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@import compass

html, body

  background: #111

#container

  background: #111
  position: absolute
  left: 50%
  top: 50%

#stats

  position: absolute
  right: 10px
  top: 10px

/* Info */

@import url( https://fonts.googleapis.com/css?family=Quantico )

@keyframes show-info
  0%
    transform: rotateY(120deg)
  100%
    transform: rotateY(0deg)

.info

  transition: all 180ms ease-out
  transform-style: preserve-3d
  transform: perspective(800px)

  font-family: 'Quantico', sans-serif
  position: absolute
  font-size: 12px
  opacity: 0.8
  color: #fff
  width: 220px
  left: 0px
  top: 20px

  &:hover

    box-shadow: 0 0 0 4px rgba(255,255,255,0.05)
    opacity: 1.0

  h1, h2, h3

    line-height: 1
    margin: 5px 0

  a

    transition: all 200ms ease-out
    border-bottom: 1px dotted rgba(255,255,255,0.4)
    text-decoration: none
    opacity: 0.6
    color: #fff

    &:hover

      opacity: 0.99

  .about,
  .more

    transform-origin: 0% 50%
    transform: rotateY(120deg)

    margin-bottom: 1px
    background: rgba(0,0,0,0.8)
    padding: 12px 15px 12px 20px

  .about

    animation: show-info 500ms cubic-bezier(0.230, 1.000, 0.320, 1.000) 600ms 1 normal forwards
    padding-bottom: 15px

    a
      opacity: 0.9

    h1

      letter-spacing: -1px
      font-weight: 300
      font-size: 19px
      opacity: 0.95

    h2

      font-weight: 300
      font-size: 13px
      opacity: 0.8

    h3

      text-transform: uppercase
      margin-top: 10px
      font-size: 11px

      &:before

        margin-right: 2px
        font-size: 14px
        content: '\203A'

  .more

    animation: show-info 500ms cubic-bezier(0.230, 1.000, 0.320, 1.000) 500ms 1 normal forwards
    padding: 5px 15px 10px 20px

    a
    
      text-transform: uppercase
      margin-right: 10px
      font-size: 10px

<script src="https://gist.github.com/mrdoob/838785/raw/a19a753b441d6ad41707c58f06dbe17f3470423c/RequestAnimationFrame.js"></script>
<script src="https://raw.github.com/mrdoob/stats.js/master/build/stats.min.js"></script>

var NUM_PARTICLES = ( ( ROWS = 100 ) * ( COLS = 300 ) ),
    THICKNESS = Math.pow( 80, 2 ),
    SPACING = 3,
    MARGIN = 100,
    COLOR = 220,
    DRAG = 0.95,
    EASE = 0.25,
    
    /*
    
    used for sine approximation, but Math.sin in Chrome is still fast enough :)http://jsperf.com/math-sin-vs-sine-approximation

    B = 4 / Math.PI,
    C = -4 / Math.pow( Math.PI, 2 ),
    P = 0.225,

    */

    container,
    particle,
    canvas,
    mouse,
    stats,
    list,
    ctx,
    tog,
    man,
    dx, dy,
    mx, my,
    d, t, f,
    a, b,
    i, n,
    w, h,
    p, s,
    r, c
    ;

particle = {
  vx: 0,
  vy: 0,
  x: 0,
  y: 0
};

function init() {

  container = document.getElementById( 'container' );
  canvas = document.createElement( 'canvas' );
  
  ctx = canvas.getContext( '2d' );
  man = false;
  tog = true;
  
  list = [];
  
  w = canvas.width = COLS * SPACING + MARGIN * 2;
  h = canvas.height = ROWS * SPACING + MARGIN * 2;
  
  container.style.marginLeft = Math.round( w * -0.5 ) + 'px';
  container.style.marginTop = Math.round( h * -0.5 ) + 'px';
  
  for ( i = 0; i < NUM_PARTICLES; i++ ) {
    
    p = Object.create( particle );
    p.x = p.ox = MARGIN + SPACING * ( i % COLS );
    p.y = p.oy = MARGIN + SPACING * Math.floor( i / COLS );
    
    list[i] = p;
  }

  container.addEventListener( 'mousemove', function(e) {

    bounds = container.getBoundingClientRect();
    mx = e.clientX - bounds.left;
    my = e.clientY - bounds.top;
    man = true;
    
  });
  
  if ( typeof Stats === 'function' ) {
    document.body.appendChild( ( stats = new Stats() ).domElement );
  }
  
  container.appendChild( canvas );
}

function step() {

  if ( stats ) stats.begin();

  if ( tog = !tog ) {

    if ( !man ) {

      t = +new Date() * 0.001;
      mx = w * 0.5 + ( Math.cos( t * 2.1 ) * Math.cos( t * 0.9 ) * w * 0.45 );
      my = h * 0.5 + ( Math.sin( t * 3.2 ) * Math.tan( Math.sin( t * 0.8 ) ) * h * 0.45 );
    }
      
    for ( i = 0; i < NUM_PARTICLES; i++ ) {
      
      p = list[i];
      
      d = ( dx = mx - p.x ) * dx + ( dy = my - p.y ) * dy;
      f = -THICKNESS / d;

      if ( d < THICKNESS ) {
        t = Math.atan2( dy, dx );
        p.vx += f * Math.cos(t);
        p.vy += f * Math.sin(t);
      }

      p.x += ( p.vx *= DRAG ) + (p.ox - p.x) * EASE;
      p.y += ( p.vy *= DRAG ) + (p.oy - p.y) * EASE;

    }

  } else {

    b = ( a = ctx.createImageData( w, h ) ).data;

    for ( i = 0; i < NUM_PARTICLES; i++ ) {

      p = list[i];
      b[n = ( ~~p.x + ( ~~p.y * w ) ) * 4] = b[n+1] = b[n+2] = COLOR, b[n+3] = 255;
    }

    ctx.putImageData( a, 0, 0 );
  }

  if ( stats ) stats.end();

  requestAnimationFrame( step );
}

init();
step();

#container

.info
  %hgroup.about
    %h1 30,000 Particles
    %h2 A study creating performant particles with Canvas 2D
    %h3 Use your mouse