A new type of pixel filter called “Blackman-Harris” has been added to Cycles by Lukas Stockner to compliment the existing Gaussian and Box filters.
From the commit:
This commit adds the Blackman-Harris windows function as a pixel filter to Cycles. On some cases, such as wireframes or high-frequency textures, Blackman-Harris can give subtle but noticable improvements over the Gaussian window.
See also: The initial patch with some discussions.
What is a pixel filter you ask? As I understand it, it’s basically the function used to apply anti-aliasing in your render. Different filters use slightly different math and methods to calculate how each pixel appears relative to its neighboring pixels, which makes edges look smoother.
The difference between the three types is most noticable on thin details like wires: