Y = FILTER2(B,X) filters the data in X with the 2-D FIR
filter in the matrix B. The result, Y, is computed
using 2-D convolution and is the same size as X.
Y = FILTER2(B,X,'shape') returns Y computed via 2-D
convolution with size specified by 'shape':
'same' - (default) returns the central part of the
convolution that is the same size as X.
'valid' - returns only those parts of the convolution
that are computed without the zero-padded
edges, size(Y) < size(X).
'full' - returns the full 2-D convolution,
size(Y) > size(X).
See also CONV26SJI0ZJ.