delta_e 0.2.1

DeltaE is a pure-Rust implementation of the CIEDE2000 algorithm
Documentation 
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use super::{f32, Lab};

pub struct DE2000;

impl DE2000 {
    /// Returns the difference between two `Lab` colors.
    ///
    /// ### Example
    ///
    /// ```
    /// extern crate delta_e;
    /// extern crate lab;
    ///
    /// use delta_e::DE2000;
    /// use lab::Lab;
    ///
    /// fn main() {
    ///     let color_1 = Lab {
    ///         l: 38.972,
    ///         a: 58.991,
    ///         b: 37.138,
    ///     };
    ///
    ///     let color_2 = Lab {
    ///         l: 54.528,
    ///         a: 42.416,
    ///         b: 54.497,
    ///     };
    ///
    ///     let delta_e = DE2000::new(color_1, color_2);
    ///     println!("The color difference is: {}", delta_e);
    /// }
    /// ```

    pub fn new(color_1: Lab, color_2: Lab) -> f32 {
        let ksub_l = 1.0;
        let ksub_c = 1.0;
        let ksub_h = 1.0;

        let delta_l_prime = color_2.l - color_1.l;

        let l_bar = (color_1.l + color_2.l) / 2.0;

        let c1 = (color_1.a.powi(2) + color_1.b.powi(2)).sqrt();
        let c2 = (color_2.a.powi(2) + color_2.b.powi(2)).sqrt();

        let c_bar = (c1 + c2) / 2.0;

        let a_prime_1 =
            color_1.a + (color_1.a / 2.0) * (1.0 - (c_bar.powi(7) / (c_bar.powi(7) + 25f32.powi(7))).sqrt());
        let a_prime_2 =
            color_2.a + (color_2.a / 2.0) * (1.0 - (c_bar.powi(7) / (c_bar.powi(7) + 25f32.powi(7))).sqrt());

        let c_prime_1 = (a_prime_1.powi(2) + color_1.b.powi(2)).sqrt();
        let c_prime_2 = (a_prime_2.powi(2) + color_2.b.powi(2)).sqrt();

        let c_bar_prime = (c_prime_1 + c_prime_2) / 2.0;

        let delta_c_prime = c_prime_2 - c_prime_1;

        let s_sub_l = 1.0 +
                      ((0.015 * (l_bar - 50.0).powi(2)) / (20.0 + (l_bar - 50.0).powi(2)).sqrt());

        let s_sub_c = 1.0 + 0.045 * c_bar_prime;

        let h_prime_1 = get_h_prime_fn(color_1.b, a_prime_1);
        let h_prime_2 = get_h_prime_fn(color_2.b, a_prime_2);

        let delta_h_prime = get_delta_h_prime(c1, c2, h_prime_1, h_prime_2);

        let delta_upcase_h_prime = 2.0 * (c_prime_1 * c_prime_2).sqrt() *
                                   (degrees_to_radians(delta_h_prime) / 2.0).sin();

        let upcase_h_bar_prime = get_upcase_h_bar_prime(h_prime_1, h_prime_2);

        let upcase_t = get_upcase_t(upcase_h_bar_prime);

        let s_sub_upcase_h = 1.0 + 0.015 * c_bar_prime * upcase_t;

        let r_sub_t = get_r_sub_t(c_bar_prime, upcase_h_bar_prime);

        let lightness: f32 = delta_l_prime / (ksub_l * s_sub_l);

        let chroma: f32 = delta_c_prime / (ksub_c * s_sub_c);

        let hue: f32 = delta_upcase_h_prime / (ksub_h * s_sub_upcase_h);

        (lightness.powi(2) + chroma.powi(2) + hue.powi(2) + r_sub_t * chroma * hue).sqrt()
    }

    /// Returns the difference between two RGB colors.
    ///
    /// ### Example
    ///
    /// ```
    /// extern crate delta_e;
    ///
    /// use delta_e::DE2000;
    ///
    /// fn main() {
    ///     let color_1 = [234, 76, 76];
    ///     let color_2 = [76, 187, 234];
    ///
    ///     let delta_e = DE2000::from_rgb(&color_1, &color_2);
    ///     println!("The color difference is: {}", delta_e);
    /// }
    /// ```

    pub fn from_rgb(color_1: &[u8; 3], color_2: &[u8; 3]) -> f32 {
        let lab_1 = Lab::from_rgb(color_1);
        let lab_2 = Lab::from_rgb(color_2);

