Coarse overview of a burn — overview

Aggregate a burn() result onto a grid of square tiles of block cells and return the mean coverage per tile (in [0, 1] for non-overlapping polygons). Tiles cut off by the grid boundary are normalised by their actual cell count. Cost is proportional to the number of records plus the number of tiles: a run that spans several tiles contributes partial lengths to its two end tiles, and the fully covered tiles between them are filled with a difference array, so nothing is expanded to cells whatever the block size.

Usage

overview_burn(x, tiles = 256L, block = NULL)

Arguments

x: A controlledburn object returned by burn().
tiles: Target number of tiles along the longer grid axis; ignored if block is given.
block: Tile size in cells. block = 1 gives the exact coverage, the same as materialise_chunk() for polygons, with NA for 0.

Value

A numeric matrix (tile rows by tile columns) with attributes extent (c(xmin, xmax, ymin, ymax) of the tile grid, which extends past the burn grid on the right and bottom when block does not divide its dimension) and block. Untouched tiles are NA.

Details

The result is a matrix in raster order (row 1 is the top), ready for ximage::ximage(ov, extent = attr(ov, "extent")).

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(1000L, 1000L))
  ov <- overview_burn(r, tiles = 20)
  dim(ov)
  attr(ov, "extent")
}
#> [1]  0 10  0 10