create a composite raster using gdal's VRT and warp via {gdalraster}

vrt_composite.R

#' Create a composite raster with a VRT pixel function
#' @param src_files a character vector with the paths to the source rasters.
#' @param outfile a character path to the output raster.
#' @param fun a character with the pixel function to apply.
#' @param t_srs a character with the target SRS. If empty string "", the spatial
#' reference of src_files[1] will be used.
#' @param vrt_options a character vector with additional options to pass to the
#' gdalbuildVRT command.
#' @param warp_options a character vector with additional options to pass to the
#' gdalwarp command.
#' @param config_options a named character vector with additional GDAL configuration
#' options.
#' @param quiet a logical indicating whether to suppress progress bar.
#' @return a character path to the output raster.
vrt_composite <- function(
    src_files,
    outfile,
    fun = c("median", "mean"),
    t_srs = "",
    vrt_options = NULL,
    warp_options = c(
      "-multi",
      "-overwrite",
      "-co", "COMPRESS=DEFLATE",
      "-co", "PREDICTOR=2",
      "-co", "NUM_THREADS=ALL_CPUS"
    ),
    config_options = c(
      GDAL_VRT_ENABLE_PYTHON = "YES",
      VSI_CACHE = "TRUE",
      GDAL_CACHEMAX = "30%",
      VSI_CACHE_SIZE = "10000000",
      GDAL_HTTP_MULTIPLEX = "YES",
      GDAL_INGESTED_BYTES_AT_OPEN = "32000",
      GDAL_DISABLE_READDIR_ON_OPEN = "EMPTY_DIR",
      GDAL_HTTP_VERSION = "2",
      GDAL_HTTP_MERGE_CONSECUTIVE_RANGES = "YES",
      GDAL_NUM_THREADS = "ALL_CPUS"
    ),
    quiet = FALSE) {
  fun <- rlang::arg_match(fun)

  purrr::iwalk(
    config_options,
    ~ gdalraster::set_config_option(.y, .x)
  )


  init_vrt_path <- tempfile(fileext = ".vrt")
  init_vrt <- gdalraster::buildVRT(
    init_vrt_path,
    src_files,
    cl_arg = vrt_options,
    quiet = TRUE
  )


  tvrt <- xml2::read_xml(init_vrt_path)
  bands <- xml2::xml_find_all(tvrt, "//VRTRasterBand")

  purrr::walk(bands, function(x) {
    xml2::xml_set_attr(x, "subClass", "VRTDerivedRasterBand")
    # Add pixel function elements
    xml2::xml_add_child(x, "PixelFunctionType", "median")
    xml2::xml_add_child(x, "PixelFunctionLanguage", "Python")
    xml2::xml_add_child(x, "PixelFunctionCode", glue::glue("
import numpy as np

def median(in_ar, out_ar, xoff, yoff, xsize, ysize, raster_xsize, raster_ysize, buf_radius, gt, **kwargs):
    out_ar[:] = np.nan{fun}(in_ar, axis=0)
"))
  })

  # Write modified VRT
  med_vrt <- tempfile(fileext = ".vrt")
  xml2::write_xml(tvrt, med_vrt)


  gdalraster::warp(
    med_vrt,
    outfile,
    t_srs = t_srs,
    cl_arg = warp_options,
    quiet = quiet
  )

  return(outfile)
}

Thanks so much for all of this will dig into this more asap!