I bar please in Ultrascope. Or Resolve. BMD, come on, how hard it this?

[Mike Most]

While it is true that the i axis (it’s NOT “I-bar,” that is a term that whomever wrote that Apple document made up and a whole lot of other people obviously took as some kind of gospel) falls in the area of the display that flesh tones often fall into, one has little to do with the other. It is simply based on the limitations of bandwidth, and the desire when the NTSC system was first introduced to allow for more information in the orange/cyan axis than the purple/green axis, due primarily to the fact that human vision has more acuity in those colors. By skewing the color difference signals toward the orange/cyan axis (it’s actually 33 degrees rotated from the X/Y axis, so if you want to mark your own I-axis just rotate it -33 degrees with respect to Y) they created a situation where the purple/green axis could be more severely bandwidth restricted, since it was “less critical” in terms of the way the human visual system perceives images. In other words, the presence of the I axis and the location of common Caucasian flesh tones being in a similar area on a vectorscope is not completely a coincidence, But it is also the case that this wasn’t done to give some kind of flesh tone guide, it was done because that’s the way human vision works. And I would also point out that in the PAL color system – used in more than 80% of the world for over 50 years – the I and Q axes do not exist (they use the U and V axes, which correspond exactly the the X and Y axes). So an assumption that the I axis is there specifically for flesh tones – in particular Caucasian flesh tones – is incorrect, based on equally incorrect information printed in an Apple document.

There are truths and there are untruths. Just because Apple says something doesn’t make it true, even if they are the richest company in the world. And asking the Blackmagic engineers to put an axis on their vectorscope display that has no relation to anything other than standard definition NTSC video, simply because you happen to like seeing it, is essentially asking them to be incorrect in terms of engineering. If you want to see an I-axis, draw one and use it if you like. But you shouldn’t demand that engineers do things that are technically incorrect or effectively obsolete, as that is not their job. That’s all I’m going to post in this thread, because it’s clear that people are going to do and think whatever they want whether it makes any technical sense or not, and regardless of the facts.

[Alexis Hurkman]

I hate to chime in late to a thread that appears to have reached its natural conclusion, but I missed it earlier, and since Apple documentation has been brought up, it seemed interesting to add another perspective, since I wrote the passages under discussion, and it may shine a light on both some design decisions in FCP and Apple Color, and how terminology gets coined and, right or wrong, disseminated. My apologies in advance for the length of this missive.

When FCP 3.0 was in development, the then-new color correction tools and video scopes being added were brand new to the majority of desktop video editors, and the engineering team was working to try and make this unfamiliar paradigm of lift/gamma/gain style controls and the accompanying scopes comprehensible to a new audience. It was a deliberate design decision to include one half of the in-phase axis indication line all by itself as an indicator of the general hue of skin tone, since to my knowledge the not-so-coincidental dual use of that indicator had been a documented rule of thumb of videoscope use for years prior.

In an effort to make the purpose of this line more transparent, I and some others decided to call this the “Flesh tone line,” a decision I now somewhat regret as it muddies the history of this indicator; and yet as this was the only one of the in-phase and quadrature lines that the team elected to draw upon the FCP vectorscope, I stand by the decision as it made immediate sense to the new user, and the purpose of this indicator as intended had nothing to do with signal alignment, and everything to do with providing a flesh tone signpost to people new to reading scopes.

Regarding the “I-bar” terminology, this was the term I decided upon when writing the Apple Color documentation, as I expected a more experienced audience would appreciate an acknowledgment of the original purpose of this line. Also, the Color team implemented all four in-phase and quadrature indicators, so it seemed appropriate, “I” for in-phase, bar because it was a line.

I did not make this term up; after scouring different terminology from various sources, I found and settled on “I-bar” as the shortest term for purposes of documentation (try typing “i-axis indication line” ten times fast). Unfortunately, I can’t cite my source anymore as this was years ago, but nobody in a position to offer a technical review of that manual, nor any colorist who’s done either technical or casual reviews of books I’ve written since, has ever informed me that “I-bar” is wrong, and I’ve been using it consistently ever since. If, in fact, it turns out that my deadline had made me delusional and “I-bar” was the fevered ravings of a caffiene-addled technical writer, then I still stand by it since it’s less to type and is a fine abbreviation, but I cannot in truth take the credit.

