Towards Better Chroma Subsampling - SMPTE Presentation Notes
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Step #2: B'-Y' and R'-Y' components are rescaled smaller and later rescaled to their original size.

The color space conversion can roughly be visualized as this:

Original Luma (Y’) channel only Color difference components
(this is rescaled smaller and then larger)
Final subsampled image

For the majority of real world scenes, chroma subsampling works very well and is visually lossless.

*The examples in this presentation are all 4:1:1 (4X horizontal subsampling, no subsampling vertically).

**This particular example uses linear/triangle resampling for the chroma. Other methods will yield different results.