adjacency: Cell adjacency

In vaster: Tools for Raster Grid Logic

Cell adjacency

Description

Find neighbouring cells by index, using only the grid dimension.

Usage

adjacency(dimension, cell, directions = "queen")

Arguments

dimension	integer vector of ncol, nrow
cell	integer vector of cell indices (1-based)
directions	character, one of "queen", "rook", or "bishop"

Details

Given cell indices into a grid of known dimension, return the indices of neighbouring cells. Out-of-bounds neighbours (at grid edges) are NA.

The directions argument controls which neighbours are returned:

• "queen" (default): all 8 neighbours (rook + bishop)

• "rook": 4 cardinal neighbours (up, down, left, right)

• "bishop": 4 diagonal neighbours

Column order for "queen" is: up, down, left, right, upleft, upright, downleft, downright. For "rook": up, down, left, right. For "bishop": upleft, upright, downleft, downright. Direction names refer to the raster convention where cell 1 is at the top-left.

Value

integer matrix with one row per cell and one column per neighbour direction. Column names indicate the direction. Out-of-bounds neighbours are NA.

Corner vertex values from area-based rasters

A key use case is converting area-based cell values to corner vertex values for mesh generation. Each corner vertex is shared by up to four cells. For example, the top-left corner of a cell is shared with the cell above, the cell to the left, and the cell diagonally above-left:

  upleft  |  up
  --------x------     vertex 'x' = mean of upleft, up, left, self
  left    | self

The queen-connected neighbours give all the cells needed to compute every corner vertex. Averaging the appropriate neighbours provides weighted corner values from flat pixel areas, which is the basis for constructing continuous meshes from raster data (as used by quadmesh and anglr).

Examples

## 4x3 grid (4 columns, 3 rows), 12 cells
adjacency(c(4, 3), cell = 6)
adjacency(c(4, 3), cell = 6, directions = "rook")
adjacency(c(4, 3), cell = 6, directions = "bishop")

## corner cell has fewer valid neighbours
adjacency(c(4, 3), cell = 1)

## multiple cells at once
adjacency(c(4, 3), cell = 1:12)

## ------------------------------------------------
## Corner vertex interpolation from area values
## ------------------------------------------------
dm <- c(4, 3)
elev <- c(10, 12, 14, 16, 11, 13, 15, 17, 10, 11, 13, 14)

## The top-left corner of each cell is shared by self, the
## cell above, the cell to the left, and the cell diagonally
## above-left. Average these for the vertex value:
nb <- adjacency(dm, seq_along(elev))
vals <- cbind(elev, elev[nb[, "up"]],
              elev[nb[, "left"]], elev[nb[, "upleft"]])
corner <- rowMeans(vals, na.rm = TRUE)
matrix(corner, nrow = 3, byrow = TRUE)

vaster documentation built on March 10, 2026, 5:08 p.m.