polygonize: Vectorise a raster into polygons

In vectra: Columnar Query Engine for Larger-than-RAM Data

Vectorise a raster into polygons

Description

Converts a .vec raster into polygon features, the inverse of rasterize(). The raster is read one tile-row strip at a time; within each strip equal-valued cells along a row collapse to a rectangle, and (with dissolve = TRUE) the rectangles are then merged by value through spatial_dissolve() so each distinct value becomes a single polygon spanning the whole raster. The result is a lazy vectra_node carrying a value column and hex-WKB geometry.

Usage

polygonize(
  x,
  band = 1L,
  dissolve = TRUE,
  na_rm = TRUE,
  values = "value",
  crs = NA,
  flush_rows = NULL
)

Arguments

x	A vectra_raster (from vec_open_raster()) or a path to a .vec raster.
band	Band to vectorise (1-based). Default 1.
dissolve	If TRUE (default) merge cells of equal value into one polygon per value; if FALSE emit one square polygon per cell.
na_rm	Drop nodata cells (TRUE, default) or vectorise them as a value.
values	Name of the output value column. Default "value".
crs	Coordinate reference system recorded on the node. Defaults to the raster's EPSG, else unknown.
flush_rows	Rows buffered before a spill flush. NULL (the default) instead flushes once a spill buffer's size crosses the streaming memory budget (a fraction of vectra_mem(), set with options(vectra.memory = )); an explicit value caps each buffer at that many rows.

Details

Extraction is the monoid fold tier (one strip at a time); the by-value dissolve rides the sort / partition tier of spatial_dissolve(), localising each value to a shard before the union. Geometry assembly is delegated to sf (an optional dependency).

Value

A vectra_node with the value column and a hex-WKB geometry column, materialise it with collect_sf().

See Also

rasterize() for the inverse, contours() for iso-lines, collect_sf() to materialise as sf.

Examples

m <- matrix(c(1, 1, 2, 2, 1, 1, 2, 2, 3, 3, 3, 3, 3, 3, 3, 3), 4, 4,
            byrow = TRUE)
f <- tempfile(fileext = ".vec")
vec_write_raster(m, f, dtype = "f64", extent = c(0, 0, 4, 4))

polys <- polygonize(f)
collect_sf(polys)
unlink(f)

vectra documentation built on July 10, 2026, 5:08 p.m.