material-colors 0.4.2

#[cfg(all(not(feature = "std"), feature = "libm"))]
#[allow(unused_imports)]
use crate::utils::no_std::FloatExt;
use crate::{
    color::{y_from_lstar, WHITE_POINT_D65},
    utils::math::lerp,
};
use core::f64::consts::PI;

/// In traditional color spaces, a color can be identified solely by the
/// observer's measurement of the color. Color appearance models such as CAM16
/// also use information about the environment where the color was
/// observed, known as the viewing conditions.
///
/// For example, white under the traditional assumption of a midday sun white
/// point is accurately measured as a slightly chromatic blue by CAM16.
/// (roughly, hue 203, chroma 3, lightness 100)
///
/// This class caches intermediate values of the CAM16 conversion process that
/// depend only on viewing conditions, enabling speed ups.
#[derive(Debug)]
pub struct ViewingConditions {
    pub white_point: [f64; 3],
    pub adapting_luminance: f64,
    pub background_lstar: f64,
    pub surround: f64,
    pub discounting_illuminant: bool,

    pub background_ytowhite_point_y: f64,
    pub aw: f64,
    pub nbb: f64,
    pub ncb: f64,
    pub c: f64,
    pub n_c: f64,
    pub drgb_inverse: [f64; 3],
    pub rgb_d: [f64; 3],
    pub fl: f64,
    pub f_lroot: f64,
    pub z: f64,
}

impl ViewingConditions {
    pub fn standard() -> Self {
        Self::s_rgb()
    }

    pub fn s_rgb() -> Self {
        Self::make(None, None, None, None, None)
    }

    /// Convenience constructor for [`ViewingConditions`].
    ///
    /// # Parameters affecting color appearance include:
    /// * `whitePoint` - coordinates of white in Xyz color space.
    /// * `adaptingLuminance` - light strength, in lux.
    /// * `backgroundLstar` - average luminance of 10 degrees around color.
    /// * `surround` - brightness of the entire environment.
    /// * `discountingIlluminant` - whether eyes have adjusted to lighting.
    ///
    /// # Panics
    ///
    /// Will panic if the surround is equal or between 0.0 and 2.0
    pub fn make(
        white_point: Option<[f64; 3]>,
        adapting_luminance: Option<f64>,
        background_lstar: Option<f64>,
        surround: Option<f64>,
        discounting_illuminant: Option<bool>,
    ) -> Self {
        let white_point = white_point.unwrap_or(WHITE_POINT_D65);
        let adapting_luminance = adapting_luminance.unwrap_or(-1.0);
        let background_lstar = background_lstar.unwrap_or(50.0);
        let surround = surround.unwrap_or(2.0);
        let discounting_illuminant = discounting_illuminant.unwrap_or(false);

        let adapting_luminance = if adapting_luminance > 0.0 {
            adapting_luminance
        } else {
            200.0 / PI * y_from_lstar(50.0) / 100.0
        };
        // A background of pure black is non-physical and leads to infinities that
        // represent the idea that any color viewed in pure black can't be seen.
        let background_lstar = (0.1_f64).max(background_lstar);
        // Transform test illuminant white in Xyz to 'cone'/'rgb' responses
        let xyz = white_point;
        let (r_w, g_w, b_w) = (
            xyz[2].mul_add(-0.051461, xyz[0].mul_add(0.401288, xyz[1] * 0.650173)),
            xyz[2].mul_add(0.045854, xyz[0].mul_add(-0.250268, xyz[1] * 1.204414)),
            xyz[2].mul_add(0.953127, xyz[0].mul_add(-0.002079, xyz[1] * 0.048952)),
        );

