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Myndex
SAPC-APCA
CSS2021–2025
Myndex / SAPC-APCA

SAPC-APCA

APCA (Accessible Perceptual Contrast Algorithm) is a new method for predicting contrast for use in emerging web standards (WCAG 3) for determining readability contrast. APCA is derived form the SAPC (S-LUV Advanced Predictive Color) which is an accessibility-oriented color appearance model designed for self-illuminated displays.

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596GitHub stars
REPOSITORY AT A GLANCE

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What it is

APCA (Accessible Perceptual Contrast Algorithm) is a new method for predicting contrast for use in emerging web standards (WCAG 3) for determining readability contrast. APCA is derived form the SAPC (S-LUV Advanced Predictive Color) which is an accessibility-oriented color appearance model designed for self-illuminated displays.

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Where it is used

No documented product use has been verified yet.

What people are saying

No recommendation phrase was found in the indexed comments.

No classified warning is attached to this page. Read the discussions before treating silence as approval.

Also in these discussions

No other repository is repeatedly linked in the same discussions.

This week on Hacker News

Nov 7–Nov 14 UTC · Direct repository links and separately labeled related project stories. Indexed through 11/14/2026, 5:04:57 PM UTC.

No direct repository links were found in the indexed HN discussions over the past seven days. Older discussions are available below.

Mentions over time

HN mentions by year6 dated mentions

Observed mentions: 2021-12-09 18:42:58 UTC to 2025-07-23 17:10:49 UTC.

01232021-01-01 00:00:00 UTC to 2022-01-01 00:00:00 UTC (exclusive): 1 mentions; 0 submissions, 1 comments20212022-01-01 00:00:00 UTC to 2023-01-01 00:00:00 UTC (exclusive): 3 mentions; 1 submissions, 2 comments2023-01-01 00:00:00 UTC to 2024-01-01 00:00:00 UTC (exclusive): 1 mentions; 0 submissions, 1 comments20232024-01-01 00:00:00 UTC to 2025-01-01 00:00:00 UTC (exclusive): 0 mentions; 0 submissions, 0 comments2025-01-01 00:00:00 UTC to 2026-01-01 00:00:00 UTC (exclusive): 1 mentions; 0 submissions, 1 comments2025

Counts come from the repository’s dated source evidence. Index cutoff: 2026-11-02 08:16:59 UTC.

View mention counts and date ranges
UTC ranges include their start and exclude their end. Select a period to filter the source evidence.
PeriodStart (UTC)End (exclusive, UTC)MentionsSubmissionsComments
20212021-01-01 00:00:00 UTC2022-01-01 00:00:00 UTC101
20222022-01-01 00:00:00 UTC2023-01-01 00:00:00 UTC312
20232023-01-01 00:00:00 UTC2024-01-01 00:00:00 UTC101
20242024-01-01 00:00:00 UTC2025-01-01 00:00:00 UTC000
20252025-01-01 00:00:00 UTC2026-01-01 00:00:00 UTC101

What Hacker News says

6 sources
seanwilson·Jul 23, 2025
> Tools like APCA improve contrast modeling, but they still don’t account for halation, ambient light mismatch, or eye strain. Doesn't APCA advocate a maximum contrast? Best link I could find right now is https://github.com/Myndex/SAPC-APCA/discussions/106 but it lacks explanation. If it doesn't account for the above, any ideas what APCA means by maximum contrast then and why the above was overlooked?
In My Color Combo Passes WCAG – So Why Does It Still Hurt to Look At?
Myndex·Sep 5, 2023
Hi @jjcm and @seanwilson First of all thank you for these comments, I do proactively seek out comments like these as there are too few at the official APCA discussion forum https://github.com/Myndex/SAPC-APCA/discussions That said: A px is not a device pixel, it is referenced to the canvas abstraction layer as the canonical CSS reference px. It represents 1.278 arc minutes of visual angle as subtended onto the retina. Or 0.0213° 𝑽𝜽 This is the case with a 96ppi monitor at 28" away. Visual Angle in arc minutes is what is used in research, and in fact what the Snellen eye chart is based around, 20/20 is based on a capital E that is 5 arc minutes high, where each line is 1 arcmin, and each space between the three horizontal lines is 1 arcmin, where we then find a spatial frequency of 30 cycles per degree. But that is for minimum acuity .
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Minimum acuity is NOT best readability, which needs to be ~2.5 or times larger than the acuity size (critical size). For a standard display a reference distance away, that means an x-height of 9.4px, which is 12 minutes of arc 𝑽𝜽. Critical size and critical contrast are recited and empirically tested for decades by eminent readability researchers Whittacker, Lovi-Kitchin, Ian Bailey, Legge, et alia. The font sizes for APCA are based on this, and assuming a reference font like Helvetica with a 0.52 x-height ratio. ALSO, if you want a "catch all" contrast value,Lc 75 is more appropriate IMO. NOT SET IN STONE YET This kind of discussion is good at the forum, so that it can be tracked and considered in the conversations about guidelines. ALSO If you want an easy way to use APCA and be fully backwards compatible with the old WCAG 2.x, then try BridgePCA at https://bridgepca.com
In Accessible Palette: stop using HSL for color systems (2021)
jjcm·May 20, 2022
For a more future oriented version (as setting a font to black wont work with dark pages), check out the CSS Color Module Level 5 spec for color-contrast(), which will automatically choose a color that has a proper contrast ratio: https://www.w3.org/TR/css-color-5/#funcdef-color-contrast One caveat is it's currently only supported in Safari technical preview. Another thing worth mentioning is the relative brightness algo used in the repo for the linked plugin is using the WCAG 2 formula for relative luminance. This works pretty well in light mode for non-colored backgrounds, but doesn't incorporate perceived luminosity differences of different hues very well. A better algorithm to use here is the Advanced Perceptual Contrast Algorithm (APCA), which is the one used in the WCAG 3 working spec. It more accurately accounts for hue: https://gi
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thub.com/Myndex/SAPC-APCA
In Change the colour of grey text back to black on website using poor contrast text
mananaysiempre·May 18, 2022
Note the licensing note in the repository[1] of the APCA™ index the author’s company[2] promotes: > The files currently in this repository are presently considered pre-release, and as such do not have a permanent license attached. In this repositiory, all files present are under a time-barred beta license, and intended for use with web-based content only, and not for any other use without written permission. The non-pre-release stuff[3] offered to W3C is better but still seems to be intended to end up non-FOSS: > Files in this repository are licensed to the W3/AGWG under their cooperative agreement for use with WCAG accessibility guidelines for web-based content only, and not for any other use. Similarly, the online calculators[4,5] referenced in the author’s posts (not this one) are > PROPRIETARY AND NOT LICENSED FOR THIRD PARTY USE. (Th
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e question being, of course, how much of this is copyrightable at all and to what extent these terms are just sowing legal FUD.) [1] https://github.com/Myndex/SAPC-APCA/ [2] https://www.myndex.com/CVD/ [3] https://github.com/Myndex/apca-w3/ [4] https://www.myndex.com/APCA/ [5] https://www.myndex.com/BPCA/
In Please Stop Using Grey Text

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