Chroma reconstruction 4:2:0 to 4:2:2 or to 4:4:4 is a key feature of Video Artifact Premium. Some users do not understand how it is possible. It is like a magic. But the method is simple.
Video Artifact uses warp sharpening in U and V color channels with a thin edge mask from Y. This way cleans edge pixels. There are another ways to do it, for example using blurring or anti-aliasing, but the VA method also works perfectly with noised sources.
See the image below. It is a chroma channel reconstruction from a Panasonic G7 video.
Chroma reconstruction from 4:2:0 to 4:4:4 with denoise in Video Artifact: Before / After, 2x zoom.
Warp sharping produces clean edges from aliased ones. The result is a smooth line after a sawtooth. Image may look very strange if used for whole image, so edge masking is used to process only image edges and only in U and V. Visually the result looks like native 4:4:4. It can be used for green screening where it is important.
Another 4:4:4 reconstruction part is a motion denoiser that summarizes many frames after a motion compensation. After 4:2:0 image (if stored in 4:4:4) we have many image fragment averaging, so flat parts will be restored, too.
Similar way is used in astronomical photography to restore stars from noised sources. Motion images give us a possibility to get sub-pixel details after motion interpolation. So Video Artifact also can restore complete destroyed details after video compression like the next examples:
Details are restored, 4x crop.
Noised details are restored, 3x crop.
Of course, if we shot two close colored pixels in 4:2:0 using a static camera position, they will be complete blurred and there is no way to get them back. Only parts those can be restored are be restored. So no magic is here.
For best results different settings for U and V channels are required. Also a pre-denoise may give better looking edges.
The chroma restore filter helps also to de-blur chroma from some Sony NEX cameras.