---
layout: default
title: Dynamic Metrics
---
{%

capture url_root
  %}{% if site.safe == false %}/{% else %}/inter/{% endif
%}{%
endcapture %}{%

for file in site.static_files %}{%
  assign _path = file.path | remove_first: "/inter" %}{%
  if _path == "/dynmetrics/index.css" %}{%
    assign index_css_v = file.modified_time | date: "%Y%m%d%H%M%S" %}{%
  elsif _path == "/res/bindings.js" %}{%
    assign bindings_js_v = file.modified_time | date: "%Y%m%d%H%M%S" %}{%
  elsif _path == "/res/graphplot.js" %}{%
    assign graphplot_js_v = file.modified_time | date: "%Y%m%d%H%M%S" %}{%
  endif %}{%
endfor

%}
<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 UI Dynamic Metrics provides guidelines
    for <em>good</em> typography.
    You simply provide the optical font size, and the tracking and leading
    (letter spacing and line height) will be calculated using the following
    formula:
  </p>
  <p>
    <formula>
      tracking =
      <const>a</const> + <const>b</const> ×
      <const>e</const><sup>(<const>c</const> × fontSize)</sup>
    </formula>
    <formula>
      leading = <const>l</const> × fontSize
    </formula>
    <formula>
      <const>e</const> ≈ <num>2.718</num>
    </formula>
  </p>
  <p class="small-window">
    <small>View this on a larger screen to enable interactive control.</small>
  </p>
</div></div>

<!-- <div class="row small-window"><div>
  Hello
</div></div> -->

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

  <div class="samples">
    <p contenteditable 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>
      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.
    </p>
  </div>

  <div class="sidebar controls">
    <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>
    <div class="control">
      <img title="Style" class="icon" src="../samples/icons/style.svg">
      <select data-binding="style">
        <option value="regular" default>Regular</option>
        <option value="italic">Italic</option>
        <option value="medium" default>Medium</option>
        <option value="medium-italic">Medium Italic</option>
        <option value="bold" default>Bold</option>
        <option value="bold-italic">Bold Italic</option>
        <option value="black" default>Black</option>
        <option value="black-italic">Black Italic</option>
      </select>
    </div>
    <div class="control">
      <img title="Base tracking" class="icon" src="../samples/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>
    <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="Constant l controls line height">l</label>
      <input type="range" min="1" max="2" step="0.1" data-binding="var-l">
      <input type="number" min="1" max="2" step="0.1" data-binding="var-l">
    </div>
    <hr class="without-bottom-margin">
    <canvas class="graphplot">Canvas not Supported</canvas>
    <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

// 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({
  // set-2018-02-20
  6:    0.055,
  7:    0.044,
  8:    0.034,
  9:    0.024,
  10:   0.018,
  11:   0.012,
  12:   0.007,
  13:   0.003,
  14:   0.001,
  15:  -0.002,
  16:  -0.004,
  17:  -0.006,
  18:  -0.008,
  20:  -0.01,
  24:  -0.013,
})

// setIdealValues({
//   // set-2018-02-19
//   6:    0.059,
//   7:    0.043,
//   8:    0.032,
//   9:    0.022,
//   10:   0.014,
//   11:   0.009,
//   12:   0.004,
//   13:   0,
//   14:  -0.003,
//   15:  -0.005,
//   16:  -0.0075,
//   17:  -0.0084,
//   18:  -0.0095,
//   20:  -0.012,
//   24:  -0.014,
// })

// setIdealValues({
// // set-2018-02-18
//   6:   0.06,
//   7:   0.04,
//   8:   0.03,
//   9:   0.02,
//   10:  0.01,
//   11:  0.005,
//   12:  0.002,
//   13:  0.001,
//   14:  0,
//   15: -0.002,
//   16: -0.005,
//   17: -0.007,
//   18: -0.009,
//   20: -0.011,
//   24: -0.011,
// })


// 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 = -0.016, b = 0.21, c = -0.18; // di=0.000532 on set-2018-02-20

var l = 1.4


// InterUIDynamicTracking takes the font size in points or pixels and returns
// the compensating tracking in EM.
//
function InterUIDynamicTracking(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)
  // [6 - 38]  0.05798 .. -0.01099  (midpoint = 12.533)

  // y = 0.025 - (ln(x) * 0.01)
  // return 0.025 - Math.log(fontSize) * 0.01
}


function InterUIDynamicLineHeight(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.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()
}

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 + InterUIDynamicTracking(fontSize, weightClass)
  this.lineHeight = InterUIDynamicLineHeight(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.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 sampleTemplate
var samples = [] // Sample[]
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() {
  var samplesEl = $('.samples')
  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))

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


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

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

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(function(x) {
    return InterUIDynamicTracking(x, weightClass)
  })
}


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 = $('.samples').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-bold')
  cl.remove('font-style-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('.control input')
bindings.bindAllInputs('.control select')

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

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


// start
initSamples()
updateSamples()

})();</script>