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This is kind of a pointless exercise given that any basic image editing programs (e.g. Microsoft Paint) can do this much more efficiently; Nevertheless, I felt compelled to figure out a way of programming this in R (I was originally interested in filling in land areas on a map that I created in R). You’ll see from my example above that I didn’t quite get it right – there is still some blank white space within the regions that I filled. Part of this problem is remedied by exporting a higher resolution image (floodfill argument “res“), but this slows things down considerably.

In order to have this function work directly on an open graphics device, I exported a PNG image and then re-imported it and trimmed off the margins. What remains is an image of the plot region itself  which I convert to a matrix and look-up dataframe, where each cell’s color and neighboring cells are defined. It is this dataframe that forms the basis of my searching algorithm. I’m guessing I have made some sort of small mistake in how I trimmed the margins of the image, thus creating the slight offset in the filled region. Anyway, feel free to suggest improvements!

#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))
 
}

#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")

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