Contrast Ratio Math and Related Visual Issues

[Myndex]

Note (Jan 2023):

This thread is mainly the very early research notes and discussions that lead to the development of certain perceptually uniform contrast methods. While of interest from a historical perspective, the most current understanding of perceptually uniform contrast can be found at the Contrast Resources page, and current discussions in the APCA forum.

The initial conclusions did not present themselves until the first several months of posts, around the time of initial fixes described in May 2019, however the present day body of knowledge is substantially improved beyond that point.

As such, this thread remains here as a matter of historical interest only, and should not be referenced directly. For a reference, a good current overview is this article

Thank you for reading,

— Andy

Andrew Somers
_{W3C AGWG Invited Expert
Research Lead, Visual Contrast
Color Scientist @ Myndex Research}

---

Original Post of April 2019:

The W3C's specification for determining sRGB contrast as discussed in "Understanding WCAG 2.0 and 2.1, Minimum Contrast 1.4.3" is not perceptually uniform and as a result creates "contrast ratios" that are not meaningful. The end result is incorrect contrast choices for some web colors. Compounding the problem are the number of "contrast tools" based on this math all over the web, and all of which are returning invalid data.

The end result are websites that may comply with the W3C's math for contrast, but are otherwise difficult to read. The bad math coupled with contrast tools have provided designers with color schemes of poor accessibility. This needs to be addressed!

PROBLEM SUMMARY

Edit May 2019: after first-round research, we've found that the issue is not so much using "simple contrast" as it is the manner (or lack of) considering ambient lighting, the nature of illuminated¹ displays, and/or a lack of math that better models human non-linear perception.

(L1 + 0.05)/(L2 + 0.05) (aka "simple contrast") is only really useful to determine monitor max on/off (#FFF / #000) Edit 6/19 I'll restate this as "does not accurately model an illuminated¹ display in real world ambient conditions": . It fails badly in the midrange colors as it does not adjust for human nonlinear perception. As such it should not be used to programmatically determine legibility of colors, esp. in the middle and darker ranges.

Weber contrast, Michelson Contrast, Bartleson-Breneman Perceptual Contrast Length (PCL), or other possible candidates are better choices for programatic legibility assessment. I am currently conducting studies on a "best" programatic contrast assessment algorithm for UI/Web design and will update this issue as I do. I am presently leaning toward a variation of PCL as it prevents the near-black contrast expansion. Using this and applying an exponent to the luminance data looks promising.

Edit June 2019: results of research/experiments below show that a "standard" Weber contrast does not provide better performance by itself — it requires a modification for a more accurate model of a computer display).

Note¹: By "illuminated" I mean both emissive (led), and transmissive backlit (LCD) display types.

Web Links to some of the pages of the document:
https://www.w3.org/TR/WCAG21/#dfn-contrast-ratio
https://www.w3.org/TR/UNDERSTANDING-WCAG20/visual-audio-contrast-contrast
https://www.w3.org/TR/2008/REC-WCAG20-20081211/#relativeluminancedef

PROBLEM EXAMPLE:

I prepared a webpage that demonstrates the problem here:
http://myndex.com/WEB/W3Contrastissue

(Edit: the full set of experiments is at https://www.myndex.com/WEB/Perception )

Here is a reduced resolution screenshot of part of that test page.

In the above experiment, we set a number of panels to color-pairs with a contrast ratio of 4.5:1, this counts as a "PASS" for the W3C spec of minimum contrast for small text. Interspersed among these panels are color-pairs that the W3C criteria counts as a "FAIL" even for large text, with a contrast of 2.9:1

As you can see, many of the "PASS" color pairs are actually hard to read and of low contrast, while all of the "FAIL" pairs are substantially easier to read and of higher perceptual contrast.

The point here is that the "contrast ratios" created by the equations listed in the WCAG documents are not useful or meaningful for determining perceptual luminance contrast.

