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separable filter

A 2D kernel that factors into two 1D kernels, computable as two cheaper 1D passes.

A 2D kernel $H$ is separable when $H(i,j)=h_{\text{row}}(i)h_{\text{col}}(j)$ , letting a 2D convolution run as two 1D passes at cost $2K$ per pixel instead of $K^2$ .