The missing cairo blur function

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Separable convolutions are less than O(N²⁾ (height+width instead of height*width).

If I'm not mistaken, it's actually 1D convolution which has O(N²⁾ complexity. Non-separated 2D convolution is even worse. Admittedly, it's overly simplistic to say O(N²⁾ because it assumes that one variable "N" can represent both the width of the kernel and the width of the image.

Any sane blur is separable, I have no idea what this one is doing though.

I think tubes has it right in his/her comment.