Part of the reason this is happening is using simple contrast (L1/L2) Simple Contrast fails to account for non linear human perception in values between #000 and #FFF. Also troubling is the use of outdated standards documents or drafts. I list these issues on the webpage:
http://myndex.com/WEB/W3Contrastissue

The upshot of all this is that if "contrast ratios" are going to be promoted as a means to define color for accessible design, then there needs to be a clear path to assess contrast based on human perception.

Looking for a Solution (EDIT 4/25/19: Better solutions in later posts)

One idea is to process the luminance with an exponent (^1.6) then take 1/3rd the contrast result, using either weber contrast or perceptual contrast length.

The purpose of the exponent is to shift contrast for black/dark text vs white/light text, this adjusts for our perception that light text on a medium or darker background has a higher perceived contrast than dark text on a medium background. The purpose for taking 1/3rd of the result is to bring the output numbers into line with the W3C standard indicating a 4.5:1 contrast.

A more ideal solution would be to commission a study with human subjects of various visual impairments to fine tune a model for programatic contrast assessment.

-Andrew Somers
Title Supervisor
General Titles & Visual Effects
Hollywood, Ca.

[patrickhlauke]

xref w3c/wcag#360 (comment)

[Myndex]

xref #360 (comment)

Thank you Patrick but as you can see I already posted in that thread. That is a separate and somewhat minor issue. The issue I discuss in THIS thread is specifically about the minimum contrast 1.4.3, and is not minor as it has far-reaching consequences such as a ton of apps that now incorrectly present colors as "accessible" when in fact they are not. And this issue has led to a great deal of misunderstanding regarding color choices and contrast.

The thread you linked to deals with a minor error in the relative luminance equation, and while the W3C picked the wrong equation that is not what is causing the much more serious problem that I outline in this separate issue.

The issue HERE is using a simple contrast (L1/L2) on linear luminance to define color & luminance contrast. But this does not provide any meaningful value for perceived contrast.

I posted this as an issue for discussion while I continue my research (search for an accurate programatic contrast assessment) and pull request separately.

[Myndex]

Some additional thoughts regarding 1.4.3

FWIW MY BACKGROUND: I work in the film and television industry in Hollywood as an editor/colorist/VFX & Title Supervisor. I work with color and visual perception issues every day.

I am going to continue to post in this thread while I delve further into this before generating a pull request. It is a concern for me because this W3C document is considered authoritative, and has made its way into government regulations. It is important that it be correct, and it is not at present. PLEASE COMMENT if you have thoughts or insights as to why some of these choices were made. Thank you.

On Terms:

Simple contrast "is not useful for real-world luminances, because of their much higher dynamic range and the logarithmic response characteristics of the human eye."[1]

and using simple contrast seems to have led to the higher (4.5:1) contrast specifications:

What is the cite and specific justification for the claimed need for a 4.5:1 contrast ratio? Studies by Legge, Rubin, Bangor etc. found that "Contrast by itself had no significance for either vision group." [unimpaired or impaired] [2]

However font size and polarity are very important, and contrast does interact with very small font sizes to a degree, especially in negative polarity. The Bangor study indicated that font sizes below 18 px resulted in a need for increased contrast, but these study participants were ether legally blind (20/200) or very impaired (20/100).

It is NOT about contrast as much as size and possibly polarity. While it is true that increasing contrast can help legibility for small fonts for visually impaired, increasing the font size offers a better improvement.

As I mentioned in my first post Weber or Michaelson are possibly better here, and one of those are what is used in nearly all the research & standards. However, I am working with Bartleson-Breneman perceptual contrast at the moment. EDIT: they are used in research for consistency and for the at-threshold JND, but are not perceptual uniform at supra-threshold which is what is needed for design guidance.

To make this point more clear: The "simple" ratio of #FFF to #808080 is 4.6:1 (3.95:1 if you add in the W3C's 5% bonus luminance). But #808080 to #040404 is a ratio of 178.88:1 (5:19:1 using the 5% extra).

