On 21/07/2025 18.54, Paul B Mahol wrote: > On Mon, Jul 21, 2025 at 10:12 PM Mark Filipak <markfilipak.i...@gmail.com> > wrote: > >> On 21/07/2025 17.50, Paul B Mahol wrote: >> >>> Number of Cb pixels === Number of Cr pixels, ALWAYS! >> >> I know that's what a lot of people assume, but it doesn't jibe with what >> I'm seeing. Perhaps that's part of the confusion I see. >> >> Do you have an authoritative source for what you write? >> > > Yes, I have. Me! > > I'm the authority.
Excellent! I'd welcome your comments on the following: Q1. I show five conjectures: 4:4:2 and 4:4:1 and 4:2:1 and 4:1:0 and 4:0:0. In your judgment, do those conjectured notations exist? Are they correct? Q2. For 4:2:2 and 4:2:0 and 4:1:1, I see a difference between Poynton's descriptions and Wikipedia's. As I see it, Poynton shows chroma samples that are a half-pixel offset to the left. What is your judgment? References. Poynton: https://www.poynton.ca/PDFs/Chroma_subsampling_notation.pdf QuestTel: https://questtel.com/wiki/chroma-sub-mapping-types Wikipedia: https://en.wikipedia.org/wiki/Chroma_subsampling KEY: : • : depicts a luma pixel. : • • : 2 consecutive luma pixels. : • • • • : 4 consecutive luma pixels. : = : a chroma pixel. : = = : a unit of 2 chroma pixels. : = = = = : a unit of 4 chroma pixels. : © : a chroma sample. : © = : a sample and a pixel, plus another pixel to the right. : ©= = : a sample 1/2 pixel to the left of its 2 pixels. : ©= =©= = : the same, plus a second one. : © = = = : a sample and a pixel, plus 3 more pixels to the right. : ©= = = = : a sample 1/2 pixel to the left of its 4 pixels. : © = = = = : a sample 1 and 1/2 pixels to the left of its 4 pixels. : ——[1]—— : asserts that there is no chroma subpicture there [1]. Shown next is the top-left corner of the sampled film. +————————— | • • • • <== Four consecutive pixels in line 0 of subpicture-0. | • • • • <== Four consecutive pixels in line 0 of subpicture-1. | The above illustrates how pixels populate subpictures and how subpictures are combined. 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. | • • • • | © = © = | © = © = | • • • • | © © © © | © © © © 4:4:1, conjecture. | • • • • | © = = = | © = = = | • • • • | © © © © | © © © © 4:4:0, Wikipedia. | • • • • | ——[1]—— | ——[1]—— | • • • • | © = © = | © = © = 4:2:2, Wikipedia. | • • • • | © = © = | © = © = | • • • • |©= =©= = |©= =©= = 4:2:2 (BT.601), Poynton. | • • • • |©= =©= = |©= =©= = [2] | • • • • | © = © = | © = © = 4:2:1, conjecture. | • • • • | © = = = | © = = = | • • • • | © = = = | © = © = 4:2:1, QuestTel (vaguely | • • • • | © = = = | © = © = described, not pictured). | • • • • | © = © = | © = © = 4:2:1 (JPEG variant, irregular), | • • • • | ——[1]—— | © = = = Wikipedia. | • • • • | © = © = | © = © = 4:2:0 (MPEG1, H.261), Poynton, | • • • • | ——[1]—— | ——[1]—— Wikipedia. | • • • • |©= =©= = |©= =©= = 4:2:0 (MPEG2 frames), Poynton, | • • • • | ——[1]—— | ——[1]—— QuestTel. [2] | • • • • | © = = = | © = = = 4:1:1, QuestTel, Wikipedia. | • • • • | © = = = | © = = = | • • • • © = = = = | ©= = = = 4:1:1 (480i), Poynton. | • • • • © = = = = | ©= = = = [2] | • • • • | © = = = | © = = = 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 samples are shown left-shifted by one-half pixel, presumably due to sampling one-half pixel to the left. Since the actual pixels cannot be moved, their values must be derived, usually by mixing. Note that mixing is most accurately accomplished in frame pictures and that the left edges of spans become slightly blurry. Note also that chroma spans are assumed to be truncated as needed at the left edge of film frames, e.g. in 4:1:1. _______________________________________________ ffmpeg-user mailing list ffmpeg-user@ffmpeg.org https://ffmpeg.org/mailman/listinfo/ffmpeg-user To unsubscribe, visit link above, or email ffmpeg-user-requ...@ffmpeg.org with subject "unsubscribe".
