If the oppo is doing nearest neighbour chroma upsampling it is not being performed as a final step. From my understand this would result in each chroma value being horizontal duplicated in the captured chroma channels - this is not the case. Also doesn’t explain the difference in the luma channel. So something else is going on …
Doing some digging, it appears that one of the more important properties of IPT/ICtCp/ IPTPQc2 (all referred to as IPT for the rest of this post - not entire sure exactly which is being used where / what are the differences), is that it has a much more constant luminance with better decorrelation of the chroma and luma channels than YCbCr.
It appears that this property is very important w.r.t. minimising the introduction of errors when performing chroma sampling. Essentially it appears that, if an IPT colorspace is involved, this is the stage where chroma sampling operations is best to, and are performed when IPT / DV content is used.
Now onto the method used for chroma sampling. For HDR video, a method for chroma up-sampling appears to be defined with a 4-tap filter with fixed coefficients, first applied vertically and then horizontally (for down-sampling a 3-tap filter, horizontal then vertical). These coefficients are known, and while I haven’t found anything specifically stating this is used for all DV content the filter is provided as an informative reference in one of the DV related ETSI specifications.
For those that are interested here are some references
- What is ICtCp - Introduction? - Dolby Laboratories - whitepaper linked to before by @cpm, talks about the benefits of advantages of ICtCp for chroma operations.
- Conversion and Coding Practices for HDR/WCG ICTCP 4:2:0 Video - published paper by people from Dolby. Talks abut the benefit of ICtCp compared to Y’CbCr in reducing chroma sampling arifacts. Figure 6 is a really good example.
- Perceptual Effects of HDR Subsampling for Dolby Vision, BluRay and BBC - another paper that shows the benfit of doing chroma sampling in ICtCp / DV. Also has a Dolby Vision processsing chain showing a pipeline of gamut, colorsapce, and chroma formats conversions.
- Weighted Chroma Downsampling and Luma-Referenced Chroma Upsampling for HDR/WCG Video Coding - paper on chroma sampling in ICtCp - shows a pipeline and chroma sampling filters defined from a MPEG common test condition.
- Compound Content Management Specification - helpful specification for DV (not that it once mentions Dolby or DV). Annex C also defined the same filers as the previous link
TL;DR
All chroma sampling for DV content is almost certainty performed in a IPT colorspace - likely with a fixed 4-tap filter for up-sampling.
@cpm Your potential pipeline appears to need some modifications …