#FFF is luminance 100, mid-grey #808080 is 21.59, and #040404 is 0.12

So ignoring the oddly-applied/misapplied "flare" value, white to mid grey is a ratio of 100:21.6 and mid grey to black is a ratio of 21.7:0.12

BUT because the first much smaller ratio is also associated with high luminance it is much easier to read and has much better PERCEPTUAL contrast:

4.6:1 (3.95:1)

And the black one with 179:1 contrast (LOL, 5.19:1 with the cheat)

I find no justification for the 4.5:1 contrast ratio for 20/40 vision as indicated in the W3's standard. Is it set that way (along with the excessive flare luminance add) to attempt make up for the other deficiencies?

See also reference [3] below, a EU paper on this subject.

Contrast Sensitivity is a separate measurement from visual acuity. From the referenced Arditi paper: "visual acuity measurements alone are insufficient to characterize basic spatial visual function..." But I don't see where you get multiplying the ISO well established 3:1 standard by 1.5 ?? Looking at acuity vs contrast graphs is see a difference in CS as low as 5% for a 20/40 person. And as I recall the common 3:1 luminance contrast ratio included near normal vision (20/40 is near normal).

Here's a graph, (for reference logMAR 0.3 is approximately 20/40.)

In short, it appears to me the 4.5:1 contrast standard is somewhat arbitrary, and there are other more important means to improve accessibility, namely font size, appropriate polarity, and total luminance.

LUMINANCE:

NITS TO THE RESCUE! (by nits I mean cd/m^2, 1 cd/m^2 is 1 nit ... but maybe I also mean ME, nit-picking on this issue, LOL).

The sRGB spec states an 80 nit monitor, however people commonly adjust them to 120 nit to 160 nit, even more (300+ is common. Some phones do 1200). If the monitor is brighter, and the material is black text on white, the light from the monitor results in pupil contraction which improves perceived sharpness.

I'll opine that it is more important to have a monitor that is adjusted bright enough for its environment. In fact it would be a good idea to lobby the ISO for an amendment to the sRGB spec to adjust away from 80 cd/m2 to a specific luminance based on the environment. 1996 was a long time ago, and display technology has changed substantially — we shouldn't have to adjust the ROOM lighting to match the monitor, it's easier to adjust the monitor. A standard stating the max display luminance for a given ambient light would go a long way toward real accessibility/accommodation.

SUMMARY:

The main thing I am lobbying for here is a revised programatic contrast assessment that is perceptually correct. But as I research this, I see there are other concerns that should be considered.

• New contrast assessment method, with a revision of the standard to accurately model perceptual needs. I.e. no simple ratios, etc.
• Among the contrast revisions, provide guidance on excessive contrast — too high of a contrast causes reading problems and eye strain as well.
• Add guidance on display polarization (light on dark vs dark on light) to the standard.
• Along with polarization, guidance for local adaptation which is part of the issue with current contrast assessments. (Local adaptation is one reason that white on a darker background looks more contrasty than grey on a dark background even if the luminance ratio is the same).
• Emphasize font size for accessibility, with guidance to avoid ever overriding the user agent's root font size.
• Lobby ISO to amend sRGB to provide a standard for adjusting a display to the room. 80 nits is really a bit dim in a lot of environments.

Thank you for reading. I hope to have a solid contrast assessment model this week.

Andy

Refs:
[1] https://www.schorsch.com/en/kbase/glossary/contrast.html
[2] https://pdfs.semanticscholar.org/4f9f/f4bcbc0eb8d1040228e5d84cd0c0e75962c7.pdf
[3] https://www.anec.eu/images/Publications/technical-studies/ANEC-final-report-1503-1700-Lenoir-et-al.pdf

Edited May 22 for typos and some clarity.

