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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)
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")