        // Scale input surround, domain (0, 2), to CAM16 surround, domain (0.8, 1.0)
        assert!((0.0..=2.0).contains(&surround));

        let f = 0.8 + (surround / 10.0);
        // "Exponential non-linearity"
        let c = if f >= 0.9 {
            lerp(0.59, 0.69, (f - 0.9) * 10.0)
        } else {
            lerp(0.525, 0.59, (f - 0.8) * 10.0)
        };
        // Calculate degree of adaptation to illuminant
        let d = if discounting_illuminant {
            1.0
        } else {
            f * (1.0f64 / 3.6f64).mul_add(-((-adapting_luminance - 42.0) / 92.0).exp(), 1.0)
        };
        // Per Li et al, if D is greater than 1 or less than 0, set it to 1 or 0.
        let d = d.clamp(0.0, 1.0);
        // chromatic induction factor
        let nc = f;

        // Cone responses to the whitePoint, r/g/b/W, adjusted for discounting.
        //
        // Why use 100.0 instead of the white point's relative luminance?
        //
        // Some papers and implementations, for both CAM02 and CAM16, use the Y
        // value of the reference white instead of 100. Fairchild's Color Appearance
        // Models (3rd edition) notes that this is in error: it was included in the
        // CIE 2004a report on CIECAM02, but, later parts of the conversion process
        // account for scaling of appearance relative to the white point relative
        // luminance. This part should simply use 100 as luminance.
        let rgb_d = [
            d.mul_add(100.0 / r_w, 1.0) - d,
            d.mul_add(100.0 / g_w, 1.0) - d,
            d.mul_add(100.0 / b_w, 1.0) - d,
        ];

        // Factor used in calculating meaningful factors
        let k = 1.0 / 5.0f64.mul_add(adapting_luminance, 1.0);
        let k4 = k * k * k * k; // pow(k, 4)
        let k4_f = 1.0 - k4;

        // Luminance-level adaptation factor
        let fl = k4.mul_add(
            adapting_luminance,
            0.1 * k4_f * k4_f * (5.0 * adapting_luminance).cbrt(),
        );
        // Intermediate factor, ratio of background relative luminance to white relative luminance
        let n = y_from_lstar(background_lstar) / white_point[1];

        // Base exponential nonlinearity
        // note Schlomer 2018 has a typo and uses 1.58, the correct factor is 1.48
        let z = 1.48 + n.sqrt();

        // Luminance-level induction factors
        let nbb = 0.725 / n.powf(0.2);

        let ncb = nbb;

        // Discounted cone responses to the white point, adjusted for post-saturationtic
        // adaptation perceptual nonlinearities.
        let rgb_afactors = [
            (fl * rgb_d[0] * r_w / 100.0).powf(0.42),
            (fl * rgb_d[1] * g_w / 100.0).powf(0.42),
            (fl * rgb_d[2] * b_w / 100.0).powf(0.42),
        ];

        let rgb_a = [
            (400.0 * rgb_afactors[0]) / (rgb_afactors[0] + 27.13),
            (400.0 * rgb_afactors[1]) / (rgb_afactors[1] + 27.13),
            (400.0 * rgb_afactors[2]) / (rgb_afactors[2] + 27.13),
        ];

        let aw = (40.0f64.mul_add(rgb_a[0], 20.0 * rgb_a[1]) + rgb_a[2]) / 20.0 * nbb;

        Self {
            white_point,
            adapting_luminance,
            background_lstar,
            surround,
            discounting_illuminant,
            background_ytowhite_point_y: n,
            aw,
            nbb,
            ncb,
            c,
            n_c: nc,
            drgb_inverse: [0.0, 0.0, 0.0],
            rgb_d,
            fl,
            f_lroot: fl.powf(0.25),
            z,
        }
    }
}

#[cfg(test)]
mod tests {
    use super::ViewingConditions;
    use float_cmp::assert_approx_eq;

    #[test]
    fn test_viewing_conditions() {
        let result1 = ViewingConditions::make(None, None, None, None, None);

        assert_approx_eq!(f64, result1.adapting_luminance, 11.725677948856951);
        assert_approx_eq!(f64, result1.background_lstar, 50.0);
        assert_approx_eq!(f64, result1.surround, 2.0);

        assert!(!result1.discounting_illuminant);
    }
}