[3.9] What is edge enhancement?

When films are transferred to video in preparation for DVD encoding, they are commonly run through digital processes that attempt to clean up the picture. These processes include digital video noise reduction (DVNR) and image enhancement. Enhancement increases contrast (similar to the effect of the "sharpen" or "unsharp mask" filters in PhotoShop), but can tend to overdo areas of transition between light and dark or different colors, causing a "chiseled" look or a ringing effect like the haloes you see around streetlights when driving in the rain.

Video noise reduction is a good thing, when done well, since it can remove scratches, spots, and other defects from the original film. Enhancement, which is rarely done well, is a bad thing. The video may look sharper and clearer to the casual observer, but fine tonal details of the original picture are altered and lost.

Note that ringing can also be caused by the player and by the TV. Scan velocity modulation (SVM), for example, causes ringing.