[mraccess77]

Hi @Myndex

• I agree that there are some combinations that pass that should fail and that there are some situations that fail that should pass. However, in general the algorithm works well and has made a big difference to ensure that text has better contrast.
• While I agree that lower acuity doesn't necessarily mean less contrast sensitivity -- once you get into the 2070 20200 levels folks tend to have eye conditions with multiple factors and there is a larger correlation between the two
• 20/40 may not be visually impaired at least by most US standards. 20/70 is generally considered visually impaired by many and I believe the criteria were written to help many of us who use vision in the range of 20/70 - 20/400. For someone with 20/200 contrast very much tends to be an important issue and these guidelines were written for folks in this range. 4.5 is very much a tipping for point for many of us and I do not agree that 4.5 is too high for someone who is visually impaired or legally blind. Many legally blind people use their vision and require this type of minimum contrast. So if the understanding text is not clear around this area then we need to update it.

[Myndex]

Hi @mraccess77

Thank you for commenting, it helps me to see when I am not explaining or describing completely. But perhaps more important it leads to some new discoveries. In answering your post I did some experiments that add insight. More below.

First, just to provide a little more background, I want to mention that I have personal experience with 20/200 vision. Several years ago (in my late 40s) I developed early onset cataracts which brought my vision to worse than 20/200 in one eye, the other diminished a bit less. I now have IOL implants, but those surgeries caused vitreal detachments and retinal detachments, requiring a vitrectomy in one eye, and in the other, continuing issues due to large vitreal floaters that still can interfere with reading. Also, I still need glasses (i.e. it is trivial for me to remove them to introduce poor vision).

As such WCAG is a topic I have a close personal interest in.

As I mentioned in an above post, I am an imaging professional in Hollywood with a career that spans decades and a background that includes broadcast engineering, colorist, VFX Supervisor, and perhaps most relevant, title designer.

I came across this WCAG issue while developing a CSS framework. For the color module I am trying to create a simple color subset that is both easy to read and aesthetically pleasing. This led me down the color and vision theory rabbit hole, where I stumbled on a contrast calculator and saw an odd comment by the coder who mentioned that his "calculate" button did not pass the WCAG standard. The button is completely readable with more than adequate contrast, thus started my present research journey.

I have since been doing nothing but research this issue in depth. My posts here are based on that research and my extensive experience in digital imaging.

• mraccess77 said: I agree that there are some combinations that pass that should fail and that there are some situations that fail that should pass. However, in general the algorithm works well and has made a big difference to ensure that text has better contrast.

I cannot agree here. Estimating roughly, 40% of the color pairs the WCAG math calls "PASS" are poor in quality and should fail. And somewhere in the area of 51% of colors they fail could conceivably pass.

Wrong nearly half the time is one huge fail. I believe it is the result of incorrect assumptions and cherry picking various bits of standards and cobbling them together into something that is truly inaccurate and unsuitable for the purpose.

I'm going to quote Whittle from his paper [1] (emphasis added)

With regard to the mathematical description, an important distinction is between ratios and contrast expressions that also involve differences. This concerns what is meant by ‘contrast’. Weber contrast and Michelson contrast are the commonest expressions for it, not the simple ratio L/Lb. Weber contrast = (L- Lb)/Lb, usually written ÆL/Lb. When the backgrounds are weak a ‘dark light’ constant must be included: ÆL/(Lb+L0). Michelson contrast = |ÆL|/(L+Lb). When people talk of contrast coding in the early stages of the visual pathway, they usually mean that the firing rate of sensory neurons is a function of, perhaps proportional to, Weber contrast or Michelson contrast. Such a code combines differencing (calculating L-Lb) with normalisation (attenuating by some function of the absolute stimulus level).

