Talk:Chroma subsampling

message: Glad for the info. Yes, Chroma formats should be merged with Chroma subsampling. The formats by themselves don't mean much without some explanation of how subsampling occurs.
Great article - seems to be the only resource on the web that handles this topic. But I have a problem. Look at the picture on the left, wouldn't you say that the 4:2:0 picture is wrong? We're talking about horizontal lines. 4:2:0 switches the U and V component every uneven line. so i would say (from logic), there should be a long blue line. and then (in the next line) a long red line. --Abdull 14:19, 23 Mar 2005 (UTC)
replies: text:  Nope, i take back the previous statement. As i thik i figured out correctly, one of the reddish or blueish block represents something like "needed area that is covered with full information". Right? Maybe the blueish and reddish rectangles can be explained? --Abdull 14:25, 23 Mar 2005 (UTC)
thread_title: Old stuff not previously in a section

message: YUV refers to an analog encoding scheme, whereas this article really talks about Y'CbCr (digital).  One difference is that the scale factors on the U and V are different than those on Cb and Cr.
Similarly, Y' does not directly represent the luminance from color science.  
See http://poynton.com/papers/YUV_and_luminance_harmful.html
:While I'm not aware of the difference between YUV and Y'CBCr myself (though I've certainly heard both used) it should be noted that the term YUV is very widely used in digital video texts/papers etc.  Even if they do do so incorrectly.  So even if the article is changed that miusage should be noted. AlyM 15:51, 6 July 2006 (UTC)
thread_title:  YUV Notation should really be avoided! 

message: Note that the luma (Y') of video engineering deviates from the luminance (Y) of color science (as defined by CIE).  Luma is formed as the weighted sum of gamma-corrected (tristimulus) RGB components.  Luminance is formed as a weighed sum of linear (tristimulus) RGB components.
In practice, the CIE symbol Y is often incorrectly used to denote luma.  In 1993, SMPTE adopted Engineering Guideline EG 28, clarifying the two terms.  The luma of video engineering is to be denoted by the symbol Y', whereas the luminance of color science is to be denoted by the symbol Y.  Note that the prime symbol ' is used to indicate gamma correction.  In current practice, the symbol Y is ofter used to refer to the luma of video engineering and not the luminance of color science.  The exact intention has to be discerned from the context of the term's usage.
Similarly, the chroma/chrominance of video engineering differs from the chrominance of color science.  
*This is mostly derived from Charles Poynton's writings
[]
Glennchan 22:44, 13 July 2006 (UTC)
thread_title:  Terminology 

message: I went ahead and made some fairly major changes to correct technical inaccuracies.  My intention was not to step on anyone's toes.
To do:
*I propose getting rid of the bitstream stuff - to me, it's too obvious
*Good diagrams showing the schemes and where the chroma samples are taken from
*Add article on chroma bug, link to it
*Correct inaccuracies in the luminance (video), YIQ, YUV articles; then link to them
*Perhaps add information on chroma interpolation and reconstruction
*Perhaps add information on chroma filtering
Glennchan 06:48, 14 July 2006 (UTC)
thread_title:  Major Changes 

message: I felt the presentation is a bit confusing as it tries to make a correspondance between the bitstream and the sampling format.
The bitstream structure can be discussed separately.
The book Video Demystified by Keith Jack, provides a good explanation of chroma subsampling and the corresponding chapters of the book are freely available at http://books.elsevier.com/bookscat/samples/1878707566/1878707566.pdf
Yeah I think it should be changed... although I really gutted the article already.  :D  I have no idea how the bitstream stuff is relevant to most people reading wikipedia... and as you point out its confusing / not organized.  Perhaps push it into its own section, or remove it entirely.Glennchan 02:55, 20 July 2006 (UTC)
thread_title:  Confusing 

