Contour Polygons From a Density Grid

Converts a pre-computed 2D density grid (as returned by AutoSpectralRcpp::fast_kde2d_cpp() or MASS::kde2d()) directly into filled contour polygons using isoband::isobands(), ready to hand to ggplot2::geom_polygon(). This bypasses ggplot2::stat_contour_filled(), which would otherwise re-derive the same grid from a long-format melt of the density matrix before running the identical isobanding step.

Usage

.contour.polygons.from.grid(x, y, z, breaks)

Arguments

x

Numeric vector of grid coordinates along the matrix rows of z.

y

Numeric vector of grid coordinates along the matrix columns of z.

z

Numeric matrix of density values, indexed [x, y] (rows correspond to x, columns to y) – the orientation returned by both fast_kde2d_cpp() and MASS::kde2d(). Transposed internally to match isoband::isobands()'s [y, x] convention.

breaks

Numeric vector of band boundaries; consecutive pairs define each filled band, matching the breaks argument of ggplot2::geom_contour_filled().

Value

A data frame with columns x, y, subgroup (isoband's own per-piece/hole identifier, for the subgroup aesthetic) and level (an ordered factor spanning every band implied by breaks, low to high, so colour-to-band mapping stays fixed regardless of which bands are empty in a given plot). Pass as: geom_polygon(data = ..., aes(x, y, group = level, subgroup = subgroup, fill = level)). Returns NULL if breaks has fewer than two elements or every band is empty.