new function to calculate the percentage of change between rasters

NAMESPACE

@@ -1,6 +1,7 @@
 # Generated by roxygen2: do not edit by hand
 
 export(area.calc)
+export(differ.rast)
 export(dist.centroids)
 export(load.data)
 export(spat.alpha)

R/diff_rast.R

@@ -0,0 +1,49 @@
+#' Difference between raster objects
+#'
+#' @param r1 A SpatRaster object.
+#' @param r2 A SpatRaster object.
+#' @param perc Boolean. Default is TRUE to calculate the percentage
+#' of change between r1 and r2. FALSE gives the absolute number
+#' instead.
+#' @param filename Character. Save results if a name is provided.
+#'
+#' @return A SpatRaster object with the difference between
+#' r1 and r2.
+#' @export
+#'
+#' @examples
+#' \donttest{
+#' library(terra)
+#' rich1 <- terra::rast(system.file("extdata", "rich_ref.tif",
+#' package = "divraster"))
+#' rich2 <- terra::rast(system.file("extdata", "rich_fut.tif",
+#' package = "divraster"))
+#' differ.rast(rich1, rich2)
+#' }
+differ.rast <- function(r1, r2, perc = TRUE, filename = "") {
+  # Check that the rasters have the same resolution and extent
+  if (!terra::compareGeom(r1, r2, stopOnError = FALSE)) {
+    stop("r1 and r2 must have the same extent and resolution")
+  }
+
+  # Calculate the difference
+  diff <- r2 - r1  # Ensure this returns a raster
+
+  # Calculate percentage difference if requested
+  if (perc) {
+    # Calculate percentage difference
+    perc_diff <- (diff / r1) * 100
+    perc_diff[is.na(r1) | r1 == 0] <- NA  # Handle NA and zero values properly
+    names(perc_diff) <- "Percentage Difference"
+    diff <- perc_diff
+  } else {
+    names(diff) <- "Absolute Difference"
+  }
+
+  # Save the result if a filename is provided
+  if (filename != "") {
+    terra::writeRaster(diff, filename = filename, overwrite = TRUE)
+  }
+
+  return(diff)
+}