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Here you can Sed posuere consectetur est at lobortis. Donec ullamcorper nulla non metus auctor fringilla. Maecenas sed diam eget risus varius blandit sit amet non magna. Donec id elit non mi porta gravida at eget metus. Praesent commodo cursus magna, vel scelerisque nisl consectetur et.

              <!-- Original code here https://gist.github.com/twolfson/5b1a1ed21a77bc0fdd215e80ee7225cb -->
  p Click and drag a circle to see the our projection vector update!
  svg#scene(width="400", height="400", viewBox="0 0 400 400", xmlns="https://www.w3.org/2000/svg")
    //- DEV: We put our line first so it's hidden behind our vertex z-indicies
    line#line--a-b.line(x1="120", y1="170", x2="280", y2="170",
      stroke-width="10", stroke="#000")
    line#line--a-c.line(x1="60", y1="260", x2="120", y2="170",
      stroke-width="10", stroke="#000")
    line#line--a-c-prime.line(x1="60", y1="170", x2="120", y2="170",
      stroke-width="10", stroke="#F00")
    circle#vertex--a.vertex(cx="120", cy="170", r="20")
    circle#vertex--b.vertex(cx="280", cy="170", r="20")
    circle#vertex--c.vertex(cx="60", cy="260", r="20")
    circle#vertex--c-prime.vertex.vertex--disabled(cx="60", cy="170", r="20")
              body {
  /* https://github.com/corysimmons/typographic/blob/2.9.3/scss/typographic.scss#L34 */
  font-family: 'Helvetica Neue', 'Helvetica', 'Arial', 'sans-serif';

/* Medium screenshot specific offsets, no height seems to be present */
@media screen and (width: 1024px) {
  .container {
    width: 768px; /* 1024 - 2*(8 - 120 - ~6) -- visual inspection of orange dot in Medium */
    margin: 0 auto;
    padding-top: 51px; /* 43px for Codepen text + 8px gutter width */

#scene {
  border: 1px solid #000;

.vertex {
  cursor: pointer;
  stroke: #000;
  stroke-width: 5px;
.vertex--disabled {
  cursor: not-allowed;

#vertex--a {
  fill: orange;
#vertex--b {
  fill: blue;
#vertex--c-prime {
  fill: limegreen;
#vertex--c-prime {
  fill: grey;
  stroke: grey;
              const Draggabilly = window.Draggabilly;
document.addEventListener('DOMContentLoaded', function handleReady () {
  // Resolve our circle
  // TODO: Add assertion for element resolve
  let sceneEl = document.querySelector('#scene');
  let vertexAEl = sceneEl.querySelector('#vertex--a');
  let vertexBEl = sceneEl.querySelector('#vertex--b');
  let vertexCEl = sceneEl.querySelector('#vertex--c');
  let vertexCPrimeEl = sceneEl.querySelector('#vertex--c-prime');
  let lineABEl = sceneEl.querySelector('#line--a-b');
  let lineACEl = sceneEl.querySelector('#line--a-c');
  let lineACPrimeEl = sceneEl.querySelector('#line--a-c-prime');

  // Add our drag bindings
  function bindCircleDraggabilly(circleEl) {
    // Initialize our bindings
    let draggie = new Draggabilly(circleEl);

    // Update our bindings to support SVG
    // https://github.com/desandro/draggabilly/blob/v2.1.1/draggabilly.js#L266
    // https://github.com/desandro/draggabilly/blob/v2.1.1/draggabilly.js#L184-L194
    draggie._getPosition = function() {
      let x = parseFloat(this.element.getAttribute('cx'), 10);
      let y = parseFloat(this.element.getAttribute('cy'), 10);
      // Clean up 'auto' or other non-integer values
      this.position.x = isNaN(x) ? 0 : x;
      this.position.y = isNaN(y) ? 0 : y;
    // https://github.com/desandro/draggabilly/blob/v2.1.1/draggabilly.js#L424-L427
    draggie.positionDrag = function () {
      this.element.setAttribute('cx', this.startPosition.x + this.dragPoint.x);
      this.element.setAttribute('cy', this.startPosition.y + this.dragPoint.y);
    // https://github.com/desandro/draggabilly/blob/v2.1.1/draggabilly.js#L418-L422
    draggie.setLeftTop = function () {
      this.element.setAttribute('cx', this.position.x);
      this.element.setAttribute('cy', this.position.y);

