---
layout: default
title: Dynamic Metrics
---
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<link rel="stylesheet" href="index.css?v={{ index_css_v }}">
<script src="{{url_root}}res/bindings.js?v={{ bindings_js_v }}"></script>
<script src="{{url_root}}res/graphplot.js?v={{ graphplot_js_v }}"></script>

<div class="row first"><div>
  <h1>Dynamic Metrics</h1>
  <p>
    There's of course no absolute right or wrong when it comes to expressing
    yourself with typography, but Inter Dynamic Metrics provides guidelines
    for how to best use Inter.
    You simply provide the optical font size,
    and the tracking and line height is calculated for you through the following
    formula:
  </p>
  <p>
    <formula>
      tracking =
      <const>a</const> + <const>b</const> ×
      <const title="Base of natural logarithm; ≈2.718">e</const><sup>(<const>c</const> × <const>z</const>)</sup>
    </formula>
    <!--formula>
      line height = <num data-binding="var-l">1.4</num> × <const>z</const>
    </formula-->
    <formula title="Values for Inter">
      <g><const title="Constant a">a</const> = <num data-binding="var-a">-0.0223</num></g> &nbsp;&nbsp;
      <g><const title="Constant b">b</const> = <num data-binding="var-b">0.185</num></g> &nbsp;&nbsp;
      <g><const title="Constant c">c</const> = <num data-binding="var-c">-0.1745</num></g> &nbsp;&nbsp;
      <g><const>z</const> = font size</g>
    </formula>
  </p>
  <p class="wide-window">
    The controls below can be used to play around with the
    <const>a</const>, <const>b</const> and <const>c</const> constants to discover
    how they affect tracking.
  </p>
  <p class="narrow-window">
    <small>View this on a larger screen to enable interactive control.</small>
  </p>
</div></div>

<div class="white full-width row with-sidebar">

  <div class="samples">
    <p class="sample">
      <span class="di"><span></span></span>
      <span class="info"
        title="size, tracking, (ideal tracking) — progress bar shows distance from ideal"
      >15 &nbsp; 0.0 &nbsp; ( 0.0 )</span>
      <span contenteditable class="content">
      Such a riot of sea and wind strews the whole extent of beach with whatever has been lost or thrown overboard, or torn out of sunken ships. Many a man has made a good week’s work in a single day by what he has found while walking along the Beach when the surf was down.
      </span>
    </p>
  </div>

  <div class="sidebar controls">

    <div class="control">
      <img title="Style" class="icon" src="icons/style.svg">
      <select data-binding="style">
        <option value="thin">Thin</option>
        <option value="extra-light">Extra Light</option>
        <option value="light">Light</option>
        <option value="regular" default selected>Regular</option>
        <option value="medium">Medium</option>
        <option value="semi-bold">Semi Bold</option>
        <option value="bold">Bold</option>
        <option value="extra-bold">Extra Bold</option>
        <option value="black">Black</option>
        <option disabled>————————————</option>
        <option value="thin-italic">Thin Italic</option>
        <option value="extra-light-italic">Extra Light Italic</option>
        <option value="light-italic">Light Italic</option>
        <option value="italic">Italic</option>
        <option value="medium-italic">Medium Italic</option>
        <option value="semi-bold-italic">Semi Bold Italic</option>
        <option value="bold-italic">Bold Italic</option>
        <option value="extra-bold-italic">Extra Bold Italic</option>
        <option value="black-italic">Black Italic</option>
      </select>
    </div>
    <div class="control">
      <img title="Base tracking" class="icon" src="icons/letter-spacing.svg">
      <input type="range" min="-0.05" max="0.06" step="0.001" data-binding="base-tracking">
      <input type="number" min="-0.15" max="1" step="0.001" data-binding="base-tracking">
    </div>
    <div class="control">
      <img title="Line height" class="icon" src="icons/line-height.svg">
      <input type="range" min="1" max="2" step="0.01" data-binding="var-l">
      <input type="number" min="1" max="2" step="0.01" data-binding="var-l">
    </div>

