palette/
relative_contrast.rs

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
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
use crate::{
    bool_mask::{HasBoolMask, LazySelect},
    num::{Arithmetics, PartialCmp, Real},
};

/// A trait for calculating relative contrast between two colors.
///
/// W3C's Web Content Accessibility Guidelines (WCAG) 2.1 suggest a method
/// to calculate accessible contrast ratios of text and background colors for
/// those with low vision or color deficiencies, and for contrast of colors used
/// in user interface graphics objects.
///
/// These criteria are recommendations, not hard and fast rules. Most
/// importantly, look at the colors in action and make sure they're clear and
/// comfortable to read. A pair of colors may pass contrast guidelines but still
/// be uncomfortable to look at. Favor readability over only satisfying the
/// contrast ratio metric. It is recommended to verify the contrast ratio
/// in the output format of the colors and not to assume the contrast ratio
/// remains exactly the same across color formats. The following example checks
/// the contrast ratio of two colors in RGB format.
///
/// ```rust
/// use std::str::FromStr;
/// use palette::{Srgb, RelativeContrast};
/// # fn main() -> Result<(), palette::rgb::FromHexError> {
///
/// // the rustdoc "DARK" theme background and text colors
/// let background: Srgb<f32> = Srgb::from(0x353535).into_format();
/// let foreground = Srgb::from_str("#ddd")?.into_format();
///
/// assert!(background.has_enhanced_contrast_text(foreground));
/// # Ok(())
/// # }
/// ```
///
/// The possible range of contrast ratios is from 1:1 to 21:1. There is a
/// Success Criterion for Contrast (Minimum) and a Success Criterion for
/// Contrast (Enhanced), SC 1.4.3 and SC 1.4.6 respectively, which are concerned
/// with text and images of text. SC 1.4.11 is a Success Criterion for "non-text
/// contrast" such as user interface components and other graphics. The relative
/// contrast is calculated by `(L1 + 0.05) / (L2 + 0.05)`, where `L1` is the
/// luminance of the brighter color and `L2` is the luminance of the darker
/// color both in sRGB linear space. A higher contrast ratio is generally
/// desirable.
///
/// For more details, visit the following links:
///
/// [Success Criterion 1.4.3 Contrast (Minimum) (Level AA)](https://www.w3.org/WAI/WCAG21/Understanding/contrast-minimum)
///
/// [Success Criterion 1.4.6 Contrast (Enhanced) (Level AAA)](https://www.w3.org/WAI/WCAG21/Understanding/contrast-enhanced)
///
/// [Success Criterion 1.4.11 Non-text Contrast (Level AA)](https://www.w3.org/WAI/WCAG21/Understanding/non-text-contrast.html)
#[doc(alias = "wcag")]
#[deprecated(
    since = "0.7.2",
    note = "replaced by `palette::color_difference::Wcag21RelativeContrast`"
)]
pub trait RelativeContrast: Sized {
    /// The type of the contrast ratio.
    type Scalar: Real + PartialCmp;

    /// Calculate the contrast ratio between two colors.
    #[must_use]
    fn get_contrast_ratio(self, other: Self) -> Self::Scalar;

    /// Verify the contrast between two colors satisfies SC 1.4.3. Contrast
    /// is at least 4.5:1 (Level AA).
    #[must_use]
    #[inline]
    fn has_min_contrast_text(self, other: Self) -> <Self::Scalar as HasBoolMask>::Mask {
        self.get_contrast_ratio(other)
            .gt_eq(&Self::Scalar::from_f64(4.5))
    }

    /// Verify the contrast between two colors satisfies SC 1.4.3 for large
    /// text. Contrast is at least 3:1 (Level AA).
    #[must_use]
    #[inline]
    fn has_min_contrast_large_text(self, other: Self) -> <Self::Scalar as HasBoolMask>::Mask {
        self.get_contrast_ratio(other)
            .gt_eq(&Self::Scalar::from_f64(3.0))
    }