    // Return our overridden binding
    return draggie;
  let vertexADraggie = bindCircleDraggabilly(vertexAEl);
  let vertexBDraggie = bindCircleDraggabilly(vertexBEl);
  let vertexCDraggie = bindCircleDraggabilly(vertexCEl);

  // Eagerly calculate our positions so we can have a shared redraw

  // When our vertices move, update our scene
  // DEV: We could be more intelligent about what updated but it's saner to perform a declarative render
  let aVertex = vec2.create();
  let bVertex = vec2.create();
  let cVertex = vec2.create();
  let abVector = vec2.create();
  let acVector = vec2.create();
  let abUnitVector = vec2.create();
  let acPrimeVector = vec2.create();
  function updateScene() {
    // Update our lines
    lineABEl.setAttribute('x1', vertexADraggie.position.x);
    lineABEl.setAttribute('y1', vertexADraggie.position.y);
    lineABEl.setAttribute('x2', vertexBDraggie.position.x);
    lineABEl.setAttribute('y2', vertexBDraggie.position.y);
    lineACEl.setAttribute('x1', vertexADraggie.position.x);
    lineACEl.setAttribute('y1', vertexADraggie.position.y);
    lineACEl.setAttribute('x2', vertexCDraggie.position.x);
    lineACEl.setAttribute('y2', vertexCDraggie.position.y);
    lineACPrimeEl.setAttribute('x1', vertexADraggie.position.x);
    lineACPrimeEl.setAttribute('y1', vertexADraggie.position.y);

    // Unwrap our positions into vectors
    vec2.set(aVertex, vertexADraggie.position.x, vertexADraggie.position.y);
    vec2.set(bVertex, vertexBDraggie.position.x, vertexBDraggie.position.y);
    vec2.set(cVertex, vertexCDraggie.position.x, vertexCDraggie.position.y);
    vec2.sub(abVector, bVertex, aVertex);
    vec2.sub(acVector, cVertex, aVertex);

    // Lay out our mathematical logic
      x offshoot

      x's = axisProjection (from offshoot)
    // Knowns:
    //   cos(θ) = direction * |axisProjection| / |offshoot|
    //   axisUnitVector = direction * axisProjectionUnitVector
    //   axisUnitVector = axisVector / |axis|
    //   axisProjectionVector = axisProjectionUnitVector * |axisProjection|
    //   dot = |axis||offshoot|cos(θ)
    // Formula:
    //   dot = |axis||offshoot|cos(θ)
    //     = |axis||offshoot|(direction * |axisProjection|/|offshoot|)
    //     = |axis||axisProjection| * direction
    //   Therefore: |axisProjection| = dot/(|axis|*direction)
    //   goal: axisProjectionVector
    //     = axisProjectionUnitVector * |axisProjection|
    //     = direction * axisUnitVector * dot/(|axis| * direction)
    //     = axisVector/|axis| * dot/|axis|
    //     = axisVector * (dot/|axis|^2)

    // Perform our calculation of the projection vector
    // axisVector = abVector
    // offshootVector = acVector
    let abAcDotProduct = vec2.dot(abVector, acVector);
    // DEV: We prefer squared length as there's no square root during calculation so more precision
    let abSquaredLength = vec2.squaredLength(abVector);
    vec2.scale(acPrimeVector, abVector, abAcDotProduct / abSquaredLength);

    // Update our line and projection dot
    lineACPrimeEl.setAttribute('x2', aVertex[0] + acPrimeVector[0]);
    lineACPrimeEl.setAttribute('y2', aVertex[1] + acPrimeVector[1]);
    vertexCPrimeEl.setAttribute('cx', aVertex[0] + acPrimeVector[0]);
    vertexCPrimeEl.setAttribute('cy', aVertex[1] + acPrimeVector[1]);
  vertexADraggie.on('dragMove', updateScene);
  vertexBDraggie.on('dragMove', updateScene);
  vertexCDraggie.on('dragMove', updateScene);

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