Trait palette::RelativeContrast

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pub trait RelativeContrast {
    type Scalar: FromF64 + PartialOrd;

    fn get_contrast_ratio(&self, other: &Self) -> Self::Scalar;

    fn has_min_contrast_text(&self, other: &Self) -> bool { ... }
    fn has_min_contrast_large_text(&self, other: &Self) -> bool { ... }
    fn has_enhanced_contrast_text(&self, other: &Self) -> bool { ... }
    fn has_enhanced_contrast_large_text(&self, other: &Self) -> bool { ... }
    fn has_min_contrast_graphics(&self, other: &Self) -> bool { ... }
}
Expand description

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.

use std::str::FromStr;
use palette::{Srgb, RelativeContrast};

// the rustdoc "DARK" theme background and text colors
let my_background_rgb: Srgb<f32> = Srgb::from(0x353535).into_format();
let my_foreground_rgb = Srgb::from_str("#ddd")?.into_format();

assert!(my_background_rgb.has_enhanced_contrast_text(&my_foreground_rgb));

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 desireable.

For more details, visit the following links:

Success Criterion 1.4.3 Contrast (Minimum) (Level AA)

Success Criterion 1.4.6 Contrast (Enhanced) (Level AAA)

Success Criterion 1.4.11 Non-text Contrast (Level AA)

Required Associated Types

The type of the contrast ratio.

Required Methods

Calculate the contrast ratio between two colors.

Provided Methods

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fn has_min_contrast_text(&self, other: &Self) -> bool

Verify the contrast between two colors satisfies SC 1.4.3. Contrast is at least 4.5:1 (Level AA).

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fn has_min_contrast_large_text(&self, other: &Self) -> bool

Verify the contrast between two colors satisfies SC 1.4.3 for large text. Contrast is at least 3:1 (Level AA).

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fn has_enhanced_contrast_text(&self, other: &Self) -> bool

Verify the contrast between two colors satisfies SC 1.4.6. Contrast is at least 7:1 (Level AAA).

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fn has_enhanced_contrast_large_text(&self, other: &Self) -> bool

Verify the contrast between two colors satisfies SC 1.4.6 for large text. Contrast is at least 4.5:1 (Level AAA).

Verify the contrast between two colors satisfies SC 1.4.11 for graphical objects. Contrast is at least 3:1 (Level AA).

Implementors