theme/

color.rs

//! Color math: the oklch → sRGB → HSL converters behind the token constructors,
//! WCAG contrast for the palette tests, and small paint helpers.

use gpui::{Hsla, hsla};

/// WCAG AA for body text: the most contrast [`kept`] asks text to keep.
pub const LEGIBLE: f32 = 4.5;

/// A neutral (chroma 0) oklch tone as Hsla. Chroma 0 means r == g == b exactly,
/// so this goes straight to an achromatic Hsla (skipping the hue math avoids
/// float-noise saturation).
pub fn neutral(lightness: f32) -> Hsla {
    let [v, _, _] = oklch_to_srgb(lightness, 0.0, 0.0);
    hsla(0.0, 0.0, v, 1.0)
}

/// An exact achromatic tone from an 8-bit channel value (`grey(13)` ≡ `#0d0d0d`)
/// — for surfaces matched against reference-screenshot samples.
pub fn grey(value: u8) -> Hsla {
    hsla(0.0, 0.0, value as f32 / 255.0, 1.0)
}

/// Convert an oklch color (CSS notation: L 0..1, C, H in degrees) to gpui Hsla.
pub fn oklch(l: f32, c: f32, h_deg: f32) -> Hsla {
    let [r, g, b] = oklch_to_srgb(l, c, h_deg);
    let (h, s, l) = rgb_to_hsl(r, g, b);
    hsla(h, s, l, 1.0)
}

/// oklch → sRGB (each 0..1, clamped/gamut-clipped per channel).
/// Reference: Björn Ottosson's OKLab definition (the same matrices CSS Color 4 uses).
pub fn oklch_to_srgb(l: f32, c: f32, h_deg: f32) -> [f32; 3] {
    let [r, g, b] = oklch_to_linear(l, c, h_deg);
    [gamma_encode(r), gamma_encode(g), gamma_encode(b)]
}

/// oklch → *linear* sRGB, unclamped: a component outside 0..1 is a color the
/// display cannot make, which is what [`fit_chroma`] tests for.
fn oklch_to_linear(l: f32, c: f32, h_deg: f32) -> [f32; 3] {
    let h = h_deg.to_radians();
    oklab_to_linear(l, c * h.cos(), c * h.sin())
}

/// OKLab → *linear* sRGB, unclamped.
fn oklab_to_linear(l: f32, a: f32, b: f32) -> [f32; 3] {
    // OKLab → LMS (cube roots undone)
    let l_ = l + 0.396_337_78 * a + 0.215_803_76 * b;
    let m_ = l - 0.105_561_346 * a - 0.063_854_17 * b;
    let s_ = l - 0.089_484_18 * a - 1.291_485_5 * b;
    let (l3, m3, s3) = (l_ * l_ * l_, m_ * m_ * m_, s_ * s_ * s_);

    // LMS → linear sRGB
    let r = 4.076_741_7 * l3 - 3.307_711_6 * m3 + 0.230_969_93 * s3;
    let g = -1.268_438 * l3 + 2.609_757_4 * m3 - 0.341_319_4 * s3;
    let b = -0.004_196_086_3 * l3 - 0.703_418_6 * m3 + 1.707_614_7 * s3;

    [r, g, b]
}

fn gamma_encode(x: f32) -> f32 {
    let x = x.clamp(0.0, 1.0);
    if x <= 0.003_130_8 {
        12.92 * x
    } else {
        1.055 * x.powf(1.0 / 2.4) - 0.055
    }
}

fn gamma_decode(x: f32) -> f32 {
    let x = x.clamp(0.0, 1.0);
    if x <= 0.040_449_936 {
        x / 12.92
    } else {
        ((x + 0.055) / 1.055).powf(2.4)
    }
}

/// Linear sRGB → OKLab.
fn linear_to_oklab([r, g, b]: [f32; 3]) -> [f32; 3] {
    let l = (0.412_221_47 * r + 0.536_332_55 * g + 0.051_445_995 * b).cbrt();
    let m = (0.211_903_5 * r + 0.680_699_5 * g + 0.107_396_96 * b).cbrt();
    let s = (0.088_302_46 * r + 0.281_718_85 * g + 0.629_978_7 * b).cbrt();
    [
        0.210_454_26 * l + 0.793_617_8 * m - 0.004_072_047 * s,
        1.977_998_5 * l - 2.428_592_2 * m + 0.450_593_7 * s,
        0.025_904_037 * l + 0.782_771_77 * m - 0.808_675_77 * s,
    ]
}

