Materialise a controlledburn sparse result into a dense matrix

Converts the sparse run/edge/line tables from burn() into a dense matrix. With target, only the cells inside a sub-window of the burn grid are materialised: the sparse tables are first filtered and re-indexed by crop_burn() and only the sub-window is allocated, so a chunk of a very large burn grid can be realised without ever allocating the full grid.

Usage

materialise_chunk(x, fun = c("sum", "id"), target = NULL)

materialize_chunk(x, fun = c("sum", "id"), target = NULL)

Arguments

x

A controlledburn object returned by burn().

fun

character, the pixel function:

"sum"

(default) Polygon runs contribute 1, edges contribute their coverage fraction, lines contribute length, points contribute 1. Values accumulate additively across overlapping geometries. Background is 0. Mixed geometry kinds are summed as-is, although the measures have different units.

"id"

Write the geometry id ($runs$id, $edges$id, etc.) into each cell. When geometries overlap, the last id wins. Background is NA.

target

optional numeric c(xmin, xmax, ymin, ymax); when given, materialise only this sub-window of the burn grid.

Value

A numeric matrix with dimensions nrow x ncol of the burn grid, or of the snapped target window.

Details

The sub-window is snapped outward to the burn grid's cell boundaries (see crop_burn()), so the returned matrix covers at least target. Use crop_burn(x, target) to recover the snapped grid's extent and dimension (for plotting with rasterImage(), say). The materialised chunk is identical to the corresponding sub-matrix of the full materialisation.

Examples

if (requireNamespace("geos", quietly = TRUE)) {
  poly <- geos::as_geos_geometry("POLYGON ((1 1, 9 1, 9 9, 1 9, 1 1))")
  r <- burn(poly, extent = c(0, 10, 0, 10), dimension = c(100L, 100L))
  full <- materialise_chunk(r)
  chunk <- materialise_chunk(r, target = c(2, 5, 3, 7))
  dim(chunk)
  crop_burn(r, c(2, 5, 3, 7))$extent
}
#> [1] 2 5 3 7