Kirsch operator

The operator takes a single kernel mask and rotates it in 45 degree increments through all 8 compass directions: N, NW, W, SW, S, SE, E, and NE. The edge magnitude of the Kirsch operator is calculated as the maximum magnitude across all directions:

${\displaystyle h_{n,m}=\max _{z=1,\dots ,8}\sum _{i=-1}^{1}\sum _{j=-1}^{1}g_{ij}^{(z)}\cdot f_{n+i,m+j}}$

where z enumerates the compass direction kernels g:

${\displaystyle \mathbf {g^{(1)}} ={\begin{bmatrix}+5&+5&+5\\-3&0&-3\\-3&-3&-3\end{bmatrix}},\ }$${\displaystyle \mathbf {g^{(2)}} ={\begin{bmatrix}+5&+5&-3\\+5&0&-3\\-3&-3&-3\end{bmatrix}},\ }$${\displaystyle \mathbf {g^{(3)}} ={\begin{bmatrix}+5&-3&-3\\+5&0&-3\\+5&-3&-3\end{bmatrix}},\ }$${\displaystyle \mathbf {g^{(4)}} ={\begin{bmatrix}-3&-3&-3\\+5&0&-3\\+5&+5&-3\end{bmatrix}}}$ and so on.

The edge direction is defined by the mask that produces the maximum edge magnitude.

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  Original
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  Maximum gradient in the 8 directions

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  Image filtered with g⁽¹⁾
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  Image filtered with g⁽²⁾
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  Image filtered with g⁽³⁾
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  Image filtered with g⁽⁴⁾

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  Image filtered with g⁽⁵⁾
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  Image filtered with g⁽⁶⁾
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  Image filtered with g⁽⁷⁾
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  Image filtered with g⁽⁸⁾