot of window shown below code)

library(pixmap)
# load bitmap using pixmap
x <- read.pnm(system.file("pictures/logo.ppm", package="pixmap")[1])

# draw bitmap on screen
plot(x)

Routline <- trace(grid=0.02)
Rinside <- trace(grid=0.02)

With a set of vector outlines, it is now possible to play with drawing the character using R. The following code redraws just the outlines, then resizes and fills the outlines (PNG snapshot of window below code).

# Redraw just the trace
grid.newpage()
grid.lines(Routline$x, Routline$y)
grid.lines(Rinside$x, Rinside$y)
# Resize, change line width, fill, ...
pushViewport(viewport(width=unit(1, "inches"),
                      height=unit(1, "inches")))
grid.polygon(Routline$x, Routline$y, gp=gpar(fill="black"))
grid.polygon(Rinside$x, Rinside$y, gp=gpar(fill="white"))
popViewport()

You can do things like annotate the vertices to make it easy to modify vertex locations. The following code draws the character outlines with vertex numbers added (PNG snapshot at bottom of page).

# Draw outline with numbers to ease editing
grid.newpage()
grid.lines(Routline$x, Routline$y)
grid.points(Routline$x, Routline$y, size=unit(2, "mm"), pch=16)
grid.text(1:unit.length(Routline$x),
          Routline$x + unit(2, "mm"),
          Routline$y + unit(2, "mm"))
grid.lines(Rinside$x, Rinside$y)
grid.points(Rinside$x, Rinside$y, size=unit(2, "mm"), pch=16)
grid.text(1:unit.length(Rinside$x),
          Rinside$x + unit(2, "mm"),
          Rinside$y + unit(2, "mm"))

There is work going on to provide X-splines as a drawing primitive in R; this will allow you to smooth parts of the outline. (The shape parameter is set to 0 at sharp corners and 1 at vertices to approximate; the annotated outlines are used to decide which vertices to smooth. PNG snapshot below code.)

# Draw as a spline (to smooth)
# Need to set some points as "sharp" corners
# (will not work even on r-devel at time of writing!)
pushViewport(viewport(x=.1, y=.8))
grid.xspline(Routline$x, Routline$y,
             shape=
             c(0, 0, 0, 0,
               1, 1, 0, 0,
               1, 0, 1, 1, 
               1, 1, 0, 0),
             open=FALSE,
             gp=gpar(fill="black"))
grid.xspline(Rinside$x, Rinside$y,
             shape=
             c(0, 0, 0, 1,
               1, 0, 0),
             open=FALSE,
             gp=gpar(fill="white"))
popViewport()