        DE2000::new(lab_1, lab_2)
    }
}

fn get_h_prime_fn(x: f32, y: f32) -> f32 {
    let mut hue_angle;

    if x == 0.0 && y == 0.0 {
        return 0.0;
    }

    hue_angle = radians_to_degrees(x.atan2(y));

    if hue_angle < 0.0 {
        hue_angle += 360.0;
    }

    hue_angle
}

fn get_delta_h_prime(c1: f32, c2: f32, h_prime_1: f32, h_prime_2: f32) -> f32 {
    if 0.0 == c1 || 0.0 == c2 {
        return 0.0;
    }

    if (h_prime_1 - h_prime_2).abs() <= 180.0 {
        return h_prime_2 - h_prime_1;
    }

    if h_prime_2 <= h_prime_1 {
        h_prime_2 - h_prime_1 + 360.0
    } else {
        h_prime_2 - h_prime_1 - 360.0
    }
}

fn get_upcase_h_bar_prime(h_prime_1: f32, h_prime_2: f32) -> f32 {
    if (h_prime_1 - h_prime_2).abs() > 180.0 {
        return (h_prime_1 + h_prime_2 + 360.0) / 2.0;
    }

    (h_prime_1 + h_prime_2) / 2.0
}

fn get_upcase_t(upcase_h_bar_prime: f32) -> f32 {
    1.0 - 0.17 * (degrees_to_radians(upcase_h_bar_prime - 30.0)).cos() +
    0.24 * (degrees_to_radians(2.0 * upcase_h_bar_prime)).cos() +
    0.32 * (degrees_to_radians(3.0 * upcase_h_bar_prime + 6.0)).cos() -
    0.20 * (degrees_to_radians(4.0 * upcase_h_bar_prime - 63.0)).cos()
}

fn get_r_sub_t(c_bar_prime: f32, upcase_h_bar_prime: f32) -> f32 {
    -2.0 * (c_bar_prime.powi(7) / (c_bar_prime.powi(7) + 25f32.powi(7))).sqrt() *
    (degrees_to_radians(60.0 * (-(((upcase_h_bar_prime - 275.0) / 25.0).powi(2))).exp())).sin()
}

fn radians_to_degrees(radians: f32) -> f32 {
    radians * (180.0 / f32::consts::PI)
}

fn degrees_to_radians(degrees: f32) -> f32 {
    degrees * (f32::consts::PI / 180.0)
}

#[cfg(test)]
mod tests {
    use super::{DE2000, Lab};

    fn round(val: f32) -> f32 {
        let rounded = val * 10000 as f32;
        rounded.round() / 10000 as f32
    }

    fn assert_delta_e(expected: f32, lab1: &[f32; 3], lab2: &[f32; 3]) {
        let color_1 = Lab {
            l: lab1[0],
            a: lab1[1],
            b: lab1[2],
        };

        let color_2 = Lab {
            l: lab2[0],
            a: lab2[1],
            b: lab2[2],
        };

        assert_eq!(round(DE2000::new(color_1, color_2)), expected);
    }

    // Tests taken from Table 1: "CIEDE2000 total color difference test data" of
    // "The CIEDE2000 Color-Difference Formula: Implementation Notes,
    // Supplementary Test Data, and Mathematical Observations" by Gaurav Sharma,
    // Wencheng Wu and Edul N. Dalal.
    //
    // http://www.ece.rochester.edu/~gsharma/papers/CIEDE2000CRNAFeb05.pdf

