[FFmpeg-user] Chroma subsampling formats

[Jim DeLaHunt]

Mark:

On 2025-07-19 12:32, Mark Filipak wrote:
> I've made quite a few simplifying revisions. What do 'you' think of it?
>
> Below is my preliminary message to Charles Poynton, which I plan to send
> in a week if there are no further revisions.
>
> --Mark.
>
> =====
>
> Dear Dr. Poynton,
>
> Thank you for providing valuable guidance to so many people for so many
> years.
>
> You may know that Wikipedia contains a rather tortured description of
> chroma subsampling. I seek to streamline the description and to expand
> the scope to notations not currently covered.

As I read this draft, I don't see you asking any questions. You are just 
stating things as you understand them. What kind of response do you want 
from Poynton?  I suggest you insert here a list of questions which 
afford concise answers. Number the questions.

e.g.

Q1. Is 4:4:2 subsampling correctly depicted in my diagram "4:4:2 
conjecture" below?

Q2. For 4:2:2 subsampling, I read a difference between your description 
and Wikipedia's. I have depicted them below in diagrams "4:2:2, 
Wikipedia" and "4:2:2 (BT.601), Poynton". As I read it, you imply that 
4:2:2 subsampling involves sampling at a half-pixel offset from the 
luminance pixel locations, while Wikipedia implies sampling at luminance 
pixel locations. Do my diagrams depict you correctly? Am I correct that, 
in your understanding, 4:2:2 subsampling involves sampling at a 
half-pixel offset?

[I do not claim that this is the right wording. I am only giving an 
example of a list of questions.]


> I'm associated with the ffmpeg-user mailing list. And though I speak
> solely for myself, I have the entirety of ffmpeg as an audience. I would
> be delighted to receive the opinions of an authority. All comments are
> welcome. I will not post any replies to ffmpeg-user or anywhere else
> without your expressed permission. However, I have posted this message
> to ffmpeg-user for comments.
>
> Warm Regards,
> Mark Filipak.
>
> References.
> Poynton:https://www.poynton.ca/PDFs/Chroma_subsampling_notation.pdf
> Wikipedia:https://en.wikipedia.org/wiki/Chroma_subsampling
>
> KEY:
> • : denotes a luma pixel.
> © : denotes a chroma pixel.
> ©== : denotes a single Cb Cr pair that spans 2 pixels.
> ©====== : denotes a single Cb Cr pair that spans 4 pixels.
> ——[1]—— : denotes that there is no chroma subpicture there [1].

Your notation also apparently represents half pixel offsets like this:

| ©== ©==
|©== ©==

When I saw this difference, I thought it was a formatting mistake. I 
suggest you add this notation to your key.

> Shown next is the top-left corner of the sampled film.
> +—————————
> | • • • •    <== The sample loci in subpicture 0 (frame or field).
> | • • • •    <== The sample loci in subpicture 1 (frame or field).
> | The above is repeated across the pixel data's columns and rows.
>
> Shown next is the loci of Y, Cb, and Cr separated as element 'planes'.
> |    Y            Cb           Cr
> | • • • •    | © © © ©    | © © © ©    4:4:4, Poynton, Wikipedia.
> | • • • •    | © © © ©    | © © © ©
>
> | • • • •    | © © © ©    | © © © ©    4:4:2, conjecture.
> | • • • •    | ©== ©==    | ©== ©==

…[elided]…

> | • • • •    | ©== ©==    | ©== ©==    4:2:2, Wikipedia.
> | • • • •    | ©== ©==    | ©== ©==
>
> | • • • •    |©== ©==     |©== ©==     4:2:2 (BT.601), Poynton.
> | • • • •    |©== ©==     |©== ©==     [2]

…[elided]…

> | • • • •   ©======       |  ©======   4:1:1 (480i), Poynton.
> | • • • •   ©======       |  ©======   [2]

There is no vertical line to the left of the Cb section in the diagram 
for "4:1:1 (480i), Poynton". Is that difference intentional? If so, what 
does it signify?

> | • • • •    | ©======    | ©======    4:1:0, conjecture.
> | • • • •    | ——[1]——    | ——[1]——
>
> | • • • •    | ——[1]——    | ——[1]——    4:0:0, (monochrome) conjecture.
> | • • • •    | ——[1]——    | ——[1]——
>
> | • • •      | ©====      | ©====      3:1:1 (Sony), Poynton,
> | • • •      | ©====      | ©====      Wikipedia.
>
> [1] The chroma subpicture is void. If both chroma subpictures 0 and 1
> are void (e.g. 4:0:0), then there are no chroma subpictures and the
> composited pixels carry luma only. Otherwise, the decoder copies chroma
> subpicture 0 to chroma subpicture 1. Note that though it is possible,
> the condition: chroma subpicture 0 void and chroma subpicture 1
> populated, has not been seen in the wild.
>
> [2] The chroma pixels are shown left-shifted by one-half pixel,
> presumably due to sampling one-half pixel to the left. Since they cannot
> be moved, the pixels must be obtained by mixing (e.g. C.n = (C.n +
> C.n+1)/2) in un-coded pictures. Note that the appearance of the left
> edges of chroma spans are slightly blurred by simple mixing. Note also
> that chroma spans are assumed to be truncated as needed at the left edge
> of film edges (e.g. in 4:1:1) and that the mixing is most accurately
> accomplished in frame pictures.

I hope you are successful in getting expert clarification, and in 
updating the Wikipedia article.

I suggest you ask a followup question of Poynton: for any differences 
with the WIkipedia article which he identifies, ask if he can suggest 
reliable sources[3] which documents those differences.  What you are 
doing here sounds like what Wikipedia would call Original Research[4]. 
Wikipedia deals in verifiability, rather than the truth[5], for deep 
philosophical reasons.  Thus, your improvements to Wikipedia are of most 
value if you can back them up with citations to reliable sources.

[3] <https://en.wikipedia.org/wiki/Wikipedia:Reliable_sources>
[4] <https://en.wikipedia.org/wiki/Wikipedia:No_original_research>
[5] <https://en.wikipedia.org/wiki/Wikipedia:Verifiability,_not_truth>

Best regards,
        —Jim DeLaHunt