Better Chroma Subsampling

1- Chroma subsampling creates out of gamut values say, when non-
neutral color difference components are reconstructed onto black
pixels. The article gives one example (red text on a black
background). This could be avoided with in-range chroma
reconstruction.

The resulting Y'CbCr values can lie outside R'G'B' gamut. That's what
I mean by out of gamut.

2- Regarding constant luminance, I define luma and luminance
differently. Luminance takes into account gamma correction whereas
luma is just an approximation (which works well enough). It's
explained in the article.

3- It's not entirely necessary to specify how the chroma samples are
interpolated. It's ok to leave it up to the output device to choose
the interpolation method based on tradeoffs between quality and cost.
(Though in some other applications like studio video, the rec. 709
standards has filter guidelines so that presumably multi-generation
performance will be good.)

Glenn