In the general case, the two-dimensional convolution operation
requires (width * height) multiplications for each output pixel.
Separable filters are a special case of general convolution in which
the filter
can be expressed in terms of two vectors
such that for each 
If the filter is separable, the convolution operation may be performed
using only (width + height) multiplications for each output pixel.
Equation 9 becomes:
To apply the separable convolution, we first apply 
as though
it were a width by 1 filter. We then apply 
as though it
were a 1 by height filter.