    <hr>

    <div class="control">
      <label title="Constant a">a</label>
      <input type="range" min="-0.05" max="0" step="0.001" data-binding="var-a">
      <input type="number" min="-0.05" max="0" step="0.0001" data-binding="var-a">
    </div>
    <div class="control">
      <label title="Constant b">b</label>
      <input type="range" min="0" max="1" step="0.01" data-binding="var-b">
      <input type="number" min="0" max="1" step="0.001" data-binding="var-b">
    </div>
    <div class="control">
      <label title="Constant c">c</label>
      <input type="range" min="-1" max="0" step="0.01" data-binding="var-c">
      <input type="number" min="-1" max="0" step="0.001" data-binding="var-c">
    </div>

    <hr>

    <div class="control">
      <label title="Number of ideal matches">ni</label>
      <input title="Number of ideal matches" type="number" readonly data-binding="ideal-count">
      <label title="Distance from ideal">di</label>
      <input title="Distance from ideal" type="number" class="wide" readonly data-binding="ideal-distance">
    </div>

    <hr class="without-bottom-margin">

    <canvas class="graphplot">Canvas not Supported</canvas>

    <hr class="when-selection without-top-margin">
    <h3 class="when-selection">CSS</h3>
    <textarea class="when-selection" readonly id="code-output"></textarea>

    <hr class="without-top-margin">
    <h3>Ideal values</h3>
    <textarea id="ideal-values"></textarea>
    <hr class="without-top-margin">
  </div>

</div>


<script type="text/javascript">(function(){

// internal variables that are user-controllable, but are not part of
// Dynamic Metrics
var baseTracking = 0
var weightClass = 400

function round(num, prec) {
  return parseFloat(num.toFixed(prec))
}

// Ideal values designed one by one, by hand.
// font size => tracking
var idealValues = {}
var idealValuesList = []
var idealValuesTextArea = $('textarea#ideal-values')

function setIdealValues(valueMap) {
  idealValues = valueMap
  idealValuesList = []
  Object.keys(idealValues).forEach(function(fontSize) {
    idealValuesList.push([fontSize, idealValues[fontSize]])
  })
  if (idealValuesTextArea.value == '') {
    idealValuesTextArea.value = idealValuesList.map(function(t){
      return t[0] + '\t' + t[1]
    }).join('\n')
  }
}

function parseValues(s) {
  var values = {}
  s = s.replace(/^[\s\r\t\n]+|[\s\r\t\n]+$/g, '') // trim whitespace
  s.split(/\s*\r?\n\s*/).map(function(line) {
    var t = line.split(/\s+/)
    if (t.length == 2) {
      t[0] = parseInt(t[0])
      t[1] = parseFloat(t[1])
      values[t[0]] = t[1]
    }
  })
  return values
}

setIdealValues({
  // // 2018
  // 6:   0.021,
  // 7:   0.017,
  // 8:   0.013,
  // 9:   0.01,
  // 10:  0.007,
  // 11:  0.005,
  // 12:  0.002,
  // 13:  0,
  // 14: -0.002,
  // 15: -0.004,
  // 16: -0.005,
  // 17: -0.007,
  // 18: -0.008,
  // 20: -0.01,
  // 24: -0.013,
  // 30: -0.016,
  // 40: -0.02,
  // 80: -0.02,

  // 2019-02-02
  // 6:    0.066,
  // 7:    0.05,
  // 8:    0.036,
  // 9:    0.025,
  // 10:   0.015,
  // 11:   0.007,
  // 12:   0,
  // 13:  -0.005,
  // 14:  -0.01,
  // 15:  -0.014,
  // 16:  -0.017,
  // 17:  -0.02,
  // 18:  -0.022,
  // 20:  -0.026,
  // 24:  -0.03,
  // 30:  -0.033,
  // 40:  -0.034,
  // 80:  -0.034,

  // // 2019-02-06
  // 6:    0.04,
  // 7:    0.032,
  // 8:    0.024,
  // 9:    0.017,
  // 10:   0.01,
  // 11:   0.005,
  // 12:   0,
  // 13:  -0.004,
  // 14:  -0.008,
  // 15:  -0.011,
  // 16:  -0.014,
  // 17:  -0.017,
  // 18:  -0.019,
  // 20:  -0.023,
  // 24:  -0.029,
  // 30:  -0.034,
  // 40:  -0.037,
  // 80:  -0.038,

