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This Pen shows how a computer renders a cubic bezier curve, as used in design and motion graphics programs. This Pen is part of an an article titled Mastering the Bezier Curve in Sketch.

~ Made by Peter Nowell @pnowelldesign, with help from Antoine and Julien Quint @graouts @pom


<svg width="550px" height="350px" viewBox="0 0 550 350" version="1.1" style="display:block; margin:100px auto 0 auto;">
    <!-- Generator: Sketch 3.0.4 (8053) -->
    <title>Cubic Bezier example</title>
    <g id="Bezier-Curves" stroke="none" stroke-width="1" fill="none" fill-rule="evenodd">
        <g id="Cubic-Bezier-example" transform="translate(-72.000000, -72.000000)">

          <!-- Animate a circle radius from 0 to 1 to have the t parameter -->
          <circle id="t" fill="none" stroke="none">
              <animate attributeType="XML" attributeName="r" from="0" to="1" dur="7s" repeatCount="indefinite"/>

          <!--Main Curve --> 
          <path d="M100,400 C100,153 300,99.9999999 600,192" id="main-curve" stroke="#000000" stroke-width="2" fill-opacity="0.179206295" fill="#D8D8D8" stroke-dasharray="657.7994384765625" stroke-dashoffset="657.7994384765625">
            <animate id="introMainCurve" attributeName="stroke-dashoffset" attributeType="XML" from="657.7994384765625" to="0" begin="0s" dur="7s" repeatCount="indefinite" calcMode="spline" keySplines="0.37 0.39 0.56 0.39" keyTimes="0;1" />
          <!--Left Handle--> 
          <path d="M99,153.474639 L99,276.741808 L99,338.375393 L99,400.008977" id="left-handle" stroke="#5C60EC" stroke-width="2" stroke-linecap="square"></path>
          <!--Right Handle-->
          <path d="M299.938027,99.7211217 L374.703522,122.790841 L449.469018,145.860561 L599.000008,192" id="right-handle" stroke="#5C60EC" stroke-width="2" stroke-linecap="square"></path>
          <!--Upper Dotted Line-->
          <path d="M100,153.5 L150.124892,140.247525 L200.249783,126.99505 L300.499566,100.4901" id="upper-dotted-line" stroke="#979797" stroke-linecap="square" stroke-dasharray="1,3">
            <animate id="introUpperDottedLine" attributeType="XML" attributeName="stroke-opacity" from="0" to="1" dur="1" begin="1s" />
            <!--begin="one.begin+3s" dur="10s"-->
          <!--Lower Dotted Line-->
          <path d="M100,400 L600.501002,192.192116" id="lower-dotted-line" stroke="#979797" stroke-linecap="square" stroke-dasharray="1,3"></path>
          <!--Cubic Drawing Triangle-->
          <polyline stroke="#95cf29" stroke-width="2" id="triangle">
              <animate attributeType="XML" attributeName="points" from="100,400 100,153 300,100" to="100,153 300,100 600,192" dur="7s" repeatCount="indefinite"/>
          <!--Cubic Drawing Triangle-->
          <path d="M100,400 L100,153 L300,100" id="cubic-drawing-triangle-50" stroke="#95CF29" stroke-width="2" stroke-linecap="round" stroke-linejoin="round">
            <animate id="special"
             attributeName="d" attributeType="XML"
             from="M100,400 L100,153 L300,100" to="M100,153 L300,100 L600,192"
             begin="0s" dur="7s" repeatCount="indefinite"></animate>
          <!-- Tangent line -->
          <line stroke="#DE40CD" stroke-width="2" stroke-linecap="round" id="tangentLine"/>
          <!--Tangent Point -->
          <circle r="4" fill="#DE40CD" id="tangentPoint"/>
          <!--Control Point Dots-->
          <circle id="controlPoint1" cx="99" cy="154" r="4" fill="white" stroke="#5C60EC" stroke-width="2"></circle>
          <circle id="controlPoint2" cx="301" cy="100" r="4" fill="white" stroke="#5C60EC" stroke-width="2"></circle>
          <circle id="controlPoint0" cx="99" cy="400" r="4" fill="#5C60EC" stroke="#5C60EC" stroke-width="2"></circle>
          <circle id="controlPoint0" cx="600" cy="192" r="4" fill="#5C60EC" stroke="#5C60EC" stroke-width="2"></circle>

<!-- Link to the Article -->
<a href="" style="display:block; margin-top:80px; font-family:Avenir,Helvetica Neue,sans-serif; color:#9DAAC4; text-align:center;">&lpar;Part of the Article&rpar; Mastering the Bezier Curve in Sketch</a>




                var circle = document.getElementById("t");
var poly = document.querySelector("polyline");
var tangentLine = document.getElementById("tangentLine");
var tangentPoint = document.getElementById("tangentPoint");

function animateTangent() {
  if (poly.animatedPoints.numberOfItems == 0) {
    var t = circle.r.animVal.value;
    var u = 1 - t;
    var p1 = poly.animatedPoints.getItem(0);
    var p2 = poly.animatedPoints.getItem(1);
    var p3 = poly.animatedPoints.getItem(2);
    var x1 = p1.x * u + p2.x * t;
    var y1 = p1.y * u + p2.y * t;
    var x2 = p2.x * u + p3.x * t;
    var y2 = p2.y * u + p3.y * t;
    tangentLine.setAttribute("x1", x1);
    tangentLine.setAttribute("y1", y1);
    tangentLine.setAttribute("x2", x2);
    tangentLine.setAttribute("y2", y2);
    tangentPoint.setAttribute("cx", x1 * u + x2 * t);
    tangentPoint.setAttribute("cy", y1 * u + y2 * t);