The following is a function to “flood fill” a region on the active plotting device. Once called, the user will be asked to click on the desired target region. The flood fill algorithm then searches neighbors in 4 directions of the target cell (down, left, up, right) and checks for similar colors to the target cell. If neighboring cells are of the same color, their color is changed to a defined replacement color, and the cell number is added to a “queue” for further searches of neighbors. Once a cell has been checked, its position is added to a list of completed cells. This algorithm is referred to as “Four-way flood fill using a queue for storage”.

Here’s a visualization of the Four-way flood fill from Wikimedia Commons:

http://commons.wikimedia.org/wiki/File:Wfm_floodfill_animation_stack.gif

Function:

#arguments: 
# replCol - The color to apply to the flood region. Should be given as a hexadecimal string, as is the format of the output from rgb().
# res - The resolution (per inch) of the temporarily exported figure (7 X 7 inches) used to create a matrix of color values for the flood fill algorithm.  
#
floodfill <- function(replCol=rgb(0.5,0.7,0.7,1), res=50){
 #####################
 ### Required package
 require(png)
 
 #####################
 ### Choose target position
 print("Choose a region to flood fill")
 pos <- locator(1)
 Pars <- par()
 dev.print(file="4fill.png", device = png, width=7, height=7, units="in", res=50)
 mat <- readPNG("4fill.png")
 dim(mat)
 
 #####################
 ### Make Colors matrix ('Col')
 Col <- rgb(mat[,,1], mat[,,2], mat[,,3], mat[,,4])
 Col <- array(Col, dim=dim(mat)[1:2])
 Col <- t(Col)
 Col <- Col[,dim(Col)[2]:1]
 
 #####################
 ### Trim Colors matrix ('Col') to only plot area
 rows <- round(Pars$mai[2]/7*nrow(Col)):round((7-Pars$mai[4])/7*nrow(Col))
 cols <- round(Pars$mai[1]/7*ncol(Col)):round((7-Pars$mai[3])/7*ncol(Col))
 Col <- Col[rows, cols]
 
 #####################
 ### Make lookup table
 rowVal <- seq(Pars$usr[1], Pars$usr[2],, nrow(Col))
 colVal <- seq(Pars$usr[3], Pars$usr[4],, ncol(Col))
 tarPos <- c(which.min((rowVal-pos$x)^2), which.min((colVal-pos$y)^2))
 tarCol <- Col[tarPos[1], tarPos[2]]
 #replCol <- rgb(0,0.5,0.5,1)
 grd <- cbind(posi=seq(Col), expand.grid(row=seq(nrow(Col)), col=seq(ncol(Col)), done=0))
 grd$Col <- c(Col)
 
 #Get neighbors
 posi <- array(grd$posi, dim=dim(Col))
 #down
 down <- posi*NaN
 down[-nrow(posi),] <- posi[-1,]
 grd$down <- c(down)
 #left
 left <- posi*NaN
 left[,-1] <- posi[,-ncol(posi)]
 grd$left <- c(left)
 #up
 up <- posi*NaN
 up[-1,] <- posi[-nrow(posi),]
 grd$up <- c(up)
 #right
 right <- posi*NaN
 right[,-ncol(posi)] <- posi[,-1]
 grd$right <- c(right)
 
 #####################
 ### Search for similar colors to target cell and record "done" or checked cells
 queue <- which(grd$row==tarPos[1] & grd$col==tarPos[2])
 grd$Col[queue] <- replCol
 count <- 1
 pb <- txtProgressBar(min = count, max = nrow(grd), initial = count, style=3)
 while(length(queue) > 0){
   queue2 <- vector(mode="list", length(queue))
  setTxtProgressBar(pb, count)
  for(i in queue){
     incl <- which(c(grd$down[i], grd$left[i], grd$up[i], grd$right[i]) > 0) 
   check <- c(grd$down[i], grd$left[i], grd$up[i], grd$right[i])[incl]
   check <- check[grd$done[check] == 0]
 
   repl <- check[which(grd$Col[check] == tarCol)]
   grd$Col[repl] <- replCol
   queue2[[count]] <- repl
   count <- count + 1
   grd$done[i] <- 1
  }
  queue <- unlist(queue2)
  queue <- queue[grd$done[queue] == 0]
 }
 
 #####################
 ### Add flood fill layer to plot region
 Fill <- array(0, dim(Col))
 Fill[which(grd$Col == replCol)] <- 1
 image(x=rowVal, y=colVal, z=Fill, add=TRUE, useRaster=TRUE, col=c(0, replCol))
 
}

Example:

#install.packages("png")
library(png)
op <- par(mar=c(4,4,1,2), bg="white")
x <- 1:10
y <- 2*x
plot(x, y, t="l", col=2, xaxs="i", yaxs="i")
abline(10,-2)
abline(10,2)
abline(20,-4)
abline(20,-1)
floodfill(replCol=rgb(0.5,0.7,0.7,1), res=50) # Choose a 1st area
floodfill(replCol=rgb(t(col2rgb("pink", alpha=TRUE)), maxColorValue = 255), res=50)  # Chose a 2nd area
floodfill(replCol=rgb(0.9,0.7,0.3,1), res=50)  # Choose a 3rd area
floodfill(replCol=rgb(0,1,1,1), res=50)  # Choose a 3rd area
par(op)
 
# Export graphics device
dev.print(file="floodfill_ex.png", device = png, width=5, height=5, units="in", res=200, type="cairo")