message: Hey, I think we should try to be more specific when we talk about what formats using what.  What exactly does "digital component video" refer?  To me, it is an umbrella term for the various digital video formats that use chroma subsampling- which could be any one of the many schemes available.  (Skimming through Charles Poyntons book, digital component video interface seems to typically refer to SDI)  Perhaps we should include HDMI on there, although I don't know too much about it.  HDMI seems to support 4:2:2, 4:4:4, and the colorspaces Y'CbCr R'G'B' and xvYCC, and different bit depths.  Glennchan 19:59, 7 December 2006 (UTC)
thread_title:  Digital component video 

message: The word "macropixel" is not defined, but used. Though I can imagine what it means, it's still unclear for me if "each macropixel of two neighbouring pixels uses 4 bytes of memory" means that for the whole frame the average memory per pixel is 2 or 4 bytes. I guess 4. Macfreek 15:29, 17 December 2005 (UTC)
thread_title:  Macropixel 

message: I don't understand, even after reading the article, what 4:2:1 exactly does. How would 4:2:1 look in the image on the left?
I don't understand the reason for this notation. Why don't they give the factors of horizontal and vertical direction separatly, e.g. "2:2" could mean 2 times horizontally and 2 times vertically (like 4:2:0). Why has 0 got a meaning?
At the main entry I added a link to an image that I think better illustrates what the different chroma subsampling notations refer to.  That black and white image is more explanatory compared to what the current pink & blue tiled image on the entry does.  I would actually suggest to remove that pink and blue image, just keep linking to other one.  —Preceding unsigned comment added by Raulsaavedraf (talk • contribs) 19:12, 22 May 2008 (UTC)
thread_title:  a:b:c notation 

message: I propose including 8:4:4 under 4:2:2 instead of its own section. 8:4:4 is marketing speak (by telecine vendors). For example, if I subsampled a 4096 x 3076 image's chroma components to 2048 x 1536, it would still be called 4:2:0 instead of something like 32:16:0, even though the analog bandwidth (an obsolete measure when talking about digital video) is ~30 times that of standard definition. The numbers in the notation correspond to absolute sample values (4 = 720 samples per line, etc.) only when talking about digitizing analog standard definition video. For everything except standard definition, the numbers just mean ratios. So if the chroma components have half the number of luma samples in the horizontal direction, the subsampling is 4:2:2, no matter if it's full-HD, 4K, or whatever pixel count you can imagine. If you disagree, then I propose adding 16:16:16, 16:8:0, 16:8:8 (and any other meaningless terms) to correspond to different sampling resolutions (I guess 12:6:6 would then be for 2048 x 1536 video with horizontal chroma subsampling :^) ).  —Preceding unsigned comment added by 130.233.243.229 (talk) 22:44, 17 December 2007 (UTC)
replies: text: I removed the section. It was uncited anyway. Totsugeki (talk) 00:13, 23 February 2010 (UTC)
thread_title:  8:4:4 

message: As correctly pointed out in the Controversy section, rudimentary color subsampling is a method of the past – much like interlacing. I never use it when creating JPEG files. Yet all video formats known to me require it. What are the video codecs capable of handling 4:4:4? -- J7n  —Preceding unsigned comment added by 83.99.184.75 (talk) 22:01, 6 May 2008 (UTC)
replies: text: I don't believe MPEG4 part 2 has any support for 4:4:4, and I know that MPEG4 part 10 only supports 4:4:4 in the "High 4:4:4 Predictive" profile which isn't widely used.  Both Dirac and Theora support unsubsampled chroma. --Gmaxwell (talk) 16:41, 28 April 2009 (UTC)
thread_title:  Video codecs doing 4:4:4 

message: In X:Y:Z notation Y and Z correspond to Cr and Cb bandwidth rate, not to even and odd scanlines.
4:2:1 codecs halve Cr and quarter Cb bandwidth rather than sampling odd lines two times less often than even ones.
Bandwidth may be unrelated to sampling rate. Some codecs do really downsample the video, others sample color difference at luma rate then average them and transmit the average.  —Preceding unsigned comment added by Abolen (talk • contribs) 15:41, 22 May 2008 (UTC)
replies: text: Yes, this when you're doing the computation as waveforms, and in particular is how it originated from the analog days.  However, nowadays I believe there is nothing that prevents the Chroma subsampling from operating discretely (by literally pulling chroma values out positionally entirely in the digital domain).Tgm1024 (talk) 16:29, 18 January 2014 (UTC)
thread_title:  X:Y:Z: Cr and Cb, not even/odd scanlines 

