You can use [] to get at matrix elements, numbered left-to-right,
top-to-bottom. Alternatively, use () to address elements by
position. For example:
VIMask fred( "mask" );
for( int i = 0; i < fred.xsize(); i++ )
fred[i] = 12;
will set the first line of the matrix to 12, and:
VDMask fred( "mask" );
for( int x = 0; x < fred.xsize(); x++ )
fred(x, x) = 12.0;
will set the leading diagonal to 12.
See the member functions below for other operations on VMask.