And then from Pelli's paper [2] (emphasis added)

The contrast of the target quantifies its relative difference in luminance from the background, and may be specified as Weber contrast Lmax−LminLbackground, Michelson contrast Lmax−LminLmax+Lmin, or RMS contrast LσLμ, where Lmax, Lmin, Lbackground, Lμ, and Lσ are luminance maximum, minimum, background, mean, and standard deviation, respectively. Weber contrast is preferred for letter stimuli, Michelson contrast is preferred for gratings, and RMS contrast is preferred for natural stimuli and efficiency calculations (Bex & Makous, 2002; Pelli & Farell, 1999). Threshold contrast is the contrast required to see the target reliably. The reciprocal of threshold is called sensitivity.

The WCAG ignores this wholesale, despite it being prominent in most research and even noted in the ISO and ANSI standards. WCAG is using simple contrast (Lw/Lk) and that is one of the errors.

Among my findings, some of the sites I have the most difficultly reading are "compliant" with the WCAG. There are countless contrast checkers and other automated or semi automated accessibility checkers that use the algorithm as written, and they all fail to detect poor perceptual contrast.

And there are a LOT of combinations that pass but are hard to read. An accessibility site has this on one of their pages indicating the problem:

The contrast checkers are all using this flawed model which ignores many important aspects of perception. The results are all over the place and inconsistent. I can see why designers are ignoring the contrast recommendations: they are ambiguous and inconsistent at best. At the moment, visual judgement does a better job than the contrast calculators.

I have a page where I am conducting live experiments in this regard, aloing with some commentary on my findings as I go along. This link is:
https://www.myndex.com/WEB/W3Contrastissue

Here are some examples from today's experiments:

Today, I've been looking into luminance — it is well known and researched that increasing luminance improves readability (within a range). One thing the WCAG lacks is a specification on minimum lightness for the lightest element in a color pair.

I have more on this on the page, but as you can see, setting a minimum lightness of the lightest element to #AAA results in a consistent, readable, block of text. On the page you'll see examples where pages with 3:1 contrast and minimum #AAA on the brightest of the pair is more readable than 4.5:1 contrast on darker pairs.

• mraccess77 said: While I agree that lower acuity doesn't necessarily mean less contrast sensitivity -- once you get into the 2070 20200 levels folks tend to have eye conditions with multiple factors and there is a larger correlation between the two.

Yes, I am aware, there is definitely correlation to contrast sensitivity due to a number of vision impairments. Indeed, one can have good visual acuity and bad contrast sensitivity. The WCAG does not discuss CS though, and only lists some Snellen numbers like 20/40.

I was talking mainly of 20/40, which is what the WCAG talks about regarding AA. For the portion of the standard that relates to profoundly impaired, still the math they provide does not create useful numbers for guidance.

And that is the point I am getting at. The math is essentially wrong. Lw/Lk is not well suited for determining contrast in this context. The vast majority of research on contrast sensitivity uses WEBER or MICHELSON or both. Rarely simple contrast. But there are other math mistakes in WCAG related to sRGB and computer displays that also need to be addressed.

(EDIT by Andy: May 2019: My recent experiments and research indicates that a "classical, unmodified Weber contrast" is really not "substantially" better than the WCAG math, though there is a modified Weber from Hwang/Peli that is much better than the WCAG math, and other more modern contrast equations such as PCL).

• mraccess77 said: 20/40 may not be visually impaired at least by most US standards. 20/70 is generally considered visually impaired by many and I believe the criteria were written to help many of us who use vision in the range of 20/70 - 20/400. For someone with 20/200 contrast very much tends to be an important issue and these guidelines were written for folks in this range. 4.5 is very much a tipping for point for many of us and I do not agree that 4.5 is too high for someone who is visually impaired or legally blind. Many legally blind people use their vision and require this type of minimum contrast. So if the understanding text is not clear around this area then we need to update it.

I never said that 4.5:1 is too high, particularly in regards to profound impairment, 4.5:1 is TOO LOW when using the WCAG math. WCAG indicates 7:1 for the more visually impaired (AAA). And yes, the "understanding" text is all over the place and not clear.

