Plotting a raster with exponential or quantile color ramp diverging around zero

问题

I am using the R function levelplot() from the rasterVis package to plot a stack of three rasters with a single diverging color ramp. I would like to change the scale of a raster color ramp so that the map accentuates differences in lower values. This can be done by non-linear binning of the color breaks.

I'm using the code from a gist written by @jbaums (code included below). Any suggestions on how to adjust the color ramp in this code so that the breaks follow 2^x but the min and max values are preserved? It would seem that changing the sequences of s (below) would have the desired effect.

diverge0 <- function(p, ramp) {
  # p: a trellis object resulting from rasterVis::levelplot
  # ramp: the name of an RColorBrewer palette (as character), a character 
  #       vector of colour names to interpolate, or a colorRampPalette.
  require(RColorBrewer)
  require(rasterVis)
  if(length(ramp)==1 && is.character(ramp) && ramp %in% 
     row.names(brewer.pal.info)) {
    ramp <- suppressWarnings(colorRampPalette(brewer.pal(11, ramp)))
  } else if(length(ramp) > 1 && is.character(ramp) && all(ramp %in% colors())) {
    ramp <- colorRampPalette(ramp)
  } else if(!is.function(ramp)) 
    stop('ramp should be either the name of a RColorBrewer palette, ', 
         'a vector of colours to be interpolated, or a colorRampPalette.')
  rng <- range(p$legend[[1]]$args$key$at)
  s <- seq(-max(abs(rng)), max(abs(rng)), len=1001)
  i <- findInterval(rng[which.min(abs(rng))], s)
  zlim <- switch(which.min(abs(rng)), `1`=i:(1000+1), `2`=1:(i+1))
  p$legend[[1]]$args$key$at <- s[zlim]
  p$par.settings$regions$col <- ramp(1000)[zlim[-length(zlim)]]
  p
}

And here is some code that applies this function:

library (rasterVis)

ras1 <- raster(nrow=10,ncol=10) 
set.seed(1) 
ras1[] <- rchisq(df=10,n=10*10) 
ras2 <- ras1*(-1)/2 
s <- stack(ras1,ras2) 

p <- levelplot(s, par.settings=RdBuTheme())

diverge0(p, ramp='RdBu')

回答1:

You can probably avoid diverge0 for this, instead defining your vector of breaks and letting levelplot take care of the rest.

library(rasterVis)

You can either define breaks manually:

b <- c(-10.289, -8, -4, -2, -1, 0, 1, 2, 4, 8, 16, 20.578)

or automate it:

rng <- range(cellStats(s, range))
lim <- ceiling(log(abs(rng), 2))
b <- sort(c(0, unique(unlist(mapply(function(x, y) y*2^(0:x), lim, sign(rng))))))
b[1] <- rng[1]
b[length(b)] <- rng[2]

Then pass the breaks to at, and the tick locations/labels to colorkey$labels:

p <- levelplot(s, par.settings=RdBuTheme(), at=b, 
               colorkey=list(height=0.8, labels=list(at=b, labels=round(b, 2))))
p

来源：https://stackoverflow.com/questions/55172091/plotting-a-raster-with-exponential-or-quantile-color-ramp-diverging-around-zero

标签

raster

levelplot

rastervis