  // 2019-02-07
  6:    0.043,
  7:    0.032,
  8:    0.024,
  9:    0.016,
  10:   0.01,
  11:   0.005,
  12:   0,
  13:  -0.0025,
  14:  -0.006,
  15:  -0.009,
  16:  -0.011,
  17:  -0.013,
  18:  -0.014,
  20:  -0.017,
  24:  -0.019,
  30:  -0.021,
  40:  -0.022,
  80:  -0.022,
})


// Variables for constants involved in Dynamic Metrics

// var a = -0.012, b = 0.23, c = -0.21;  // di=0.002514 on set-2018-02-18
// var a = -0.013, b = 0.251, c = -0.222 // di=0.001742 on set-2018-02-18
// var a = -0.015, b = 0.283, c = -0.23; // di=0.00221 on set-2018-02-18
// var a = -0.0149, b = 0.298, c = -0.23; // di=0.000484 on set-2018-02-19
// var a = -0.018, b = 0.21, c = -0.18; // di=0.000532 on set-2018-02-20
// var a = -0.017, b = 0.202, c = -0.175; // 2018-09-28
// var a = -0.02, b = 0.0755, c = -0.102 // 2019-02-02
// var a = -0.038, b = 0.161, c = -0.12 // 2019-02-06
var a = -0.0223, b = 0.185, c = -0.1745 // 2019-02-07


var l = 1.4


// _InterDynamicTracking is a version of InterDynamicTracking that
// uses some global variables that are adjustable.
//
function _InterDynamicTracking(fontSize, weightClass) {
  // Note: weightClass is currently unused
  //
  // y = -0.01021241 + 0.3720623 * e ^ (-0.2808687 * x)
  // [6-35]  0.05877 .. -0.0101309  (13==0; stops growing around 30-35)
  // See https://gist.github.com/rsms/8efdbca5f8145a584ed08a7c3d6e5788
  //
  return a + b * Math.pow(Math.E, c * fontSize)
}


function InterDynamicLineHeight(fontSize, weightClass) {
  return Math.round(fontSize * l)
}


var NPOS = Number.MAX_SAFE_INTEGER

function Sample(fontSize) {
  this.rootEl = sampleTemplate.cloneNode(true)
  this.infoEl = $('.info', this.rootEl)
  this.contentEl = $('.content', this.rootEl)
  this.diEl = $('.di', this.rootEl)
  this.diEl.classList.add('unavailable')
  this.style = this.rootEl.style
  this.maxBoxWidth = 0
  this.fontSize = 0
  this.tracking = 0
  this.lineHeight = 0
  this.idealTracking = NPOS
  this.matchesIdeal = false
  this.setFontSize(fontSize)
  this.render()

  // listen for focus events on the editable content
  var s = this
  this.contentEl.addEventListener(
    'focus',
    function(){ s.onReceivedFocus() },
    {passive:true, capture:false}
  )
  this.contentEl.addEventListener(
    'blur',
    function(){ s.onLostFocus() },
    {passive:true, capture:false}
  )
}

Sample.prototype.onReceivedFocus = function() {}
Sample.prototype.onLostFocus = function() {}

Sample.prototype.idealDistance = function(fontSize) {
  // returns the distance from the ideal (or NPOS if no "ideal" data available)
  if (this.idealTracking == NPOS) {
    return NPOS
  }
  return (
    Math.max(this.tracking, this.idealTracking) -
    Math.min(this.tracking, this.idealTracking)
  )
}

Sample.prototype.setFontSize = function(fontSize) {
  this.fontSize = fontSize
  this.tracking = baseTracking + _InterDynamicTracking(fontSize, weightClass)
  this.lineHeight = InterDynamicLineHeight(fontSize, weightClass)
  this.maxBoxWidth = Math.round(fontSize * (this.tracking + 1) * 25)

  var idealTracking = idealValues[this.fontSize]
  if (idealTracking === undefined) {
    if (this.idealTracking != NPOS) {
      this.diEl.classList.add('unavailable')
    }
    this.idealTracking = NPOS
    this.matchesIdeal = false
  } else {
    if (this.idealTracking == NPOS) {
      this.diEl.classList.remove('unavailable')
    }
    this.idealTracking = idealTracking
    // match to a precision of 3
    this.matchesIdeal = this.tracking.toFixed(3) == idealTracking.toFixed(3)
    var di = 1
    if (this.matchesIdeal) {
      this.diEl.classList.add('match')
    } else {
      this.diEl.classList.remove('match')
      di = 1 - Math.min(1, this.idealDistance() * 50)
    }
    this.diEl.firstChild.style.width = (di * this.maxBoxWidth) + 'px'
  }
}

