farg 0.4.5

A Rust library for colorimetry, color space conversions, and color manipulation.
Documentation 
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152

//! CMC l:c color difference.
//!
//! Implements the [CMC l:c](https://en.wikipedia.org/wiki/Color_difference#CMC_l:c_(1984))
//! color difference formula developed by the Colour Measurement Committee of the Society
//! of Dyers and Colourists. The formula uses CIE LCh components and is **not symmetric** —
//! the first argument is treated as the reference color.

use crate::space::{Lch, Xyz};

/// Calculates the CMC l:c color difference for perceptibility (l=1, c=1).
///
/// The first argument is the reference color; the second is the sample.
/// This function is **not** order-independent.
pub fn calculate(reference: impl Into<Xyz>, sample: impl Into<Xyz>) -> f64 {
  calculate_parametric(reference, sample, 1.0, 1.0)
}

/// Calculates the CMC l:c color difference for acceptability (l=2, c=1).
///
/// The first argument is the reference color; the second is the sample.
/// This function is **not** order-independent.
pub fn calculate_acceptability(reference: impl Into<Xyz>, sample: impl Into<Xyz>) -> f64 {
  calculate_parametric(reference, sample, 2.0, 1.0)
}

/// Calculates the CMC l:c color difference with custom l and c factors.
///
/// The first argument is the reference color; the second is the sample.
/// This function is **not** order-independent.
///
/// - `l` — lightness weighting factor (1.0 for perceptibility, 2.0 for acceptability)
/// - `c` — chroma weighting factor (typically 1.0)
pub fn calculate_parametric(reference: impl Into<Xyz>, sample: impl Into<Xyz>, l: f64, c: f64) -> f64 {
  let ref_lch = Lch::from(reference.into());
  let smp_lch = Lch::from(sample.into());

  let l1 = ref_lch.l();
  let c1 = ref_lch.c();
  let h1 = ref_lch.h();

  let l2 = smp_lch.l();
  let c2 = smp_lch.c();

  let dl = l1 - l2;
  let dc = c1 - c2;

  // Compute ΔH from Lab differences
  let ref_lab = crate::space::Lab::from(ref_lch.to_xyz());
  let smp_lab = crate::space::Lab::from(smp_lch.to_xyz());
  let da = ref_lab.a() - smp_lab.a();
  let db = ref_lab.b() - smp_lab.b();
  let dh_sq = (da * da + db * db - dc * dc).max(0.0);

  let sl = if l1 < 16.0 {
    0.511
  } else {
    0.040975 * l1 / (1.0 + 0.01765 * l1)
  };
  let sc = 0.0638 * c1 / (1.0 + 0.0131 * c1) + 0.638;

  let h1_rad = h1.to_radians();
  let f = (c1.powi(4) / (c1.powi(4) + 1900.0)).sqrt();
  let t = if (164.0..=345.0).contains(&h1) {
    0.56 + (0.2 * (h1_rad + 168.0_f64.to_radians()).cos()).abs()
  } else {
    0.36 + (0.4 * (h1_rad + 35.0_f64.to_radians()).cos()).abs()
  };
  let sh = sc * (f * t + 1.0 - f);

  let term_l = dl / (l * sl);
  let term_c = dc / (c * sc);
  let term_h = dh_sq / (sh * sh);

  (term_l * term_l + term_c * term_c + term_h).sqrt()
}

#[cfg(test)]
mod test {
  use super::*;

  mod calculate {
    use super::*;

    #[test]
    fn it_returns_zero_for_identical_colors() {
      let color = Xyz::new(0.4, 0.5, 0.3);

      assert_eq!(calculate(color, color), 0.0);
    }

    #[test]
    fn it_returns_positive_for_different_colors() {
      let a = Xyz::new(0.0, 0.0, 0.0);
      let b = Xyz::new(0.9505, 1.0, 1.089);

      assert!(calculate(a, b) > 0.0);
    }

    #[test]
    fn it_increases_with_greater_difference() {
      let reference = Xyz::new(0.9505, 1.0, 1.089);
      let near = Xyz::new(0.8, 0.9, 0.95);
      let far = Xyz::new(0.2, 0.3, 0.25);

      assert!(calculate(reference, far) > calculate(reference, near));
    }
  }

  mod calculate_acceptability {
    use super::*;

    #[test]
    fn it_returns_zero_for_identical_colors() {
      let color = Xyz::new(0.4, 0.5, 0.3);

      assert_eq!(calculate_acceptability(color, color), 0.0);
    }

    #[test]
    fn it_differs_from_perceptibility() {
      let a = Xyz::new(0.1, 0.2, 0.3);
      let b = Xyz::new(0.5, 0.6, 0.7);

      let perceptibility = calculate(a, b);
      let acceptability = calculate_acceptability(a, b);

      assert!((perceptibility - acceptability).abs() > 1e-10);
    }
  }

  mod calculate_parametric {
    use pretty_assertions::assert_eq;

    use super::*;

    #[test]
    fn it_matches_perceptibility_with_same_params() {
      let a = Xyz::new(0.1, 0.2, 0.3);
      let b = Xyz::new(0.5, 0.6, 0.7);

      assert_eq!(calculate_parametric(a, b, 1.0, 1.0), calculate(a, b));
    }

    #[test]
    fn it_matches_acceptability_with_same_params() {
      let a = Xyz::new(0.1, 0.2, 0.3);
      let b = Xyz::new(0.5, 0.6, 0.7);

      assert_eq!(calculate_parametric(a, b, 2.0, 1.0), calculate_acceptability(a, b));
    }
  }
}