27  * color and a contrast ratio.
29  * <p>Contrast ratio is calculated using XYZ's Y. When linearized to match human perception, Y
33   // The minimum contrast ratio of two colors.
34   // Contrast ratio equation = lighter + 5 / darker + 5, if lighter == darker, ratio == 1.
37   // The maximum contrast ratio of two colors.
38   // Contrast ratio equation = lighter + 5 / darker + 5. Lighter and darker scale from 0 to 100.
39   // If lighter == 100, darker = 0, ratio == 21.
45   // Given a color and a contrast ratio to reach, the luminance of a color that reaches that ratio
47   // contrast ratio of the input color and the returned luminance may not reach the contrast ratio
50   // When the desired contrast ratio and the result contrast ratio differ by more than this amount,
52   // it will return a valid luminance but that luminance may not meet the requested contrast ratio.
54   // 0.04 selected because it ensures the resulting ratio rounds to the same tenth.
73   // display color spaces, methods that take a contrast ratio and luminance, and return a luminance
74   // that reaches that contrast ratio for the input luminance, purposefully darken/lighten their
75   // result such that the desired contrast ratio will be reached even if inaccuracy is introduced.
79   // lightness rounds to the same as the requested, the desired contrast ratio will be reached.
85    * Contrast ratio is a measure of legibility, its used to compare the lightness of two colors.
91    * <p>The equation is ratio = lighter Y + 5 / darker Y + 5.
100    * Contrast ratio of two tones. T in HCT, L* in L*a*b*. Also known as luminance or perpectual
103    * <p>Contrast ratio is defined using Y in XYZ, relative luminance. However, relative luminance is
110    * of a ratio, a linear difference. This allows a designer to determine what they need to adjust a
119    * Returns T in HCT, L* in L*a*b* >= tone parameter that ensures ratio with input T/L*. Returns -1
120    * if ratio cannot be achieved.
123    * @param ratio Desired contrast ratio of return value and tone parameter.
125   public static double lighter(double tone, double ratio) {  in lighter()  argument
129     // Invert the contrast ratio equation to determine lighter Y given a ratio and darker Y.  in lighter()
131     final double lightY = ratio * (darkY + 5.0) - 5.0;  in lighter()
136     final double delta = Math.abs(realContrast - ratio);  in lighter()
137     if (realContrast < ratio && delta > CONTRAST_RATIO_EPSILON) {  in lighter()
150    * Tone >= tone parameter that ensures ratio. 100 if ratio cannot be achieved.
153    * bounds return value may not reach the desired ratio.
156    * @param ratio Desired contrast ratio of return value and tone parameter.
158   public static double lighterUnsafe(double tone, double ratio) {  in lighterUnsafe()  argument
159     double lighterSafe = lighter(tone, ratio);  in lighterUnsafe()
164    * Returns T in HCT, L* in L*a*b* <= tone parameter that ensures ratio with input T/L*. Returns -1
165    * if ratio cannot be achieved.
168    * @param ratio Desired contrast ratio of return value and tone parameter.
170   public static double darker(double tone, double ratio) {  in darker()  argument
174     // Invert the contrast ratio equation to determine darker Y given a ratio and lighter Y.  in darker()
176     final double darkY = ((lightY + 5.0) / ratio) - 5.0;  in darker()
181     final double delta = Math.abs(realContrast - ratio);  in darker()
182     if (realContrast < ratio && delta > CONTRAST_RATIO_EPSILON) {  in darker()
186     // For information on 0.4 constant, see comment in lighter(tone, ratio).  in darker()
196    * Tone <= tone parameter that ensures ratio. 0 if ratio cannot be achieved.
199    * bounds return value may not reach the desired ratio.
202    * @param ratio Desired contrast ratio of return value and tone parameter.
204   public static double darkerUnsafe(double tone, double ratio) {  in darkerUnsafe()  argument
205     double darkerSafe = darker(tone, ratio);  in darkerUnsafe()