I’ve discussed the history of the I and Q axes with many folks over the years, and while it’s true that the engineering reasons behind these indicators have nothing strictly to do with flesh tones, my personal feeling is that the coincidental utility of the in-phase indicator’s position has, over time, come to outweigh its original purpose, and in fact I would consider the “I-bar” we’re currently referring to as a new thing that coopted the old, sort of like Easter coopting an earlier collection of various pagan celebrations.

To clarify, I would never and have never suggested that this line is a strict guideline for human hue. In my “color correction handbook” I wrote and illustrated more pages then my editor may have wished about the subtle variations of human skin tone, and how the in-phase indicator under discussion is merely a general signpost; like speed limits, nobody follows them exactly, but it lets you know you’re around what you ought to be doing.

Lastly, I’ve used scopes that have an I-bar, and I have a very expensive scope that doesn’t (in HD mode, as has been pointed out), and while I still think it’s nice to have, its absence has never hampered me from delivering attractive skin tones to my commercial clients. However, given the choice, I’d like to see this indicator as an option for folks who like it; in fact, I’d love to see someone develop the option for multiple programmable vectorscope indicators at user-selectable angles, but then I’m a bit nutty for options. The hue that, in NTSC, is represented by the I-bar can certainly be mathematically translated into the same hue in HD color space, and I see no reason why that wouldn’t be useful or appropriate, if it’s documented clearly that this is no longer in-phase, but in fact an analogous flesh tone guidepost that can be turned on or off.

I would suggest that video scopes at this point are simply software, and it should be no sin for developers to add new features of utility to users and to label them clearly. I’m fond of pointing out that the days of fixed ground glass graticules are over, and it would be nice to see developers find more things to do with scopes for both basic and advanced users then to simply replicate functionality from analog, trace-drawn CRT technology.

http://www.alexisvanhurkman.com | http://www.correctionforcolor.com

[Chris Tomberlin]

Thank you for commenting on this and providing some additional insight. It makes me think that perhaps I’m not suggesting something ridiculous after all by requesting BMD add a little line (that I can choose to ignore or not) as an option.

Thanks

Chris Tomberlin
Editor/Compositor/Owner
OutPost Pictures

[Dmitry Kitsov]

I am sorry it is not entirely clear to what you meant?. Did you mean that the human vision has more acuity in Orange /Cyan or in Green/Purple?

[Mike Most]

Human vision exhibits less spatial acuity in the green/magenta axis than it does in the red/cyan axis. When the YIQ system was developed, this was taken into account, as they were trying to use the available bandwidth in the most efficient manner. It was thought that by skewing the color difference axes, the one that represented the green/magenta axis could be more heavily filtered since some of the information in it would be superfluous based on that fact. In practice, though, most NTSC transmissions wound up being encoded as YUV anyway, but the I and Q axes remained as an engineering setup guide regardless, and are thus present in the standard SMPTE color bar display for that system.

All of this is a lot of geek talk, though. Alexis has basically confirmed that the original meaning of the I and Q axes was co-opted to mean something completely different in the world of Apple. Kind of like whispering something to the first person in line, then having it repeated to 20 other people in line, only to have it come out of the 20th person as something completely different than what it started out as. What bothers me is that engineering, by definition, is something that usually deals with absolutes, not opinions. Mathematical equations and human perceptual studies don’t change, and neither should the definition of things based on those criteria, even when it’s done in the name of making things simple for the masses, which is Apple’s stock in trade. No matter how complex and meaningful something is, Apple can present an “i-version” of it. Hence why we have iMovie (editing for non-editors), iPhoto (color retouching for non-artists), iWeb (web site design for non-designers), and numerous other things. And, at least in part, I guess it’s why they apparently invented the “iBar,” to allow a flesh tone guide for non-colorists.

