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I was reading this article related to edge detection and here is a edge detection mask

-1 0 1 -1 0 1 -1 0 1 

It's mentioned that this detector detects the edges in the east direction. I am confused how come the direction is east. It should have been north isn't it

It should detect something like | rather than - isn't it

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    I think you compute something like a dot product of this placed on a 3x3 part of your image. To me, this seems like a derivative in the east direction, so the edge is actually going north-south.2012-09-15

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This mask is one part of Prewitt operator. Please have a look at :

http://en.wikipedia.org/wiki/Prewitt_operator

It detects vertical edges, while its transpose detects the horizontal ones. Prewitt operator is the aplication of both masks, one after the other.