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Compute the contrast ratio between pairs of colours

Source: R/colour_math.R
contrast_ratio.Rd

Compute contrast ratios between pairs of colours, using the WCAG 2.1 formula.

Usage

contrast_ratio(colour1, colour2)

Arguments

colour1, colour2

Character vectors of hexadecimal colour values.

Value

contrast_ratio() returns a numeric vector of contrast ratios, of the same length as colour1 and colour2.

Details

Contrast ratio is defined as \(\frac{L_1 + 0.05}{L_2 + 0.05}\) where \(L_1\) and \(L_2\) are the relative luminances of the lighter and darker colours being compared.

Contrast ratios range from 1 (no colour contrast) to 21 (maximum contrast). To meet WCAG 2.1 minimum (AA) criteria for accessibility, the contrast ratios between adjacent colours must be:

  • For text, at least 4.5

  • For large text (18pt or larger, or 14pt bold), at least 3

  • For non-text plot elements (e.g. patterns, lines, points, borders etc.), at least 3

To meet WCAG 2.1 enhanced (AAA) contrast requirements, generally a ratio above 7 is required.

See WCAG 2.1 success criteria 1.4.3 Contrast (Minimum), 1.4.6 Contrast (Enhanced) and 1.4.11 Non-text Contrast.

Examples

backgrounds <- doe_cols("white", "black", "blue-01", "grey-04", "black")
foregrounds <- doe_cols("black", "white", "blue-03", "grey-03", "black")

contrast_ratio(colour1 = backgrounds, colour2 = foregrounds)
#> [1] 21.000000 21.000000  9.732217  1.268706  1.000000