Originally Posted by TomHuffman
Bill, CIE establishes general standards for color. It does not get into the weeds of how specific industries implement those standards. If the CIE has published standards for how to use dE in grayscale evaluation, then I'd love to read it.
I also reference SMPTE, which you have ignored. Please read SMPTE Engineering Guideline 432-1, especially appendix L. I will quote the relevant sections here to save folks the cost of buying it from the SMPTE store. I HIGHLY recommend that pro calibrators join SMPTE, if only for the discounts for SMPTE publications.
So that I do not create a monster post (it's going to be big, as it is), and I stay within fair use doctrine for copyrighted materials, I am only going to post excerpts. If you want to double-check that I am not quoting things out of context, please purchase the document I reference for yourself. Note: you can get some of this content, but with some odd contradictions, from www.dcimovies.com
, though do realize that SMPTE Engineering Guidelines are authoritative.
If you are correct, then the entire video industry ignores CIE. Not just me. Again, this is not personal. I don't have a beef against you or against Derek. Aside from being more than a little over-sensitive, you seem like a nice guy. This a substantive disagreement not a pissing match.
So, let's try to establish a few things:
1) What are the properties of a color error equation?
2) Should there be any differences as it applies to grayscale and why?
3) What is the best way to quantify these adjustments?
Let's turn to Appendix L, it's a good treatment of a topic I have, so far, been unsuccessful in getting you to understand:
Originally Posted by SMPTE EG-432-1-2007, pp. 72 - 73
The Munsell color space is the most uniform color space. - It was defined by having many people make many judgements of color differences using color patches. One problem with the Munsell space is that it was based on judgements of color patches and it is not based on any mathematical equations. The CIELab space is a mathematical approximation, based on XYZ tristimulus values and Equations L-1 to L-13, of the Munsell space. But the equations that define the CIELab space do not exactly describe the Munsell space. Therefore the CIELab space is not perfectly uniform, but is sufficiently uniform for most uses.
So, one of the issues we are dealing with is that we have to give up some perceptual uniformity for quantification and ease-of-modeling. The implications here is that at some places, the perceptual side will break down, but the benefits gained in terms of predictive capabilities outweigh these point failures. In other words, nothing is perfect.Further in the same paragraph:
There is a general rule of thumb that says that when comparing two color patches, which are placed near each other, which are near neutral in color, and which are in the environment specified for judging colors in the Munsell color system, a delta E*ab = 1 is at or near the threshold of visibility of the color difference for most people. If any of these conditions is changed, for example the environment is changed or the colors are very colorful instead of being near neutral or the colors are presented sequentially at very low frequency, not simultaneously, the threshold of visibility of the color difference increases. This means that for any of these different conditions, in order for a person to see the color difference, the delta E*ab will increase. Conversely, for any of these different conditions, a pair of color patches with a given delta E*ab will appear to be less different than if they were in the environment specified for judging colors in the Munsell color system. For the case in which the two colors are highly colored, the threshold of the visibility of the color difference increases to a delta E*ab of about 2. The result of this is that because the illuminance levels in a theatre are much lower than were used to define the Munsell space and because the color differences are between colors in one theatre using one projector and colors in another theatre using another projector, this delta E*ab of 4 is visually a very tight tolerance. In fact, few, if any, people will be able to detect the color difference between two colors from the same code values in two different theatres if both theatres and projectors are set up to these tolerances.
(Emphasis mine) While a reference back to the origin of the "4 dE" standard is not made in the text, its validity is based upon studies done by SMPTE. Do note, though, that this is for fully-specified Lab. Also note the reference to the "dim" environment in a typical theater. Color differences have been demonstrated to be harder to see where light levels are lower. Let's see how this translates back to the xy plane:
Originally Posted by p.73
In the a*b* diagram for each primary there are four circles for the four levels of luminance around each primary. The four circles for each primary are not all centered at the same a*b* values, but instead fall on a line from the a*b* values of the primary at its highest luminance to the neutral axis. The circle corresponding to the 0.5% luminance patch is the closest to the neutral axis. In the chromaticity diagram, the reference point of the tolerance figures is located at the same xy chromaticity coordinates independent of the luminance value. However, although all of the tolerance figures were circles of radius 4 in the a*b* diagram, the tolerance figures in the xy diagram increase in size with decreasing luminance.
