Hi Maarten,

If you put your input matrix into cells A1:IV256 then write into cells A257,
B258, C259, ... your constant 1 (one) and into cells A258, A259:B259,
A260:C260, ... the formula

=SUM(OFFSET($A$1,COLUMN()-1,0,1,256))-SUM(OFFSET($A$1,ROW()-257,0,1,256))

Empty cells won't hurt. You can hide them.

HTH,
Bernd