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October 2024

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Re: Video Standards Question
by Wire ~ 07 Oct '24

07 Oct '24

davemcc(a)mac.com wrote: BT.1886 as being the recognized/official EOTF for BT.709 video. [...] (I > realize that various implementations, or OSes may deviate from what the > recommendations/standards state. In particular, I'm aware of > "issues/quirks" inherent in AVFoundation and ColorSync, but I'm not > interested in that. I am interested in what is implied by the > standards/recommendations.) > Dave, I may be able to offer an insight. There's not likely to be a consensus about BT.1886 ipower response because BT.1886 is not a single alignment. To see why, look at the origin context and the spec. Here is the origin doc for BT.1886— it's well worth a read; enlightening and approachable: PERCEPTUAL UNIFORMITY, PICTURE RENDERING & IMAGE STATE, C. POYNTON, 2010 https://poynton.ca/notes/PU-PR-IS/Poynton-PU-PR-IS.pdf tl;dr— BT.1886 says that when you are interpreting content that was originally mastered on CRTs using new-age reference-grade flat-panel displays, don't forget that the BT.709 spec was camera referred (picture line-up generation equipment is employed as a synthetic camera for purposes of studio monitor alignment) and that two key picture controls on the monitor were locally adjusted for the mastering environment: peak-picture level and black-level. Display power response is the CRT gamma, attenuated by the level controls. BT.709 reference gamma is 2.4, but picture rendering depends on how peak-level and black-level are aligned, and final display power response varies with the black offset. BT.1886 codifies these variables, and serves to remind engineers about these factors when interpreting content produced in the CRT era. https://en.wikipedia.org/wiki/ITU-R_BT.1886 EXCERPT FROM POYNTON'S PU-PR-IS PAPER: Creative intent > The goal of video production is not to reproduce, at the viewer’s > premises, an accurate representation of the scene in front of the camera. > Rather, the goal is to reproduce an accurate representation of what the > director saw on his studio display upon approving the final product of > post-production. > BT.709 didn't account for creative intent, it specified an abstract capture device: https://en.wikipedia.org/wiki/Rec._709 [BT.709] Transfer characteristics > Rec. 709 specifies a non-linear OETF (opto-electrical transfer function) > which is known as the "camera gamma" and which describes how HDTV camera > encodes the linear scene light into a non-linear electrical signal value. > Rec. 709 doesn't specify the display EOTF (electro-optical transfer > function) which describes how HDTV displays should convert the non-linear > electrical signal into linear displayed light, that was done in ITU-R > BT.1886 > POYNTON ON BT.709 (NOTE POINT 3): BT.709 proposals > To achieve accurate representation of the director or cinematographer’s > visual experience, standardization of the reference display EOCF is > necessary. I propose to codify current practice and standardize today’s > 2.4-power function as part of BT.709 and its derivatives. For creative > purposes, there is no need to standardize OECF; however, retaining a > reference OECF is sensible for engineering reasons. > > 1. Pertinent display characteristics and reference viewing conditions > should be standardized. I propose that the studio reference display should > have reference white luminance of 100 cd·m-2 at CIE D65 . Veiling glare > should be specified at approximately 0.2% of reference white. The display > should be viewed in a 50% diffuse neutral grey surround having 5% of the > luminance of reference white. Standards groups should consider the manner > in which viewing parameters have been specified in the sRGB standard, the > opRGB (AdobeRGB) standard, and in ISO and ICC documents, and should > consider discussions that have taken place within the colour > management community. > > 2 EOCF of a studio reference display should be standardized based upon a > 2.35-power function. (Other values such as 2.36 and 2.4 have been proposed; > any value between 2.35 and 2.4 would serve.) > > >> 3 BT.709’s current OECF should be retained as a reference for engineering > purposes. BT.709 should make clear that its OECF is appro- priate for > studio scenes, and that modifications of the OECF for creative purposes – > perhaps dramatic modifications – should be routinely expected. A statement > is needed saying that encoding should be arranged such that the intended > image appearance is obtained on the reference display in the reference > viewing conditions. > > 4 Standards should discuss—or at a minimum, mention—image state as that > term is used in the colour management community. In particular, BT.709 and > its derivatives should be clarified to explain that the reference OECF > included in the standard is meant to exemplify capture of a typical studio > scene, and that the video signal (image data) is output (display) referred > IOW one size does not fit all, and that's OK. BT.1886 lays out the variables to consider for display alignment when working with BT.709 content. Read the PU-PR-IS paper for definitions of terms, plus many details, observations and examples. /wire ——— For more on monitor gamma see: THE EFFECT OF USER CONTROLS ON CRT MONITOR CHARACTERISTICS, SONY 1998 https://www.researchgate.net/publication/234930635_The_Effect_of_User_contr…

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Video Standards Question
by Dave McCollum 03 Oct '24

03 Oct '24

Hi, I realize that this post is only related to ColorSync in an ancillary way, but I figure that someone might be able to help answer my "simple" question. I believe my question is rather straightforward, but in searching various documents I haven't found anything definitive. My question is about SDR HD video and the proper EOTF to use with it. I'm aware of, and have read, BT.709-6 and BT.1886-0. In addition, I've read ITU-T Rec. H.273 and ITU-T Rec. H.Suppl.19, which discuss using coding-independent code points in video file metadata. H.273 and H.Suppl.19 both refer to BT.1886 as being the recognized/official EOTF for BT.709 video. Of course, I get the same impression, though perhaps a bit less explicitly, from reading BT.709 and BT.1886 documents themselves. Following H.273, one could use the numbered code points scheme for color information via the triplet (CP1-CP2-CP3), where CP1 is the color primaries used, CP2 is the transfer function, and CP3 is the matrix for going from R'G'B' to Y'CbCr. For example, this triplet would be (1-1-1) for BT.709 video. I would imagine that since BT.1886-0 came into effect in 2011, and especially since BT.709-6 added explicit reference to BT.1886 in 2015, that using code points (1-1-1) for BT.709 implies that BT.1886 should be used for the EOTF, by default. So, is the consensus that BT.1886 (or gamma of 2.4) should be the default EOTF for BT.709 video? (I realize that various implementations, or OSes may deviate from what the recommendations/standards state. In particular, I'm aware of "issues/quirks" inherent in AVFoundation and ColorSync, but I'm not interested in that. I am interested in what is implied by the standards/recommendations.) Thanks for any enlightenment you might provide and for your consideration. Cheers, Dave

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