hor.convdown(x, filter, boundary="periodic", fast=F)
ver.convdown(x, filter, boundary="periodic", fast=F)

x:

an image or a matrix.

filter:

a vector of filter coefficients.

boundary:

a character string or a character vector of length two giving the name(s) of the boundary rule. All the boundary rules listed for convdown are available except for "infinite". See convdown for the definitions of these rules.

fast:

logical flag indicating if fast algorithm should be used.

If ncol(x) is even, ver.convdown returns a nrow(x) by ncol(x)/2 matrix. If ncol(x) is odd, see convdown for details on the number of columns of output.