It has actually been requested/brought up before by a few users (myself included) and would be indeed welcomed.
However,
kabakZ wrote:it would be cool if we could get some advanced pixel filtering algorithms like Mitchel.
Mitchel is considered as a
legacy Pixel Filter for legacy rendering solutions that were designed differently (lacking shading samples per pixel), pre path-tracing era - among some others that are also not appropriate to a path-tracer (such as, box/triangle, catmull-rom, sinc). I would tend to think that Blackman-Harris (quite standardized in production renderers, for animation or VFX) would be the alternatively offered filtering option as being a fair middle-ground between sharp and soft and being a recent up to date filter to this date, or rather last time I checked.
kabakZ wrote:The lack of filtering
There
is a pixel-filtering, currently. It's mandatory for rendering, which I'm sure you knew already and might have been a phrasing matter.
kabakZ wrote:forces you to always render with a pixel size of 1.0 and deal with jagged edges, anti-aliasing and crisp fireflies.
It isn't forcing
everyone.
Fireflies is a different issue (
this guide might help).
kabakZ wrote:The standard Gaussian blur doesn't look great on the final render.
If this is referring to the current Pixel Filter, it may be subjectively not matching your needs. Aside from that, it is "fine" and has been for many years, for many projects done with Octane by numerous users. It doesn't change the fact that more filter(s) would be welcomed.
Worth to mention that the texture-level filtering ≠ Render Kernel Pixel Filtering, as I've seen some users mistaken one (former) for the other, likely not you.