(Again, emphasis mine) Here we see the difference between what one gets at the periphery versus at the "neutral" (i.e., gray/white) point. Again, for copyright purposes, I am attaching only the part figure L-2 where a constant dE of 4 (with dL = 0) is re-cast from the ab plane to an xy plane. What is notable is the absence of a discussion here of using a different calculation approach to dE for work in the gamut versus the grayscale.
Having worked through the introductory material in the appendix, let's look to section 6, Measurement of Projected Images (seems relevant). On page 13, you see the chromaticity and luminance targets and tolerances for reference theaters, review rooms and general exhibition theaters. Here we get a bit of irony, there is a +/- 0.002 deviation allowed for deviation in x or y, even though the perceptibility of these deviations is not constant. In practice, these should be very small, though. These are well within the measurement confidence interval for ordinary colorimeters. For luminance, the reference is 14ftL (unambigous) with a tolerance of +/- 1 ftL for review rooms (also unambiguous).
The tolerance for luminance is given in terms of a variance on gamma, rather than a variance in dE. SMPTE uses an ordinary power curve, with the computation being done in a log-log fashion. Reference slope (i.e., gamma exponent) is 2.6, with a tolerance for reference rooms of 2.548 to 2.652 (+/- 0.52). So, this is pretty important. What have we learned:
Section 6.13 gives us the method for verifying the color accuracy of the entire system using 12 measures, two each for RGBCMY. Here, the standard of dE(ab) being <= 4 is reasserted. Since white/gray is a color, presumably these also fall within this standard, though in reality, the tolerances given previously are a tighter standard.
- SMPTE uses dE(76, Lab)
- The amount of saturation/hue error that is perceptible varies with changes in adapted light level.
- Error in luminance is a "measure of concern" to SMPTE, but is measured in terms of gamma deviation, rather than dE -- until we get to section 6.13.
Originally Posted by 6.13; page 18
Within the minimum color gamut specified for the Reference Projector, all colors need to be accurately reproduced within a tolerance of 4 delta E*ab. A discussion of delta E*ab is given in Appendix L. In theory this applies to all colors, but in practice it would be impossible to display and measure all possible colors that can be encoded by the DCDM color encoding equations and displayed by a Reference Projector. Therefore Table 6-11 gives a set of colors that can be used to verify the color accuracy of a system. It is felt that if these colors are within the tolerance limits, then all colors are most likely within the tolerances. The neutral colors 6 through 10 in Table 6-7 may also be used as tests of the color accuracy of any projector in its environment.
So, given this, the obvious question is "why does CalMAN advocate anything different from SMPTE?" The reason is that until relatively recently, there were not ways for people to manipulate the gamma performance of their displays, so we wanted a method that preserves the (desirable) characteristic of color error diminishing as light level decreases, while, at the same time, giving our predominantly enthusiast customers clear guidance that was usable in a typical home environment with home tools (e.g., a 0.002 tolerance does you no good if your meter's limit under best circumstances is 0.004). In this respect, we use uv, rather than ab, because uv is better than xy, and at least a few publications (e.g., WSR) use it. Once you move beyond dE(76), you are "stuck" with Lab, so there is no uv vs. ab controversy there, only threads like this.
Originally Posted by TomHuffman
You must agree with me because that's what your software displays.
CalMAN displays a lot of things. Our goal is to provide the user as much information as possible. In terms of gamut charts, we prefer/recommend people use a uv chart, but there is only so much that education and outreach can do, as my efforts here prove.
I've said it before, and I'll say it again: dE(uv) provides a much more aggressive standard than either of the dE(76) methods. People who use it should feel confident that they are not going to encounter visible errors resulting from problems in saturation or hue. For an enthusiast, this may be too aggressive, at which point, CalMAN offers several alternatives.
If this is sufficient, then I'm not really sure what is.