To be clear, I am not "condemning" the 4.5:1 ratio per se, but I am questioning where it was derived from and the basis of the equations when those equations are not supported by standards nor research. I am also pointing out that luminance is a much bigger factor than contrast yet it is not mentioned, nor is local adaptation unless I missed seeing that. My statements here are from published research as well as my own research.

The California Council of the Blind (Lozano, 2009) states, and Federal ADA guidelines also state, that contrast for signs be 70% (note it is a percent not a ratio). The math used for the Federal standard is (B1-B2)/B1 (B1 is the lighter LRV, and B2 is the darker).[3] Now, if I use the WCAG math, 70% equals a ratio somewhere around 2.3:1 to 3.2:1. WCAG math is all over the place and does not relate to Weber's law nor anything else useful.

Nevertheless, California Council of the Blind (Lozano, 2009)[4] is on record stating that the Federal equation is flawed when B1 is less than 45. I just came across this a minute ago — I'm slightly amused as it is closely mirroring what I have been saying about the WCAG.

I describe this in more detail on the experiments page, and there are more examples.

In closing I just want to say that simply switching the equation to Weber is not the complete answer. I think we can do better, and that is the focus of my research.

Thank you again for the comments.

Andy

REFS:
[1]http://aardvark.ucsd.edu/color/whittle.pdf
[2]https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3744596/
[3]https://segd.org/sites/default/files/SEGD_2012_ADA_White_Paper_Update.pdf
[4]https://www.documents.dgs.ca.gov/dsa/access/Pt2_Final-SOR.pdfHi @mraccess77

Edited May 2019 for some minor clarity fixes.

[mraccess77]

Hi @Myndex I appreciate your efforts in addressing the shortcomings of the current algorithm. In your examples, I personally found some of the first 4.5 items easier to read than the ones with values greater than #AAA -- thus I know there will always be differences in interpretation by different people as we all see differently. Along those lines with the adaption you were discussing, halos may technically be used to meet the requirement but when you take into account the width of the stroke and surrounding colors haloed text can actually be harder for me to read. I agree that we want more people to use contrasting colors that meet users needs and if we can change the algorithm to meet those needs without lessening it get more adoption that would be a good thing. Personally, I see these changes as something that can't be changed with the current standard as the method is too normative to change with an errata but would be a great opportunity to address for the next version of the accessibility guidelines (silver). It would be good to socialize this with some other folks such as Jared Smith from WebAIM who also would like to change the future direction of the contrast calculations and the Low Vision Accessibility Task Force which is part of the Accessibility Guidelines Working group. Adding @WayneEDick and @allanj-uaag.

[bruce-usab]

Thanks @Myndex for writing this up so thoroughly!

[Myndex]

2023 edit for clarification: Weber is not the "gold standard" today, and the link to this post is an inappropriate reference made by Bengfort, as this post was part of the very early due diligence looking into the problems of WCAG 2 contrast failures. (In fact this post was made literally within the first two days of looking into and discussing this problem, four years later the body of knowledge in this area is substantially improved).

Weber is not perceptually uniform for the purpose of supra-threshold text on self-illuminated displays. Further, deeper investigation into the origins of the WCAG 2.0 contrast math shows that it is a derivation of Weber, but it is extremely important to point out that the derivation was originally intended only for 7:1, and not intended for lower contrasts such as 3:1 or 4.5:1. At 7:1, the dark color failures of WCAG 2 contrast were not as obvious.

Nevertheless the poor results of WCAG2 were used as part of the misguided 1.4.3 SC, despite the objections of major stakeholders including IBM.

Original 2019 message continues below:

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Hi @mraccess77

Hi @Myndex I appreciate your efforts in addressing the shortcomings of the current algorithm. In your examples, I personally found some of the first 4.5 items easier to read than the ones with values greater than #AAA.

