Constrained directional enhancement filter (CDEF)
I got to know about this filter from Xiph's Daala project (Video Codec):
https://people.xiph.org/~jm/daala/deringing_demo/
CDEF is Daala's deringing together with Cisco's deringing filter from Thor.
They joined efforts during the development of AV1 (which will likely end up as ietf's netvc) under the alliance of open media (aom).
https://arxiv.org/pdf/1602.05975.pdf
https://tools.ietf.org/id/draft-midtskogen-netvc-cdef-00.html
Afaik ringing artifacts have been quite the hassle in the past and CDEF appears to be a quite effective filter to counter it.
What do you think?