d3/docs/d3-interpolate.md

d3-interpolate

This module provides a variety of interpolation methods for blending between two values. Values may be numbers, colors, strings, arrays, or even deeply-nested objects. For example:

const i = d3.interpolateNumber(10, 20);
i(0.0); // 10
i(0.2); // 12
i(0.5); // 15
i(1.0); // 20

The returned function i is an interpolator. Given a starting value a and an ending value b, it takes a parameter t typically in [0, 1] and returns the corresponding interpolated value. An interpolator typically returns a value equivalent to a at t = 0 and a value equivalent to b at t = 1.

You can interpolate more than just numbers. To find the perceptual midpoint between steelblue and brown:

d3.interpolateLab("steelblue", "brown")(0.5); // "rgb(142, 92, 109)"

Or, as a color ramp from t = 0 to t = 1:

Here’s a more elaborate example demonstrating type inference used by interpolate:

const i = d3.interpolate({colors: ["red", "blue"]}, {colors: ["white", "black"]});
i(0.0); // {colors: ["rgb(255, 0, 0)", "rgb(0, 0, 255)"]}
i(0.5); // {colors: ["rgb(255, 128, 128)", "rgb(0, 0, 128)"]}
i(1.0); // {colors: ["rgb(255, 255, 255)", "rgb(0, 0, 0)"]}

Note that the generic value interpolator detects not only nested objects and arrays, but also color strings and numbers embedded in strings!

See one of:

• Value interpolation
• Color interpolation
• Transform interpolation
• Zoom interpolation