Sample.prototype.cssProperties = function() {
  return [
    ['font-size',      round(this.fontSize, 3) + 'px'],
    ['letter-spacing', round(this.tracking, 3) + 'em'],
    ['line-height',    round(this.lineHeight, 3) + 'px'],
  ]
}

Sample.prototype.setText = function(text) {
  this.contentEl.innerText = text
  this.render()
}

Sample.prototype.render = function() {
  this.style.fontSize = this.fontSize + 'px'
  this.style.letterSpacing = this.tracking + 'em'
  this.style.lineHeight = this.lineHeight + 'px'

  var SP = '\u00A0\u00A0\u00A0'
  this.infoEl.innerText = (
    this.fontSize +
    SP + this.tracking.toFixed(3) +
    // SP + this.lineHeight.toFixed(3) +
    (
      this.idealTracking != NPOS ? (
        SP + '( ' + this.idealTracking +
        (this.matchesIdeal ? ' \u2713' : '') +
        ' )'
      ) : ''
    )
  )

  this.style.maxWidth = this.maxBoxWidth + 'px'
}


var bindings = new Bindings()
var samplesEl = $('.samples')
var sampleTemplate
var samples = [] // Sample[]
var focusedSample = null // Sample | null
var graph = new GraphPlot($('canvas.graphplot'))
graph.setOrigin(-0.2, 0.8)
graph.setScale(0.049, 5)

function addChildren(targetNode, children) {
  targetNode.style.visibility = 'hidden'
  var i = 0;
  for (; i < children.length; i++) {
    targetNode.appendChild(children[i])
  }
  targetNode.style.visibility = null
}

function initSamples() {
  sampleTemplate = $('.sample', samplesEl)
  samplesEl.removeChild(sampleTemplate)

  // small and medium sizes
  var i, fontSize = 6, endFontSize = 16
  for (; fontSize <= endFontSize; fontSize++) {
    samples.push(new Sample(fontSize))
  }

  // a few select large samples
  samples.push(new Sample(17))
  samples.push(new Sample(18))
  samples.push(new Sample(20))
  samples.push(new Sample(24))
  samples.push(new Sample(30))
  samples.push(new Sample(40))
  samples.push(new Sample(80))

  // connect focus events
  var onSampleReceivedFocus = function() { setSelectedSample(this) }
  samples.forEach(function(s) {
    s.onReceivedFocus = onSampleReceivedFocus
  })

  // add to dom in one go
  addChildren(samplesEl, samples.map(function(s) { return s.rootEl }))
}

function setSelectedSample(sample) {
  if (focusedSample !== sample) {
    if (focusedSample) {
      focusedSample.rootEl.classList.remove('selected')
    }
    if (sample) {
      sample.rootEl.classList.add('selected')
    }
    focusedSample = sample
    updateSelection()
  }
}

function updateSample(sample) {
  sample.setFontSize(sample.fontSize) // updates derived values
  sample.render()
}

function updateSamples() {
  samples.forEach(updateSample)
  updateIdealMatches()
  updateGraphPlot()
  updateCodeView()
}

function updateIdealMatches() {
  // ideal-distance
  var idealCount = 0
  var distance = 0
  var ndistances = 0
  samples.forEach(function(sample) {
    if (sample.matchesIdeal) {
      idealCount++
    }
    var di = sample.idealDistance()
    if (di != NPOS) {
      distance += di
      ndistances++
    }
  })

  distance = distance / ndistances

  bindings.setValue('ideal-distance', distance.toFixed(6))
  bindings.setValue('ideal-count', idealCount)
}