If that is based on the images in my post above, I should mention that the SIZE of the first set is 30% larger, and therefore easier to read (I just realized the scaling error due to how this site handles images as I looked at the post, post edited to correct).BUT ALSO, three of the first 5 have the brightest color well above #AAA. For an apples to apples comparison, please see the live experiment on the website: https://www.myndex.com/WEB/W3Contrastissue
Text scaling on the site for the samples is equal in each group (the first group is general contrast assessments, and the second is body text).

thus I know there will always be differences in interpretation by different people as we all see differently. Along those lines with the adaption you were discussing, halos may technically be used to meet the requirement but when you take into account the width of the stroke and surrounding colors haloed text can actually be harder for me to read.

Indeed, for instance, research shows that most people do better with dark text on a light background (Positive Display) but with my vision, I much prefer light/colored text on a black background (Negative Display). Right now I am having difficulty with THIS site due to the bright background (L* 98 in the text area) yet for most people this is the ideal presentation.

I agree that we want more people to use contrasting colors that meet users needs and if we can change the algorithm to meet those needs without lessening it get more adoption that would be a good thing. Personally, I see these changes as something that can't be changed with the current standard as the method is too normative to change with an errata but would be a great opportunity to address for the next version of the accessibility guidelines (silver).

Yes I see it was just tagged as WCAG 2.2 which I was somewhat expecting. Correcting the algorithm also means changing the standard, as far as I can tell the current standard(s) seem to be compensating for the math issues — I don't know the complete history, but that is how it appears based on the reverse engineering/analysis.

For an errata, it might be useful to place a note to the effect of: "Current contrast algorithms may overvalue contrasts with pairs of darker colors. Designers should be cautioned not to rely on contrast numbers in these cases/"

I should note that problems and controversy on this very subject are visibly present in the research and some standards. It is partly why I am being so proactive here. I hope to cut through the clutter to bring some clarity (puns more or less intended).

It would be good to socialize this with some other folks such as Jared Smith from WebAIM who also would like to change the future direction of the contrast calculations and the Low Vision Accessibility Task Force which is part of the Accessibility Guidelines Working group. Adding @WayneEDick and @allanj-uaag.

Excellent. What are the deadlines for 2.2? As I mentioned in one of my posts while there is much research on simple monitor displays (i.e. black on white, white on black), there is not much in the way of research on complex, graphically rich content (that I've found anyway). I'm thinking some empirical studies would be illustrative.

ON ALGORITHMS:
Weber has been the "gold standard" for text contrast. But I am presently leaning toward a modified version of Perceptual Contrast Length (PCL) which "essentially" uses a modified L* type curve (perceptual lightness) in calculating contrast. I believe I have a way to adapt it so the contrast results are similar to what people are used to using (i.e. 3:1 etc).

• 2022 EDIT for clarity: Weber or Michelson are used in research, but neither are "perceptually uniform over the visual range" as neither takes adaptation into account. To grossly oversimplify, this means only the stimulus can change, but not the background as that directly impacts adaptation. Weber may have been the "gold standard" once upon a time, back in the day when everything was printed on white paper—it is not even close to accurate when adaptation and spatial frequency are added variables. Stevens in the 1960s was critical of Weber and found various power-laws better defined perception, and I believe Stevens was the first (or among them) to recognize the importance of spatial frequency. More recently Poynton observed that the lightness perception curve is better described as part log and part power-law. In 2019, the author found further variables that are the subject of a paper (WIP).

Original post continues:

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Another idea is using the difference between a brighter/darker L* values (as in CIE L* a* b*).

Those are all fairly simple models for contrast determination. A more advanced approach is a true color or image model like CIECAM02 or ICAM. ICAM is the work of Mark Fairchild at Rochester Inst. of Technology. A model like that could (I believe) analyze an overall page, as opposed to a pair of colors in isolation.

On the experiments page there is an example of local adaptation issues do to surrounding colors. But here's a quick example:

The blue text on grey is WCAG 4.5:1 contrast, and both bits of text are identical. But the one centered on black is more readable because the black allows local adaptation to the darker colors. So among other things, minimum padding for elements against a high contrasting color are important.

