D3.js Scales with custom easing-like interpolation

Question

I want to modify an array of numbers and output it to a new range using d3.scale but with custom interpolation function. The interpolation function should be one of the easing functions used in transitions, e.g. easeInOutQuad:

easeInOutQuad = function (x, t, b, c, d) {
    if ((t/=d/2) < 1) return c/2*t*t + b;
    return -c/2 * ((--t)*(t-2) - 1) + b;
}

So my input array [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10] would become, more less, something like [0, 2, 5, 10, 20, 40, 70, 90, 95, 98, 100] where numbers increase slower at the beginning of the array, then gradually faster towards the middle and then again slower towards the end.

My code so far:

var inputArr = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10],
    linearArr = [],
    easingArr = [],
    easing = d3.interpolate, // ?
    min = d3.min(inputArr),
    max = d3.max(inputArr),
    linearScale = d3.scale.linear()
        .domain([min,max])
        .range([0,100]),
    easingScale = d3.scale.linear()
        .domain([min,max])
        .interpolate(easing) // ?
        .range([0,100]);

for (var i = 0; i < inputArr.length; i++) {
    linearArr[i] = linearScale(inputArr[i]);
    easingArr[i] = easingScale(inputArr[i]);
}

console.log(linearArr); // 0,10,20,30,40,50,60,70,80,90,100
console.log(easingArr); // 0,10,20,30,40,50,60,70,80,90,100

Thanks for any suggestions/examples of how such an easing function could be used with d3.interpolate.

score 3 · Accepted Answer · answered Jul 04 '16 at 10:26

Thanks to this helpful example, it's solved now, so a linear array of numbers can be 'eased':

linear = [0,25,50,75,100] --> eased = [0,12.5,50,87.5,100]

Here's the code:

var steps = 5,
    zStart = 0,
    zEnd = 100,
    linearArr = [],
    easingArr = [],
    linearScale = d3.scaleLinear()
        .domain([0,1])
        .range([zStart,zEnd]),
    easingScale = d3.scaleLinear()
        .domain([0,1])
        .interpolate(easeInterpolate(d3.easeQuadInOut))
        .range([zStart,zEnd]);

for (var i = 0; i < steps; i++) {
    linearArr[i] = linearScale(i/(steps-1));
    easingArr[i] = easingScale(i/(steps-1));
}

console.log("linear (" + linearArr.length + "): " + linearArr);
console.log("easing (" + easingArr.length + "): " + easingArr);

function easeInterpolate(ease) {
    return function(a, b) {
        var i = d3.interpolate(a, b);
        return function(t) {
            return i(ease(t));
        };
    };
}

D3.js Scales with custom easing-like interpolation

1 Answers1