message: Ack... I made the diagrams but they were from codecs that don't obey the chroma siting standards.  I believe that DV has Cr and Cb offset from each other by 2 pixels (it's a DSP hack to avoid having to stick a delay into the DSP; cheap consumer bastards)... which is incredibly weird but true.  And MPEG-2 would be as described in chroma subsampling notation on poynton's website.  Unfortunately I'm too lazy to fix it, and I don't know of ANY DV codec on a PC that handles the DV to the spec.
I also haven't read the original DV and MPEG-2 spec, so I could definitely be wrong about what these standards are.  e.g. even if you look in the print version of Poynton's book, the DV chroma diagram in it is wrong; inconsistent with http://www.poynton.com/PDFs/Chroma_subsampling_notation.pdf.
Glennchan (talk) 18:07, 3 September 2008 (UTC)
thread_title:  Diagrams don't obey chroma siting standards 

message: I have for years struggled with hand-waving "experts" completely mangling the origin and definition of the a:b:c notation.  The current article is no differrent, and includes a rather obtuse way of handling the 4:2:0 case.  I have finally found a consistent and coherent explanation.  Please refer to an excellent discussion on p.6 of Chrominance Subsampling in Digital Images, by Douglas A. Kerr.  I propose that the Wiki section be completely scrubbed to include this excellent explanation.--algocu (talk) 20:42, 9 September 2008 (UTC)
replies: text: I agree agree; Doug writes good clear and correct stuff.  Go for it.  It would be good to back up with some book refs, too. Dicklyon (talk) 01:44, 10 September 2008 (UTC)
replies: text: Check Charles Poynton's chroma subsampling notation document.  In the video world at least, there are multiple configurations for 4:2:0.  Whatever the proper terminology is, it is abused in practice.  We might as well mention what 4:2:0 might refer to.  JPEG, DV, MPEG-2, also handle it differently.  Interlaced and progressive configurations also differ.99.226.215.161 (talk) 05:09, 30 September 2008 (UTC)
thread_title:  Disputed 

message: I reverted a mal-styled declaration of incorrectness, based on inconsistency of terminology with the YCbCr article.  If someone wants to work on making them more consistent, that would be welcome, but sources need to be cited, especially if someone is to claim that some usage is incorrect. Better to just say what's correct, and cited sources for that. Dicklyon (talk) 21:13, 22 November 2008 (UTC)
thread_title:  Conflicting terminology 

message: Just curious what sampling h.264 uses for HD video? --70.167.58.6 (talk) 22:27, 22 April 2009 (UTC)
replies: text: 4:2:0. (there is a h264 profile for other pixel formats, but it doesn't appear to be widely supported as far as I can tell) --Gmaxwell (talk) 16:35, 28 April 2009 (UTC)
replies: text:  It uses left chroma location. JPEG uses center and HEVC for BT.2100 (HDR) uses Top-left. Valery Zapolodov (talk) 02:51, 13 May 2021 (UTC)
thread_title:  What does h.264 use? 

message: The caption on the illustration refers to the NTSC standard having a resolution of 720 x480.  Surely this is incorrect, as the aspect ratio of NTSC is 4:3, hence 640 x 480 (same as VGA computer monitors).
Ross Fraser (talk) 10:09, 8 January 2011 (UTC)
thread_title:  NTSC resolution of 720 x 480? 

