alpha argument doesn't work as expected when plotting with Terra

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I am trying to replicate a plot that I have made in raster using the new(ish) terra R package but the alpha argument in terra::plot does not seem to work the same way as it does in raster.

I am trying to use the alpha layer to indicate uncertainty in the base (coloured) layer. Below is some simplified code indicating my problem. Using the raster package, the alpha layer changes the transparency based on values of each individual pixel. When using terra this doesn't seem to work.

I think I am using the latest version of terra (1.3.4), and I am on a Mac (version 15.5.11).

library(terra)
library(raster)

par(mfrow=c(1,2))

plot_cols<-terrain.colors(n=5)

## raster package
raster.template<-raster(xmn=1, xmx=5, ymn=1, ymx=5, nrows=5, ncols=5)

legend.ras.comm<-raster.template
legend.ras.comm[]<-rep(c(5,4,3,2,1), 5)

legend.ras.alpha<-raster.template
legend.ras.alpha[]<-rep(c(0.9,0.7,0.45,0.2,0), 5)

plot(t(legend.ras.comm), col=plot_cols, legend=FALSE, axes=FALSE, box=FALSE, main="Raster package")
plot(t(legend.ras.comm), col="#000000", alpha=legend.ras.alpha, add=TRUE, legend=FALSE)


## terra package
raster.template<-rast(xmin=1, xmax=5, ymin=1, ymax=5, nrows=5, ncols=5)

legend.ras.comm<-raster.template
legend.ras.comm[]<-rep(c(5,4,3,2,1), 5)

legend.ras.alpha<-raster.template
legend.ras.alpha[]<-rep(c(0.9,0.7,0.45,0.2,0), 5)

plot(t(legend.ras.comm), col=plot_cols, legend=FALSE, axes=FALSE, main="Terra package")
plot(t(legend.ras.comm), col="#000000", alpha=legend.ras.alpha, add=TRUE, legend=FALSE, axes=FALSE)

Plot output

Any help with this would be appreciated.

terra r-raster r
1 Answers

You can now do this with terra 1.3-22

I get, using your code:

## raster package
library(raster)
legend.ras.comm <- raster(xmn=1, xmx=5, ymn=1, ymx=5, nrows=5, ncols=5, vals=rep(c(5,4,3,2,1), 5))
legend.ras.alpha <- setValues(raster.template, rep(c(0.9,0.7,0.45,0.2,0), 5))
legend.ras.comm <- t(legend.ras.comm)
## terra package
library(terra)
legend.ter.comm <- rast(legend.ras.comm)
legend.ter.alpha <- rast(legend.ras.alpha)

par(mfrow=c(1,2))
plot_cols <- terrain.colors(n=5)

plot(legend.ras.comm, col=plot_cols, legend=FALSE, axes=FALSE, box=FALSE, main="Raster package")
plot(legend.ras.comm, col="#000000", alpha=legend.ras.alpha, add=TRUE, legend=FALSE)

plot(legend.ter.comm, col=plot_cols, legend=FALSE, axes=FALSE, main="Terra package")
plot(legend.ter.comm, col="#000000", alpha=legend.ter.alpha, add=TRUE, legend=FALSE, axes=FALSE)

enter image description here

With terra you can also use a single value that affects all colors

plot(legend.ter.comm, col=plot_cols, alpha=.75)

Or change the transparency by color. Something like this:

plot_cols_alpha <- terrain.colors(n=5, alpha=seq(.75, 1, .05))
plot(legend.ter.comm, col=plot_cols_alpha)
user_6352450
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