On Tue, Jul 15, 2025 at 7:34 PM Mark Filipak <markfilipak.i...@gmail.com> wrote:
> On 15/07/2025 14.51, Paul B Mahol wrote: > > On Tue, Jul 15, 2025 at 6:00 PM Mark Filipak <markfilipak.i...@gmail.com > > > > wrote: > > > >> Are these correct? Or, is there some official reference I can consult? > >> > > > > Maybe correct for packed formats only. > > Thanks for replying, Paul. > > Ya know, I've tried to read H.262 & H.264 about pixel planes and it all > seems so vague to me that I > finally give up looking at words that just lack meaningful content. Maybe > the answers were there and > I just didn't see them but I get along much better with pictures of > objects than with words about > objects. Specifically, where are those planes? Perhaps "plane" is a > metaphor for something. I just > don't know. > > >> Reference: https://en.wikipedia.org/wiki/Chroma_subsampling > > > > Isn't this very obvious and straightforward read? > > Not for me. > > • J: horizontal sampling reference (width of the conceptual region). > Usually, 4. > • a: number of chrominance samples (Cr, Cb) in the first row of J pixels. > • b: number of changes of chrominance samples (Cr, Cb) between first and > second row of J pixels. b > is usually either zero or equal to a (except in rare irregular cases like > 4:4:1 and 4:2:1, which do > not follow this convention). > > The explanation of 'b' is vague. I think it's wrong -- the wrong 'method'. > I think the authors made > it up -- "number of changes" -- because it somehow makes sense to them. > But "irregular cases like > 4:4:1 and 4:2:1" doesn't make sense, does it? > > I think 'b' is simply the number of chroma samples in the lines of the > second field, or it's '0' if > the second field isn't stored because it's not needed because it will be > copied from the first > field. I don't think it has anything to do with "changes". The test is: > Are my 4:4:1 and 4:2:1 > (below) correct? > > >> Y element, pixels 0..3 > >> / Cb element, pixels 0..3 > >> / / Cr element, pixels 0..3 > >> / / / > >> 4:4:1 ¤¤¤¤ ¤¤¤¤ ¤¤¤¤ —— lines in field 0 > >> ¤¤¤¤ ¤••• ¤••• —— lines in field 1 > >> > >> 4:2:1 ¤¤¤¤ ¤•¤• ¤•¤• > >> ¤¤¤¤ ¤••• ¤••• > > > Also you forgot that chroma, when subsampled, can have different > positions, > > top, top-left, center, .. > > No. I didn't forget. > Chroma is not simply copied from 4:4:4 its weight averaged. > --Mark. > > _______________________________________________ > 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". > _______________________________________________ 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".
