Separable filter


### can be separable into product of a column and row
### ** **$**M^2N^2 \rightarrow 2 \times N \times M^2**$** **
if we can decompose, we can lower the cost of convolution when there is $M \times M$ pixels and $N \times N$ size filter 
2D convolution with a separable filter is equivalent to two 1D [Convolution](https://texonom.com/convolution-54def9d34de64e199ea46393675af2ce) 