At least that’s the way I see it in my more cynical moments. ;-D

[Joseph Owens]

@Mike.

No cynicism involved, and extremely happy to see Alexis chime in on this. So few of us have living memory of composite baseband analog that we live in danger of reverting to a system of superstitions. “When you believe in things that you don’t understand….” thank you Stevie.

The In-Phase axis is a great reference, no doubt. Back in the day of setting up quad playback, where the matrixing varied from machine to machine and trim pots, sometimes the only reference we could really count on was placing the red “dot” on the 2% box, and everything else would just have to fit where it landed. Too bad. It was the closest to “fleshtone”, whatever that is, and resulted in the fewest client objections. Its actually too bad that the I/Q flags are 120 degrees from reference and that the -I burst is not actually set up to fall on its positive axis. but anyway…

As you say, In-phase and Quadrature are no longer relevant, the same as subcarrier and RS-170A, since the advent of CAV and then 601 and 259M. For those who have outboard scopes that show axes, you will also have to come to terms with the the fact that even when those guidelines are shown, they are not actually in 90 degree quadrature anymore, oweing to the difference in matrix calculations, especially between YIQ (no longer in use) and Y’CbCr, which is now ubiquitous. It does make it rather difficult for the manufacturers to be both mathematically rigorous and sympathetic to the artistic community at the same time.

As to the cyan/red and green/magenta bandwidth-limiting strategy, while it is true that the spatial frequency response of the human eye has that characteristic mentioned, from an engineering standpoint, the reality is that you can “borrow” bandwidth from how the channels are constructed. One upshot of how Y is composed, (mostly green) is that you can really knock it off in the chroma channel… because its already there waiting to be re-constituted when you make Y/I/Q back into R/G/B for display.

There are of course three channels in vestigial-sideband composite analog to cope with the historical 6MHz total bandwidth allotted to terrestrial broadcasting. Early videotape recorders needed to achieve a relative head-to-tape velocity of about 90 feet per second to achieve the response necessary. That’s just electromagnetic physics. Anyway, the three components, which is what you need to re-construct any color signal, whether its R/G/B, or H/S/L or whatever, can have different characteristics depending on the transmission. In Y/I/Q, Y is luminance, and saturation and hue are encoded in the phase and magnitude of the I/Q vectors. NTSC engineers were looking for ways to buy the best response with maximum efficiency. Luminance was pretty much mandated for backwards-compatibility with black & white — and interestingly enough, if you look at a bell-curve of human electromagnetic band response, (we see between, but not including, infrared and ultraviolet), right in the middle is green/yellow. So it should not come as a surprise that NTSC “Y” is a matrix of about 70% green, 20% red and 10% blue (numbers inexact for the mathematically-challenged). The immediate technical bonus is that your main signal can be nearly represented correctly with one channel and it gets the lion’s share of bandwidth. It is also true that humans don’t really need a lot of detail in the color component of vision since we mostly interpret the world in terms of contrast. So the I and Q components can be highly bandwidth limited, since we just don’t need it, and you can re-modulate them onto the top end of the luminance signal since the energy distribution will also allow that. NTSC (Color) is really an astonishing achievement for its time.

So two things: If the flesh-tone area was really THAT important, why wouldn’t it be made into a major axis (maybe adding some kind of dotted brackets for some kind of subjective variation)? and, having skipped over two or three years of sampling and communications theory, Fourier analysis, Nyquist theorems etc, can modern media practitioners survive without knowing and appreciating how some of the tools and methodologies evolved almost naturally as the children of necessity?

When I think about it, however, the Inphase axis and I-Bar (awareness and understanding) may also be a line of demarcation of another type… (we? us? old farts who know how things actually work and they? the young ones who don’t actually care?) Now that’s cynicism.

jPo

You mean “Old Ben”? Ben Kenobi?

[Dmitry Kitsov]

Could you elaborate on how to do so in a PC version?