Creating a high resolution png from a raster in R

In this post I'll show you two different ways to create a png from an equal-area bathymetry grid in R.

Version 1: using the raster::plot function

The first option for plotting a raster is to use the raster::plot function and write this to a sufficiently large png file. This works reasonably well but the only disadvantage is that both the title and legend are really small. The title is optional but I didn't find a way to disable the legend. The main trick to get the raster::plot function to output a high resolution png consists in setting the maxpixels parameter. Colors where selected on ColorBrewer but with a small addition that the color range was reverted and that the darkest color is repeated to ensure that rare depths (-6000 to -10000) get the same color.

if(!requireNamespace("sdmpredictors")) {
  install.packages("sdmpredictors")
}
library(raster)
x <- sdmpredictors::load_layers("BO_bathymean", equalarea = TRUE)
png("bathymetry_plot1.png", width=ncol(x), height=nrow(x))
col <- rev(c("#f7fbff", "#deebf7", "#c6dbef", "#9ecae1", "#6baed6", "#4292c6","#2171b5","#08519c", rep("#08306b",7)))
plot(x, maxpixels = ncell(x), col = col, colNA = "#818181",

     main = "Bathymetry", axes = FALSE, ylim=extent(x)[3:4])
dev.off()

Version 2: write to png using leaflet colors

While this version looks and is a bit more complicated, it produces really good looking results. The main gist of this code is that it transforms the values to a range from 0 to 1000 while making sure that extreme values at both extremes will get the same colors as more common values. This is similar to what you general would do when e.g. creating a color scale with QGIS. Once values are mapped to colors they are converted to raw bytes with the right dimensions and written to a png file. Remark that this code was inspired by some of the internal functions in the leaflet package.

if(!requireNamespace("leaflet")) {
  install.packages("leaflet")
}
if(!requireNamespace("sdmpredictors")) {
  install.packages("sdmpredictors")
}
library(sdmpredictors)
library(raster)

# create colors
colors <- leaflet::colorNumeric(rev(c("#f7fbff", "#deebf7", "#c6dbef", "#9ecae1", "#6baed6", "#4292c6","#2171b5","#08519c", "#08306b")),
                                -1:1001, na.color =  "#818181")
cols <- c(colors(-1:1001), colors(NA))
x <- sdmpredictors::load_layers("BO_bathymean", equalarea = TRUE)
# scale values and remove extreme values from the color range 
vals <- values(x)
vals <- scale(vals)
minmax <- quantile(vals, probs=c(0.01, 0.99), na.rm = TRUE)
vals <- round((((vals - minmax[1]) / (minmax[2] - minmax[1])) * 1000))
vals[vals < 0] <- 0
vals[vals > 1000] <- 1000
vals[is.na(vals)] <- 1002

# lookup colors for scaled values, convert to raw and write to file
valcolors <- cols[vals+2] # +2 because -1 and 0 are in cols (value 0 is at index 2 in cols)

rgb_data <- col2rgb(valcolors, alpha = TRUE)
raw_data <- as.raw(rgb_data)
dim(raw_data) <- c(4, ncol(x), nrow(x))

png::writePNG(raw_data, "bathymetry_plot2.png")

All source code is available on GitHub at https://github.com/samuelbosch/blogbits/blob/master/misc/raster2png.R

Main R Packages used:

Samuel Bosch

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