    /// Verify the contrast between two colors satisfies SC 1.4.6. Contrast
    /// is at least 7:1 (Level AAA).
    #[must_use]
    #[inline]
    fn has_enhanced_contrast_text(self, other: Self) -> <Self::Scalar as HasBoolMask>::Mask {
        self.get_contrast_ratio(other)
            .gt_eq(&Self::Scalar::from_f64(7.0))
    }

    /// Verify the contrast between two colors satisfies SC 1.4.6 for large
    /// text. Contrast is at least 4.5:1 (Level AAA).
    #[must_use]
    #[inline]
    fn has_enhanced_contrast_large_text(self, other: Self) -> <Self::Scalar as HasBoolMask>::Mask {
        self.has_min_contrast_text(other)
    }

    /// Verify the contrast between two colors satisfies SC 1.4.11 for graphical
    /// objects. Contrast is at least 3:1 (Level AA).
    #[must_use]
    #[inline]
    fn has_min_contrast_graphics(self, other: Self) -> <Self::Scalar as HasBoolMask>::Mask {
        self.has_min_contrast_large_text(other)
    }
}

/// Calculate the ratio between two `luma` values.
#[inline]
#[deprecated(
    since = "0.7.2",
    note = "replaced by `LinLuma::relative_contrast`, via `Wcag21RelativeContrast`"
)]
pub fn contrast_ratio<T>(luma1: T, luma2: T) -> T
where
    T: Real + Arithmetics + PartialCmp,
    T::Mask: LazySelect<T>,
{
    lazy_select! {
        if luma1.gt(&luma2) => (T::from_f64(0.05) + &luma1) / (T::from_f64(0.05) + &luma2),
        else => (T::from_f64(0.05) + &luma2) / (T::from_f64(0.05) + &luma1)
    }
}

#[cfg(feature = "approx")]
#[cfg(test)]
#[allow(deprecated)]
mod test {
    use core::str::FromStr;

    use crate::RelativeContrast;
    use crate::Srgb;

    #[test]
    fn relative_contrast() {
        let white = Srgb::new(1.0f32, 1.0, 1.0);
        let black = Srgb::new(0.0, 0.0, 0.0);

        assert_relative_eq!(white.get_contrast_ratio(white), 1.0);
        assert_relative_eq!(white.get_contrast_ratio(black), 21.0);
        assert_relative_eq!(
            white.get_contrast_ratio(black),
            black.get_contrast_ratio(white)
        );

        let c1 = Srgb::from_str("#600").unwrap().into_format();

        assert_relative_eq!(c1.get_contrast_ratio(white), 13.41, epsilon = 0.01);
        assert_relative_eq!(c1.get_contrast_ratio(black), 1.56, epsilon = 0.01);

        assert!(c1.has_min_contrast_text(white));
        assert!(c1.has_min_contrast_large_text(white));
        assert!(c1.has_enhanced_contrast_text(white));
        assert!(c1.has_enhanced_contrast_large_text(white));
        assert!(c1.has_min_contrast_graphics(white));

        assert!(!c1.has_min_contrast_text(black));
        assert!(!c1.has_min_contrast_large_text(black));
        assert!(!c1.has_enhanced_contrast_text(black));
        assert!(!c1.has_enhanced_contrast_large_text(black));
        assert!(!c1.has_min_contrast_graphics(black));

        let c1 = Srgb::from_str("#066").unwrap().into_format();

        assert_relative_eq!(c1.get_contrast_ratio(white), 6.79, epsilon = 0.01);
        assert_relative_eq!(c1.get_contrast_ratio(black), 3.09, epsilon = 0.01);

        let c1 = Srgb::from_str("#9f9").unwrap().into_format();

        assert_relative_eq!(c1.get_contrast_ratio(white), 1.22, epsilon = 0.01);
        assert_relative_eq!(c1.get_contrast_ratio(black), 17.11, epsilon = 0.01);
    }
}