    #[test]
    fn tests() {
        assert_delta_e(0.0, &[0.0, 0.0, 0.0], &[0.0, 0.0, 0.0]);
        assert_delta_e(0.0, &[99.5, 0.005, -0.010], &[99.5, 0.005, -0.010]);
        assert_delta_e(100.0, &[100.0, 0.005, -0.010], &[0.0, 0.0, 0.0]);
        assert_delta_e(2.0425, &[50.0000, 2.6772, -79.7751], &[50.0000, 0.0000, -82.7485]);
        assert_delta_e(2.8615, &[50.0000, 3.1571, -77.2803], &[50.0000, 0.0000, -82.7485]);
        assert_delta_e(3.4412, &[50.0000, 2.8361, -74.0200], &[50.0000, 0.0000, -82.7485]);
        assert_delta_e(1.0000, &[50.0000, -1.3802, -84.2814], &[50.0000, 0.0000, -82.7485]);
        assert_delta_e(1.0000, &[50.0000, -1.1848, -84.8006], &[50.0000, 0.0000, -82.7485]);
        assert_delta_e(1.0000, &[50.0000, -0.9009, -85.5211], &[50.0000, 0.0000, -82.7485]);
        assert_delta_e(2.3669, &[50.0000, 0.0000, 0.0000], &[50.0000, -1.0000, 2.0000]);
        assert_delta_e(2.3669, &[50.0000, -1.0000, 2.0000], &[50.0000, 0.0000, 0.0000]);
        assert_delta_e(7.1792, &[50.0000, 2.4900, -0.0010], &[50.0000, -2.4900, 0.0009]);
        assert_delta_e(7.1792, &[50.0000, 2.4900, -0.0010], &[50.0000, -2.4900, 0.0010]);
        assert_delta_e(7.2195, &[50.0000, 2.4900, -0.0010], &[50.0000, -2.4900, 0.0011]);
        assert_delta_e(7.2195, &[50.0000, 2.4900, -0.0010], &[50.0000, -2.4900, 0.0012]);
        assert_delta_e(4.8045, &[50.0000, -0.0010, 2.4900], &[50.0000, 0.0009, -2.4900]);
        assert_delta_e(4.7461, &[50.0000, -0.0010, 2.4900], &[50.0000, 0.0011, -2.4900]);
        assert_delta_e(4.3065, &[50.0000, 2.5000, 0.0000], &[50.0000, 0.0000, -2.5000]);
        assert_delta_e(27.1492, &[50.0000, 2.5000, 0.0000], &[73.0000, 25.0000, -18.0000]);
        assert_delta_e(22.8977, &[50.0000, 2.5000, 0.0000], &[61.0000, -5.0000, 29.0000]);
        assert_delta_e(31.9030, &[50.0000, 2.5000, 0.0000], &[56.0000, -27.0000, -3.0000]);
        assert_delta_e(19.4535, &[50.0000, 2.5000, 0.0000], &[58.0000, 24.0000, 15.0000]);
        assert_delta_e(1.0000, &[50.0000, 2.5000, 0.0000], &[50.0000, 3.1736, 0.5854]);
        assert_delta_e(1.0000, &[50.0000, 2.5000, 0.0000], &[50.0000, 3.2972, 0.0000]);
        assert_delta_e(1.0000, &[50.0000, 2.5000, 0.0000], &[50.0000, 1.8634, 0.5757]);
        assert_delta_e(1.0000, &[50.0000, 2.5000, 0.0000], &[50.0000, 3.2592, 0.3350]);
        assert_delta_e(1.2644, &[60.2574, -34.0099, 36.2677], &[60.4626, -34.1751, 39.4387]);
        assert_delta_e(1.2630, &[63.0109, -31.0961, -5.8663], &[62.8187, -29.7946, -4.0864]);
        assert_delta_e(1.8731, &[61.2901, 3.7196, -5.3901], &[61.4292, 2.2480, -4.9620]);
        assert_delta_e(1.8645, &[35.0831, -44.1164, 3.7933], &[35.0232, -40.0716, 1.5901]);
        assert_delta_e(2.0373, &[22.7233, 20.0904, -46.6940], &[23.0331, 14.9730, -42.5619]);
        assert_delta_e(1.4146, &[36.4612, 47.8580, 18.3852], &[36.2715, 50.5065, 21.2231]);
        assert_delta_e(1.4441, &[90.8027, -2.0831, 1.4410], &[91.1528, -1.6435, 0.0447]);
        assert_delta_e(1.5381, &[90.9257, -0.5406, -0.9208], &[88.6381, -0.8985, -0.7239]);
        assert_delta_e(0.6377, &[6.7747, -0.2908, -2.4247], &[5.8714, -0.0985, -2.2286]);
        assert_delta_e(0.9082, &[2.0776, 0.0795, -1.1350], &[0.9033, -0.0636, -0.5514]);
    }
}