[INFO] DeRing vs DeHalo?

[Selur]

Halo: A bright ring (halo, the thing angels have above their head)
Halos in digital video: A bright ring around edges (caused by edge enhancement)
Ringing: Noise or unwanted alteration around edges in general
Ringing in analog video: Edge-echo/ghost (but not caused by edge enhancement)
Ringing in image/audio compression: High-frequency distortion caused by quantization

They are very similar. "ringing" usually refers to Gibb's phenomenon occuring within DCT blocks (or other transforms, like in wavelet or h.264 codecs which aren't only 8x8 DCT), where "haloes" are most often caused by some kind of frequency filtering, either in analog or digital (oversharpening).

Halo is usually the white/black additional line around an edge.
Reinging is usually reffered to as moving noise around edge.

But they often look quite similar and are often used to describe the same visual effect.

Just curios what is the fundamental difference between DeRing vs DeHalo?

DeRinging is usually more aggressive than DeHalo, also DeHalo mainly aimss to remove the halo itself and not necessarily the

So what are "correct" situations to using each of those filters?

If you don't have rining, usually you should not apply de-ringing. If there's no halo do not apply dehalo.
If you only have a halo try first dehaloing filters, if they are not aggressive enough try deringing filters.
If you only have ringing, use a ringing or denoise filter. (ringing filter = basically a masked filter)

Any risks if use DeRing as essential filter for DVDs (in similar way as Deblocker)?

Like (basically) all filters they destroy/change image information, so yes blinbly applying filters without cause is not a good idea if you want to preserve details/information.

Cu Selur

Ps.: There should be tons of documentation about this on the net.