function updateGraphPlot() {
  graph.clear()
  graph.plotLine(idealValuesList, '#0d3')
  graph.plotf(
    x => _InterDynamicTracking(x, weightClass),
    'rgba(0, 0, 0, 0.5)'
  )
  if (focusedSample) {
    var graphedFontSize = Math.min(24, focusedSample.fontSize) // clamp to [-inf,24]
    graph.plotPoints([
      [graphedFontSize, focusedSample.tracking]
    ], 'rgb(45, 143, 255)')
  }
}

var codeOutput = $('#code-output')
codeOutput.addEventListener('focus', function(ev) {
  ev.preventDefault()
  ev.stopPropagation()
  codeOutput.select()
}, {passive:false,capture:true})

codeOutput.addEventListener('pointerdown', function(ev) {
  // TODO: don't do this if codeOutput is focused
  ev.preventDefault()
  ev.stopPropagation()
  codeOutput.select()
  document.execCommand("copy")
  HUDNotification.show('Copied to clipboard')
}, {passive:false,capture:true})

function updateCodeView() {
  var s = ''
  if (focusedSample) {
    var props = focusedSample.cssProperties()
    props.forEach(function(prop, i) {
      var name = prop[0], value = prop[1]
      s += name + ': ' + value + ';'
      if (i != props.length-1) {
        s += '\n'
      }
    })
  }
  codeOutput.value = s
}

function updateSelection() {
  var controlsEl = $('.controls')
  if (focusedSample) {
    controlsEl.classList.add('has-selected-sample')
  } else {
    controlsEl.classList.remove('has-selected-sample')
  }

  updateGraphPlot()
  updateCodeView()
}


bindings.configure('base-tracking', 0, 'float', function (tracking) {
  baseTracking = tracking
  updateSamples()
})

bindings.configure('var-a', a, 'float', function (v) {
  a = v
  updateSamples()
})

bindings.configure('var-b', b, 'float', function (v) {
  b = v
  updateSamples()
})

bindings.configure('var-c', c, 'float', function (v) {
  c = v
  updateSamples()
})

bindings.configure('var-l', l, 'float', function (v) {
  l = v
  updateSamples()
})

bindings.configure('ideal-count', 0, 'int', function (v) {})
bindings.configure('ideal-distance', 0, 'float', function (v) {})

bindings.configure('style', null, null, function (style) {
  var cl = samplesEl.classList
  cl.remove('font-style-regular')
  cl.remove('font-style-italic')
  cl.remove('font-style-medium')
  cl.remove('font-style-medium-italic')
  cl.remove('font-style-semi-bold')
  cl.remove('font-style-semi-bold-italic')
  cl.remove('font-style-bold')
  cl.remove('font-style-bold-italic')
  cl.remove('font-style-extra-bold')
  cl.remove('font-style-extra-bold-italic')
  cl.remove('font-style-black')
  cl.remove('font-style-black-italic')
  cl.add('font-style-' + style)
  weightClass = (
    style.indexOf('black')  != -1 ? 900 :
    style.indexOf('bold')   != -1 ? 700 :
    style.indexOf('medium') != -1 ? 500 :
    400
  )
  updateSamples()
})


bindings.bindAllInputs('[data-binding]')

// double-click base tracking to reset
$('input[type="range"][data-binding="base-tracking"]').addEventListener(
  'dblclick',
  function(ev) { bindings.setValue('base-tracking', 0) }
)

;[
  ['var-a',a],
  ['var-b',b],
  ['var-c',c],
  ['var-l',l],
  ['base-tracking', 0]
].forEach(p => {
  let bindname = p[0], defval = p[1]
  $$('input[type="range"][data-binding="'+bindname+'"]').forEach(e => {
    e.addEventListener('dblclick', ev =>
      bindings.setValue(bindname, defval))
  })
})

// allow editing of ideal values
idealValuesTextArea.addEventListener('input', function(ev) {
  setIdealValues(parseValues(idealValuesTextArea.value))
  updateSamples()
})

// listen for clicks onto "background", to deselect any selected sample
document.body.addEventListener('pointerdown', function(ev){
  if (
    ev.target === document.body ||
    ev.target === samplesEl ||
    ev.target.classList && ev.target.classList.contains('row')
  ) {
    setSelectedSample(null)
  }
})


// start
initSamples()
updateSamples()

})();</script>