Chroma Upsampling (Chroma Interpolation)

Shooting green-screen onto a 4:2:0 chroma-subsampled format, intending of course to use it for chroma-keying.  Obvious disadvantage is green-ness of green-screen only gets sampled at quarter-resolution.  Not a show-stopper, given my target deliverable is standard definition, but anyhow, towards perfectionism, is there any way to up-sample to 4:4:4 i.e. full definition colour?

It does occur to me that something more sophisticated than chroma blur ought to be possible, broadly along the lines of edge-following methods employed in resizing. What’s out there?

  • Simplest method, that most people seem to use, is chroma-blur.  That’s only the chroma, not the luma.
  • Searching around, Graham Nattress has analysed the problem and seems to have produced a more mathematical approach.  But it’s only available (at time of writing) for Final Cut (which of course is Mac-only at present).

Some tools that “promise” upsampling, but I wonder by what methods:

  • GoPro-CineForm intermediate.  The codec settings include an option to up-sample to 4:4:4
  • Adobe Premiere, but only if a Color Corrector effect employed.
    • But the crucial thing here, regarding the usefulness of this, is whether it uses any better method than chroma blur.

Some questions:

  •  Does Adobe have anything built-in to do something Nattress-like nowadays?
  • DaVinci Resolve?
  • Boris?


    • Understanding Color Sampling, by Barry Green
      • {This has some fantastically clarifying images of the color-distorting effect of subsampling etc.}
      • On the PC you can usually find a chroma blurring filter which will smooth out the blocky edges
      • …on the Mac I’d recommend looking to Nattress’s G-Chroma Smoother plug-in.
    • free chroma repair project
    • I made a chroma smoother for After Effects. It sandwiches rgb to hsl, then smart blurs only the saturation channel, so there’s zero blurring on the actual rgb. It fixes minor DCT blockyness and grainy footage.
    • Jim Arco:
      • As advertised, it takes out much of the blockyness and even improves the graininess in the low-light DV footage.
      • The “black mask” control seems very sensitive. With a setting of 100, I get strange artifacts in the shadows and with 99.99, I do not see them
    • Remove DV colour artifacts with intelligent chroma upsampler
    • G Chroma Smooth 4:1:1
    • G Chroma Smooth 4:2:0
    • G Chroma Smooth 4:2:2
    • Each of these nodes smoothes the chroma according to the specific nature of the chroma sub sampling. See below for a discussion on Chroma Sub-samplng.
    • Chroma Sampling: An Investigation, by Graeme Nattress
    • The limiting factor in the quality of DV in Final Cut Pro is related to the method by which the 4:1:1 chroma is dealt with. By using the SDI output into an uncompressed Final Cut Pro editing system, DV chroma can be quickly and easily upsampled to 4:2:2, although the upsampled chroma is visually lacking compared to a reference 4:2:2 Digital Betacam original.
    • {Great article with plenty of illustrative images}
    • AVC Intra and Chroma Key Tip, maybe!, by Jeff Heck
      • rafael:
        • The 411 and 420 filters are to be applied to 411 or 420 stuff. The filter will work in a 422 or 444 sequence.
        • The 422 filter is to be applied to 422 footage and only works in a 444 sequence.
          The sequence codec needs to have more chroma samples than the footage so can retain the interpolated (smoothed) chroma
        • for years the “Nattress Chroma Smooth/Sharpening” has been my “secret weapon” when editing DV footage. Is magic.
          Drop the filter it in any DV stuff; render to any 422 codec, and -although won’t look like DigiBeta- that won’t look like DV anymore
    •  Color Subsampling, or What is 4:4:4 or 4:2:2?

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