/// `from` moved `t` of the way to `to` (0..1) in OKLab, alpha alongside.
pub fn mix_oklab(from: Hsla, to: Hsla, t: f32) -> Hsla {
    let t = t.clamp(0.0, 1.0);
    let lab = |color: Hsla| {
        let [r, g, b] = hsl_to_rgb(color.h, color.s, color.l);
        linear_to_oklab([gamma_decode(r), gamma_decode(g), gamma_decode(b)])
    };
    let (a, b) = (lab(from), lab(to));
    let [l, x, y] = std::array::from_fn(|ix| a[ix] + (b[ix] - a[ix]) * t);
    let [r, g, bl] = oklab_to_linear(l, x, y);
    let (h, s, l) = rgb_to_hsl(gamma_encode(r), gamma_encode(g), gamma_encode(bl));
    hsla(h, s, l, from.a + (to.a - from.a) * t)
}

/// The oklch lightness of an achromatic tone: [`neutral`] inverted.
///
/// Only greys round-trip through this. A grey's three channels are equal, and
/// the three LMS matrix rows each sum to 1, so the whole OKLab transform
/// collapses to one cube root — no inverse matrix needed.
pub fn lightness(color: Hsla) -> f32 {
    gamma_decode(color.l).cbrt()
}

/// The most chroma sRGB can hold at this lightness and hue, up to `chroma`.
///
/// Near black and near white the gamut is a needle: asking for a mid-ramp
/// chroma there produces an out-of-range component, and clamping it per channel
/// shifts the hue instead of dropping the saturation. Tailwind's neutral ramps
/// taper their chroma at both ends by hand for the same reason; here the taper
/// is whatever the gamut allows, so no ramp has to be tabulated.
fn fit_chroma(l: f32, chroma: f32, hue: f32) -> f32 {
    let fits = |c: f32| {
        oklch_to_linear(l, c, hue)
            .iter()
            .all(|x| (-1e-4..=1.0 + 1e-4).contains(x))
    };
    if fits(chroma) {
        return chroma;
    }
    let (mut lo, mut hi) = (0.0, chroma);
    for _ in 0..24 {
        let mid = 0.5 * (lo + hi);
        if fits(mid) { lo = mid } else { hi = mid }
    }
    lo
}

/// Re-emit an achromatic tone at `hue`, carrying as much `chroma` as its
/// lightness can hold. Alpha rides through untouched.
pub fn tint(color: Hsla, hue: f32, chroma: f32) -> Hsla {
    if chroma <= 0.0 {
        return color;
    }
    let l = lightness(color);
    let mut out = oklch(l, fit_chroma(l, chroma, hue), hue);
    out.a = color.a;
    out
}

/// sRGB (0..1 components) → HSL, all components 0..1 (gpui's Hsla convention).
pub fn rgb_to_hsl(r: f32, g: f32, b: f32) -> (f32, f32, f32) {
    let max = r.max(g).max(b);
    let min = r.min(g).min(b);
    let l = (max + min) / 2.0;
    let delta = max - min;
    if delta < f32::EPSILON {
        return (0.0, 0.0, l);
    }
    let s = if l > 0.5 {
        delta / (2.0 - max - min)
    } else {
        delta / (max + min)
    };
    let h = if (max - r).abs() < f32::EPSILON {
        ((g - b) / delta).rem_euclid(6.0)
    } else if (max - g).abs() < f32::EPSILON {
        (b - r) / delta + 2.0
    } else {
        (r - g) / delta + 4.0
    } / 6.0;
    (h, s, l)
}

/// HSL (gpui convention, all 0..1) → sRGB components 0..1.
pub fn hsl_to_rgb(h: f32, s: f32, l: f32) -> [f32; 3] {
    if s <= f32::EPSILON {
        return [l, l, l];
    }
    let q = if l < 0.5 {
        l * (1.0 + s)
    } else {
        l + s - l * s
    };
    let p = 2.0 * l - q;
    let hue = |mut t: f32| {
        t = t.rem_euclid(1.0);
        if t < 1.0 / 6.0 {
            p + (q - p) * 6.0 * t
        } else if t < 0.5 {
            q
        } else if t < 2.0 / 3.0 {
            p + (q - p) * (2.0 / 3.0 - t) * 6.0
        } else {
            p
        }
    };
    [hue(h + 1.0 / 3.0), hue(h), hue(h - 1.0 / 3.0)]
}

