[FFmpeg-user] Chroma subsampling formats

[Mark Filipak]

Before I attempt to correspond with Charles Poynton, I want to give
folks the opportunity to correct what I've written and to make additions.

I plan to seek Charles Poynton's comments. I plan to ask specifically
about his 4:2:2, 4:2:0, and 4:1:1, and whether
Chroma_subsampling_notation.pdf assumes subpixel dither and averaging
without saying so.

--Mark.


References:
Wikipedia: https://en.wikipedia.org/wiki/Chroma_subsampling
Poynton: https://www.poynton.ca/PDFs/Chroma_subsampling_notation.pdf

KEY:
'•' : denotes a luma locus (i.e. a physical pixel location).
'©' : denotes a chroma locus.
'—' : there is no field there [1].
'©=©' : a single Cb Cr pair spans 2 pixels.
'©=©=©=©' : a single Cb Cr pair spans 4 pixels.
[1] There is no field data (i.e. no chroma pixels) for this field. If
there is a first chroma field, the decoder copies it to the second
chroma field, effectively halving vertical resolution for that chroma.
If there is no first field (e.g. 4:0:0), then there are no chroma fields
and composited pixels carry luma only.

Shown next is the top-left corner of the sampled film.
+——————————
| ¤ ¤ ¤ ¤    RGB samples
| ¤ ¤ ¤ ¤    RGB samples
|

Shown next is Y, Cb, and Cr separated as element 'planes'.
|    Y            Cb           Cr
| • • • •    | © © © ©    | © © © ©    4:4:4
| • • • •    | © © © ©    | © © © ©    Poynton, Wikipedia

| • • • •    | © © © ©    | © © © ©    4:4:2
| • • • •    | ©=© ©=©    | ©=© ©=©    My conjecture

| • • • •    | © © © ©    | © © © ©    4:4:1
| • • • •    | ©=©=©=©    | ©=©=©=©    My conjecture

| • • • •    | © © © ©    | © © © ©    4:4:0
| • • • •    |————————    |————————    Wikipedia

| • • • •    | ©=© ©=©    | ©=© ©=©    4:2:2
| • • • •    | ©=© ©=©    | ©=© ©=©    Wikipedia

|            | +-- center is coincident with Y0(?)
|            | ¦          | .-- center is coincident with Y0(?)
| • •        |©=©•        |©=©•        4:2:2 (BT.601)
| • •        |©=©•        |©=©•        Poynton

| • • • •    | ©=© ©=©    | ©=© ©=©    4:2:1
| • • • •    | ©=©=©=©    | ©=©=©=©    My conjecture

| • • • •    | ©=© ©=©    | ©=© ©=©    4:2:1 (JPEG variant)
| • • • •    |————————    | ©=©=©=©    Wikipedia

| • • • •    | ©=© ©=©    | ©=© ©=©    4:2:0 (MPEG1, H.261)
| • • • •    |————————    |————————    Wikipedia

| • •        | ©=©        | ©=©        4:2:0 (MPEG1, H.261)
| • •        |————        |————        Poynton

|            | +-- center is coincident with Y0(?)
|            | ¦          | .-- center is coincident with Y0(?)
| • •        |©=©•        |©=©•        4:2:0 (MPEG2 frames)
| • •        |————        |————        Poynton

| • • • •    | ©=©=©=©    | ©=©=©=©    4:1:1
| • • • •    | ©=©=©=©    | ©=©=©=©    Wikipedia

|            | +-- center is coincident with Y0(?)
|            | ¦          |     .-- center is coincident with Y2(?)
| • • • •   ©=©=©=©• •    | •©=©=©=©   4:1:1 (480i)
| • • • •   ©=©=©=©• •    | •©=©=©=©   Poynton

| • • • •    |©=©=©=©•    | •©=©=©=©   4:1:0
| • • • •    |————————    |—————————   My conjecture

| • • • •    |—————————   |—————————   4:0:0
| • • • •    |—————————   |—————————   My conjecture (monochrome)

| • • •      | ©=©=©      | ©=©=©      3:1:1 (Sony)
| • • •      | ©=©=©      | ©=©=©      Poynton