adjacency: Pairs of adjacent cells

Description

Identify pairs of cells that are adjacent.

Usage

adjacency(x, fromCells, toCells, directions)

Arguments

Raster* object

fromCells

a vector of cell numbers for which adjacent cells should be calculated

toCells

a vector of cell numbers from which adjacent cells are selected. You can use the adjacent function if you want all cells to be considered

directions

in how many direction cells should be connected: 4, 8 or 16; or "bishop"

Value

A two column matrix with each row containing a pair of adjacent cells.

Details

Cell numbers start with 1 in the upper-left corner and increase from left to right and from top to bottom. Number of directions: 4 connects cells with one-cell rook moves, 8 with one-cell queen moves, and 16 with knight and one-cell queen moves. "bishop" connects cells with one-cell diagonal moves. The function connects the outer meridians for global rasters with a longitude/latitude coordinate reference system.

Examples

Run this code

r <- raster(nrows=10, ncols=10)
adj <- adjacency(r, fromCells = c(1,30,55,72,100), 
				toCells = 1:ncell(r), directions=4) 
	
# put the result in a RasterLayer
r[] <- 0
r[c(1,30,55,72,100)] <- 1
r[adj[,2]] <- 2
plot(r)


# How many time does one cell value occur next to another cell value?
r <- raster(ncol=10, nrow=10)
set.seed(0)
r[] <- round(runif(ncell(r)) * 5)
a <- adjacency(r, 1:ncell(r), 1:ncell(r), 8)
v1 <- r[a[,1]]
v2 <- r[a[,2]]
m <- matrix(0, nrow=5,ncol=5)
for(i in 1:length(v1)) {m[v1[i], v2[i]] <- m[v1[i], v2[i]] +1}
m