(I'll mention in passing that pair of blue on grey is a fail in my modified PCL algorithm).

And this is where things are more complicated than a simple contrast — web content is graphically rich. Text on a background may be a pass, but if it is far different than the overall page, adaptation will affect perceived legibility.

Thank you again,

Andy

[Myndex]

Thanks @Myndex for writing this up so thoroughly!

Thank you @bruce-usab — I consider this a particularly important issue, partly because W3C standards are used not just for web but for app design and other applications as well. Because it is a freely distributed standard, it has a very wide reach. Myself I spend 12 hours a day in front of monitors, while that may be more than average displays are certainly integral to the lives of so many — we are inseparable from our technology — standards like this have a very real affect on people's lives. This standard in particular has become part of government regulations, for instance.

What is the timeline/deadlines for 2.2? I'm hoping to have some candidate contrast models soon, but also thinking there is one giant rabbit hole to crawl down considering how page complexity affects perception (adaptation) etc.

Thank you!

Andy

[Myndex]

There is a mathematical problem with this discussion line. L1 and L2 are computed using weightings that take color receptivity into account. R, G, and B have distinct weights in the relative luminance formula.

Hi @WayneEDick , thank you for commenting. I’m away from the studio, on location, so I can’t comment in depth, but:

Yes, luminance is spectrally weighted. However luminance is a linear measure of light.

Light is linear (additive) but human vision is NOT linear (essentially a power curve). So while the sRGB coefficients adjust for spectral sensitivity, luminance is NOT relative to PERCEPTION of lightness. L* is perceptual (CIELAB), and gamma encoded transfer curves are somewhat perceptual (such as luma, the Y‘ of Y‘IQ) but not luminance (Y).

But that’s not even the most relevant part. L1/L2 is called “simple contrast” and it is wrong in this context. The “standard” for contrast for TEXT is Weber, which is based on ∆L/L typically as
(LMax-Lmin)/Lbackground or (LMax-Lmin)/LMax
And is in fact still using linear luminance. But there are some better more modern implementations now such as Perceptual Contrast Length (PCL) which essentially converts to a modified L* (perceptual lightness). EDIT FOR CLARITY: WCAG2 simple contrast is essentially Weber+1, regardless, values become increasingly incorrect and lacking in perceptual uniformity as the color pair becomes darker relative to the adaptation level. Therefore they both fail for self-illuminated displays for similar reasons. This was clear a short time after making this post, but as this thread ranks highly on search engines it's useful to amend errata inline, here. End edit.

This issue came to my attention when I saw the contrast equation was wrong (as is the sRGB threshold WCAG lists), as I have outlined in my posts above. But now that this is being discussed, we can do better. I am currently investigating PCL and other methods.

For further details, I suggest Charles Poynton’s GAMMA FAQ and COLOR FAQ. Here’s a link: https://poynton.ca/GammaFAQ.html
but please feel free to ask me any other questions.

-Andy

[bruce-usab]

@Myndex, you asked:

What is the timeline/deadlines for 2.2?

The formal/approved Project Plan has a goal of this time next year for the first public working draft.

I'm hoping to have some candidate contrast models soon, but also thinking there is one giant rabbit hole to crawl down considering how page complexity affects perception (adaptation) etc.

I am not optimistic about the chances for wholesale replacement formulas for 2.2. That is possible for 3.0.

This standard in particular has become part of government regulations, for instance.

Yes. I am one of the actors in helping that happen.

There was some user testing associated with the validation of the 2.0 formula. I could not quickly find a cite for that. My recollection is that the hard data pointed to a ratio of 4.65:1 as a defensible break point. The working group was close to rounding that up to 5:1, just to have round numbers. I successfully lobbied for 4.5:1 mostly because (1) the empirical data was not overwhelmingly compelling, and (2) 4.5:1 allowed the option for white and black (simultaneously) on a middle gray.

I am sorry to say that I will offline for the next ten days or so, but I will be circling back to this!