message: The chroma blocks should be 2x smaller and consequently the resulting blocks should contain smaller color component blocks.
Or separate block patterns could be made for the color components.
reasoning: in the 4:4:4 format both Yr and Yb should appear exactly as often as Y. Now the diagram shows alternatively an Yr and Yb for every consecutive Y.  —Preceding unsigned comment added by 83.87.234.209 (talk) 22:17, 29 January 2011 (UTC)
replies: text: Which figures are you talking about?  Dicklyon (talk) 00:44, 30 January 2011 (UTC)
thread_title: The pattern examples seem wrong

message: i do not see any image to illustrate the appearance of "4:2:0 progressive sampling applied to moving interlaced material. "  All image captions are UNDER their corresponding images, right?  If so, this one is missing.
FleetingJoy (talk) 19:15, 7 July 2011 (UTC)
thread_title:  missing image 

message: I was reading near the beginning of the article, and noted that the author, for some reason, did use the term "luminance", but not "chrominance". I was reluctant to do a wholesale replacement, however. "Chrominance" seems to be a correct and accepted term, but I don't keep up with this field any more.
:: Fwiw, I was greatly inspired by the Color TV special issue of Proceedings of the IRE, iirc 1949. Defining the I and Q axes and bandwidths in terms of human vision seemed especially inspiring. Even when the cost of true I and Q decoding in a receiver had been reduced somewhat, receivers using true I and Q seemed to be all but nonexistent. One would think that better quality would encourage some high-end products to be made, but "high-fidelity video" never seemed to take hold.
 Regards, Nikevich 18:40, 23 April 2012 (UTC)
replies: text: Chroma and luma are gamma corrected (OETF) and the other two are not. See ITU-T Rec. H.273. Nowadays only chroma and luma are there, though HDR did bring back linear light. Valery Zapolodov (talk) 02:49, 13 May 2021 (UTC)
thread_title:  Chrominance (term) 

message: The explanation of J:a:b in Sampling systems and ratios   
 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.
is currently confusing, and inconsistent with some information elsewhere.
A lot of the problems relate to luma (Y) sampling.  
Firstly, there is no indication in this explanation of the sampling of Y.  But the subsequent explanations presuppose a meaning.  For example, in 4:4:4:  
 Each of the three Y'CbCr components have the same sample rate
Yet actually the above definitions only say that the two chroma channels are sampled at each pixel, but say nothing about the luma (Y).  
Secondly, I find at Wikimedia this example of the structure for "3:1.5:1.5":
I don't know whether this is ever used in practice, but nevertheless — if correct — it is helpful to have a quite different example to illustrate the logic of the naming.  
Notice that this figure shows luma sampled every 4/3 pixels, rather than every (4/4) pixel.  
Lastly, for b, why say "between first and second row of J pixels", rather than just "between first and second row" or "between adjacent rows"?  Surely the mention of "J" here adds no information, and is rather distracting.  
—DIV (120.17.225.128 (talk) 15:17, 4 October 2018 (UTC)
thread_title:  Explanation of J:a:b 

message: Is there a 4:0:0 sampling scheme?
I think I've read it somewhere, but that doesn't make it true.
But if there is, I think it should be mentioned. Algotr (talk) 01:42, 4 March 2022 (UTC)
replies: text: That is just Y only, grayscale. 2A00:1370:8184:164:CDD4:5A3C:A775:5C0C (talk) 20:33, 25 March 2022 (UTC)
thread_title:  Is there a 4:0:0 sampling scheme? 

message: Where should R'G'B' be explained?
This "chroma subsampling" article (and this corresponding talk page) mention R'G'B' several times.
Another article -- "RGB color spaces" -- mentions R'G'B' in the title of one of its references.
The first use of R'G'B' in this article links to the "RGB" article, so I clicked on it, hoping to find an explanation there, but that article never even mentions R'G'B', much less explains it.
None of these 3 articles ever explains what R'G'B' means, and what (if anything) makes it different from RGB.
Is there some other article that explains what R'G'B' means?
--DavidCary (talk) 22:32, 29 November 2022 (UTC)
replies: text: The prime means gamma-corrected (or any non-linear), so R'G'B' means any non-linear RGB signal. That would be nearly every display standard you've heard of: sRGB, P3, whatever. Artoria2e5 🌉 00:10, 17 July 2023 (UTC)
thread_title:  R'G'B'