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
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