R/vx_aggregate.R

In juoe/vxr: Fast raster operations with velox and raster

#' Aggregate to a lower resolution
#'
#' @param s Input RasterStack.
#' @param factor Aggregation factor.
#' @param mem_share Share of memory to be used
#' @param n_cores Number of cores used for parallel processing. Default is NULL, indicating no parallelization.
#' @param aggtype Type of aggregation. See ?VeloxRaster_aggregate
#' @param mem_of_total Is 'mem_share' specified as share of total memory (instead of currently available)?
#'
#' @return RasterStack
#' @export
#

vx_aggregate <- function(s, factor, mem_share = .5, n_cores = NULL, aggtype = "mean", mem_of_total = FALSE){

  # split into memory-safe chunks
  raster_chunk_list <- split_layers(s, mem_share, mem_of_total)

  # process one chunk after the other
  # use parallel processing if specified

  out_list <- lapply(raster_chunk_list, function(r_chunk){

    if(!is.null(n_cores)) {

      cl <- parallel::makeCluster(n_cores)

      r_list <- parallel::parLapply(cl,
                                    raster::unstack(r_chunk),
                                    varlist = c("factor", "aggtype"),
                                    fun = function(ri) {

                                      vx <- velox::velox(ri)
                                      vx$aggregate(factor = factor, aggtype = aggtype)
                                      vx$as.RasterStack()

                                    })

      out_chunk <- raster::stack(r_list)
      parallel::stopCluster(cl)

    } else {

      vx <- velox::velox(r_chunk)
      vx$aggregate(factor = factor, aggtype = aggtype)
      out_chunk <- vx$as.RasterStack()

    }

    out_chunk

  })

  out <- raster::stack(out_list)
  out

}