APCA

APCA (Accessible Perceptual Contrast Algorithm) is a perceptually weighted contrast metric developed by Andrew Somers, intended as a successor to WCAG 2.x luminance contrast. It is the algorithm being evaluated for inclusion in WCAG 3. iridis ships APCA-W3 0.0.98G-4g (packages/core/src/math/ContrastApca.ts).

Why APCA

WCAG 2.1's ratio is symmetric and treats foreground and background interchangeably. In practice, dark text on a light background and light text on a dark background do not need the same contrast to read equally well, the eye is asymmetric. WCAG 2.1 is also a poor predictor at the dark end of the scale, where small differences in luminance compress into large differences in ratio.

APCA addresses both. It uses different exponents for text and background, returns a signed value that records polarity, and produces a number that maps more directly to the practical question "is this text readable at this size and weight".

Lc

The output is Lc, lightness contrast, a signed value, conventionally reported with a magnitude on roughly [0, 108].

Lc sign	Meaning
Positive	light background, dark text
Negative	dark background, light text
Zero	below the noise floor; treated as no readable contrast

Magnitudes:

Surface	Lc magnitude
Small body text (under 14 pt thin / 12 pt regular)	≥ 75
Fluent body text (normal sizes)	≥ 60
Large text and headlines	≥ 45
Non-text icons and graphical components	≥ 30

The math

APCA computes per-side luminance with separate exponents, then takes the difference and applies clamping and scaling.

Soft clamp

For each side, Y_soft applies a polynomial clamp at the dark end:

Y_soft = Y                                        if Y ≥ 0.022
       = Y + (0.022 − Y) ^ 1.414                   otherwise

The clamp suppresses runaway sensitivity to tiny absolute luminances near pure black.

Asymmetric weighting

Y_text = 0.2126729 · R_lin^0.56  +  0.7151522 · G_lin^0.56  +  0.0721750 · B_lin^0.56
Y_bg   = 0.2126729 · R_lin^0.65  +  0.7151522 · G_lin^0.65  +  0.0721750 · B_lin^0.65

The exponents differ, 0.56 for text, 0.65 for background, which is what gives APCA its asymmetry. The Rec. 709 luminance coefficients are unchanged.

Difference and scale

if bg_clamped > text_clamped:
    Lc = (bg_clamped^0.56 − text_clamped^0.57) · 1.14
    if Lc < 0.001:  return 0
    Lc = Lc − 0.027
else:
    Lc = (bg_clamped^0.62 − text_clamped^0.65) · 1.14
    if Lc > −0.001:  return 0
    Lc = Lc + 0.027

return Lc · 100

Two branches: one for normal-polarity (light bg, dark text), one for reverse-polarity (dark bg, light text). Each branch applies its own exponent pair and offset. The final multiply by 100 puts Lc on the conventional scale.

The constants in ContrastApca.ts (SA98G_NORM_BG, SA98G_NORM_TXT, SA98G_CLAMP, SA98G_CLAMP_P, SA98G_SCALE, SA98G_LOW_CLIP, SA98G_OFFSET) are the canonical APCA-W3 0.0.98G-4g values from the Myndex SAPC reference.

Math primitive

contrastApca is exported as a singleton from @studnicky/iridis (packages/core/src/math/ContrastApca.ts) and called directly:

import { contrastApca } from '@studnicky/iridis';

const lc = contrastApca.apply(text, background);

Both arguments are ColorRecord instances. The primitive reads the rgb field of each (sRGB-safe, gamut-mapped if the source OKLCH lay outside sRGB) and returns a signed Lc value.

Where it appears

• enforce:contrast (packages/core/src/tasks/enforce/EnforceContrast.ts) routes pairs whose algorithm is 'apca' to contrastApca.apply(text, background) and uses the absolute value of Lc against the pair's minRatio.
• enforce:apca in the contrast plugin (packages/contrast/src/tasks/) provides a wrapper that applies APCA threshold tables to pairs without each pair restating the magnitude.
• A role schema selects APCA per pair:

contrastPairs: [
  { foreground: 'text', background: 'canvas', minRatio: 60, algorithm: 'apca' },
]