# Flood fill a region of an active device in R

July 23, 2014
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(This article was first published on me nugget, and kindly contributed to R-bloggers)

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
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!

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)) }
Created by Pretty R at inside-R.org

Example:
#install.packages("png")library(png)op <- par(mar=c(4,4,1,2), bg="white")x <- 1:10y <- 2*xplot(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 areafloodfill(replCol=rgb(t(col2rgb("pink", alpha=TRUE)), maxColorValue = 255), res=50)  # Chose a 2nd areafloodfill(replCol=rgb(0.9,0.7,0.3,1), res=50)  # Choose a 3rd areafloodfill(replCol=rgb(0,1,1,1), res=50)  # Choose a 3rd areapar(op) # Export graphics devicedev.print(file="floodfill_ex.png", device = png, width=5, height=5, units="in", res=200, type="cairo")
Created by Pretty R at inside-R.org