/// WCAG 2.1 relative luminance of an opaque color.
pub fn relative_luminance(color: Hsla) -> f32 {
    let lin = |c: f32| {
        if c <= 0.040_45 {
            c / 12.92
        } else {
            ((c + 0.055) / 1.055).powf(2.4)
        }
    };
    let [r, g, b] = hsl_to_rgb(color.h, color.s, color.l);
    0.2126 * lin(r) + 0.7152 * lin(g) + 0.0722 * lin(b)
}

/// WCAG 2.1 contrast ratio between two opaque colors (1.0 … 21.0).
///
/// Used by the palette tests to prove each light token reproduces the contrast
/// its dark counterpart had, rather than merely looking plausible.
pub fn contrast_ratio(a: Hsla, b: Hsla) -> f32 {
    let (la, lb) = (relative_luminance(a), relative_luminance(b));
    let (hi, lo) = if la >= lb { (la, lb) } else { (lb, la) };
    (hi + 0.05) / (lo + 0.05)
}

/// Whichever of `a` and `b` has the higher contrast ratio against `plate`,
/// `a` on a tie.
pub fn legible_on(plate: Hsla, a: Hsla, b: Hsla) -> Hsla {
    if contrast_ratio(plate, a) >= contrast_ratio(plate, b) {
        a
    } else {
        b
    }
}

/// `layers`, each of which may be translucent, painted in order over the
/// opaque `base`: the color the eye receives where they all overlap.
pub fn stack(base: Hsla, layers: impl IntoIterator<Item = Hsla>) -> Hsla {
    layers
        .into_iter()
        .fold(opaque(base), |under, layer| flatten(layer, under))
}

/// The contrast `text` keeps when something is painted under it: what it has
/// over `page`, up to [`LEGIBLE`]. Faint text is held to its own contrast, not
/// raised to body text's.
pub fn kept(text: Hsla, page: Hsla) -> f32 {
    let page = opaque(page);
    contrast_ratio(flatten(text, page), page).min(LEGIBLE)
}

/// `color` at full alpha: a translucent surface measured as though nothing
/// showed through it.
fn opaque(color: Hsla) -> Hsla {
    hsla(color.h, color.s, color.l, 1.0)
}

/// The color to draw `text` (which may be translucent) in over the opaque
/// `plate`: `text` where it reads at `floor` there, else `text` with its
/// lightness inverted where that does, else whichever of black and white reads
/// better.
pub fn legible(text: Hsla, plate: Hsla, floor: f32) -> Hsla {
    let reads = |color: Hsla| contrast_ratio(flatten(color, plate), plate) >= floor;
    if reads(text) {
        return text;
    }
    let inverted = hsla(text.h, text.s, 1.0 - text.l, text.a);
    if reads(inverted) {
        return inverted;
    }
    legible_on(plate, hsla(0.0, 0.0, 1.0, 1.0), hsla(0.0, 0.0, 0.0, 1.0))
}

/// Composite `fg` (which may be translucent) over an opaque `bg`, returning the
/// opaque result — the color the eye actually receives.
pub fn flatten(fg: Hsla, bg: Hsla) -> Hsla {
    let a = fg.a.clamp(0.0, 1.0);
    let [fr, fg_, fb] = hsl_to_rgb(fg.h, fg.s, fg.l);
    let [br, bg_, bb] = hsl_to_rgb(bg.h, bg.s, bg.l);
    let (h, s, l) = rgb_to_hsl(
        fr * a + br * (1.0 - a),
        fg_ * a + bg_ * (1.0 - a),
        fb * a + bb * (1.0 - a),
    );
    hsla(h, s, l, 1.0)
}

/// Linear per-component mix of two colors (paint helper for the gradient spinner).
pub fn mix(a: Hsla, b: Hsla, t: f32) -> Hsla {
    let t = t.clamp(0.0, 1.0);
    let lerp = |x: f32, y: f32| x + (y - x) * t;
    // Mix through hue naively — both spinner endpoints sit close enough on the
    // wheel that shortest-arc handling isn't needed for our palette.
    hsla(
        lerp(a.h, b.h),
        lerp(a.s, b.s),
        lerp(a.l, b.l),
        lerp(a.a, b.a),
    )
}