|	 1  1  1  0 -1 -1 -1  |
	     |	 1  1  1  0 -1 -1 -1  |
	     |	 1  1  1  0 -1 -1 -1  |
	1/9  |	 0  0  0  0  0  0  0  |
	     |	-1 -1 -1  0  1  1  1  |
	     |	-1 -1 -1  0  1  1  1  |
	     |	-1 -1 -1  0  1  1  1  |




Formula: The 7 by 7 matched filter used for the detection of the crossing.
This filter will give a maximum response in the case schematically shown in
this situation