Historically, Pixar's RenderMan defaulted to a 2x2 Gaussian filter [1], and from what I can see, that hasn't changed [2].

Essentially it uses a truncated isotropic (non-separable) Gaussian defined by exp(-2.0 * (x*x + y*y)) with a 2x2 pixel support [3], which is slightly soft and completely avoids ringing.

Gaussian also plays very nicely with filtered importance sampling [4] since it has no negative lobes.

(Though I remember a number of studios using RenderMan preferring filters with a bit of sharpening.)

[1] https://paulbourke.net/dataformats/rib/RISpec3_2.pdf#page=40

[2] https://rmanwiki-26.pixar.com/space/REN26/19661819/Filtering

[3] https://paulbourke.net/dataformats/rib/RISpec3_2.pdf#page=20...

[4] https://citeseerx.ist.psu.edu/document?repid=rep1&type=pdf&d...