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This slide's filter weight matrix performs more of a Gaussian blur that essentially makes the blurred image more closely resemble a camera lens out of focus, as the structure of the bricks themselves still is visible, compared to the blurred image in the previous slide.


As an aside, it seems that box blur is better at making blurs that look like focal blur due to shooting photos out of focus, whereas Gaussian blur is better at making the effect of frosted glass.


An interesting fact: Performing box blur on an image many times converges to gaussian blur.


For those curious as to why the box blur converges to the gaussian blur, it is due to

(The distribution of the sum of independent variables converges to the gaussian distribution)