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The Prototypo library is a javascript library that will let you control fonts using javascript. You’ll be able to create typefaces from Prototypo’s template and change their shapes.

This documentation the differents functionalities of the library and how to use them. Some examples are shown at the end.

Getting started

Setting yourself up

Basic html markup to import the library

        <!-- Your web content -->
        <script src=""></script>

Your token should look like something like this cf74462a-4a23-4b3e-40e6-95334f41168c. Here is how you’ll use it:

var prototypoFontFactory = new Ptypo.default('cf74462a-4a23-4b3a-b0e6-95334f41188c');

If your not subscribed to Prototypo just use

var prototypoFontFactory = new Ptypo.default();

If your subscribed to Prototypo the first thing you’ll need to do is go to the Prototypo App to get you library token. This token we’ll let you download the full templates from Prototypo. If your not subscribed to Prototypo your template will contain all the latin uppercases, lowercases and figures.

Your token is found in the Developers section of the account dashboard. This is how to find it.

Go there. Account link location

Then there. Developpers link location

Once you imported your script and created your font factory. It’s time to create some fonts.

Creating fonts

Font creation example

prototypoFontFactory.createFont('nameOfYourFont', Ptypo.templateNames.ELZEVIR)
    .then(function(font) {
        //You'll be able to modify your font here

Basic html and css example for font styling

<div class="my-class">Test</div>
.my-class {
    font-family: 'nameOfYourFont', serif;

Creating font with the library is just one call to the createFont function. prototypoFontFactory.create('nameOfYourFont', Ptypo.templateNames.ELZEVIR);.

Changing the parameters

Parameter change example

prototypoFontFactory.createFont('nameOfYourFont', Ptypo.templateNames.ELZEVIR)
    .then(function(font) {
        font.changeParam('thickness', 100);
            xHeight: 600,
            width: 1.1,

Changing the parameters of the font is as easy as creating it. Just call the changeParam method on the object returned by createFont. You can see what parameters are available in the API documentation.

Here you go! You’ve created a font and are ready to customize it.

API documentation

Font creation


Ptypo.default.createFont( fontName, templateName )


name type description
fontName string The fontName parameter is the name that the font will take in your css style. i.e if fontName is hello you should use font-family: 'hello'
templateName Ptypo.templateNames the templateName specifies the parametric template you will use. It can take the values Ptypo.templateNames.ELZEVIR, Ptypo.templateNames.SPECTRAL, Ptypo.templateNames.GROTESK and Ptypo.templateNames.FELL

Return value

A promise that resolve to a PtypoFont object. This object is used to manipulate the font created.

PtypoFont class


Ptypo.changeParam( paramName, paramValue )


name type description
paramName string The name of the parameter to be changed
paramValue number The new value of the parameter specified


Ptypo.changeParams( paramObject )


name type description
paramObject Object an object of the shape paramName: paramValue containing the name of the values to be changed and their values



Return value

contains the current value of the parameters. i.e Ptypo.values.width returns the current value of the width parameter

Parameter names and values

Name Min advised Max advised Behavior Affected Letters
xHeight 300 900 Change the base height of every character All letters
capDelta 100 400 Change the height of capital letters Capital letters
ascender 150 500 Change the height of ascender on ascender letters b, d, f, h, k, l
descender -500 -150 Change the height of descender on descender letters g, j, p, q, y
crossbar 0.8 1.1 Change the height of the crossbar on letters with crossbars a, e, A, B, E, F, H, P, R
width 0.5 2 Change the width of every characters All letters
slant -5 12 Change the slant of every characters All letters
thickness 50 160 Change the thickness of the strokes All letters
aperture 0.5 1.6 Change the size of the aperture of letters (see here for more info) a, e, c, s, t, r, C, G, S
curviness 0.4 1.0 Change the curviness of letters. A really curved ‘o’ has a square shape, a non curved ‘o’ has a classic diamond shape All letters
serifWidth 1 90 Change the width of serifs (How long the serifs are) All letters
serifHeight 0 70 Change the height of serifs (How thick the serifs are) All letters
midWidth 0.5 1.2 See anatomy of a serif All letters
serifMedian 0.2 1.3 See anatomy of a serif All letters
serifCurve 0 100 See anatomy of a serif All letters
serifRoundness 0 1.8 See anatomy of a serif All letters
serifArc 0 0.1 See anatomy of a serif All letters
serifTerminal 0 0.7 See anatomy of a serif All letters
serifTerminalCurve 0 1.5 See anatomy of a serif All letters

Anatomy of a serif

Serif height

serif height demo

Serif width

serif width demo

Serif middle width

serif middle width demo

Serif median

serif median demo

Serif curve

serif curve demo

Serif roundness

serif roundness demo

Serif arc

serif arc demo

Serif terminal

serif terminal demo

Serif terminal curve

serif terminal curve demo


Changing thickness with background

The first examples is all about consistency. For font to feel consistently thick dark font on light background have to slighty heavier than light font on dark background. This is due to the light characters bleeding on the dark background.

In this example we create a simple text that can be switched from light mode to dark mode. We adjust the thickness of the font accordingly. We use a transition to mask the small computing time of the font.

See the Pen bRKbxY by Francois Poizat (@FranzPoize) on CodePen.

Fitting heights to container

Ok let’s get crafty for this second example. You can check how the titles resize by resizing your window.

See the Pen vZrRVW by Francois Poizat (@FranzPoize) on CodePen.

Creating the fonts and initialization

ptypoFactory.createFont('titleFont', Ptypo.templateNames.GROTESK).then(function(font) {

ptypoFactory.createFont('subFont', Ptypo.templateNames.GROTESK).then(function(font) {

ptypoFactory.createFont('textFont', Ptypo.templateNames.ELZEVIR).then(function(font) {
  font.changeParam('thickness', 60);

We start By creating three fonts. Two for the different level of headings and one for the body. We use the batch update to initialize the fonts parameters.

Finding the right formula for the width

<span style="font-family: 'titleFont' id="test-font-width">Article</span>
//Base width for the font
ptypoFactory.createFont('titleFont', Ptypo.templateNames.GROTESK).then(function(font) {
    'xHeight': 602,
    'thickness': 160,
    'capDelta': 280,
    'ascender': 310,
    'aperture': 1.2,
    'spacing': 0.8,
    'width': 0

  console.log('w1', document.getElementById('test-font-width').clientWidth);

  //Second that helps us find the function to compute the width of the text
  //with respect to the width parameters
    'xHeight': 602,
    'thickness': 160,
    'capDelta': 280,
    'ascender': 310,
    'aperture': 1.2,
    'spacing': 0.8,
    'width': 1
  console.log('w2', document.getElementById('test-font-width').clientWidth);

Using this we get two width value for the heading, which we will call w1 and w2 respectively. Here is the theoretical width of the text given the width parameter:

$$textWidth = w1 + (w2 - w1) \times widthParam$$

So to find the widthParam we have the following formula:

$$widthParam = \frac{(textWidth - w1)}{(w2 - w1)}$$

Putting it to good use

window.addEventListener('resize', _.throttle(function(e) {
    var containerWidth = containerDiv.clientWidth;
    var width = Math.max(0, (containerWidth - baseTitleWidth) / 94)

    font.changeParam('width', width);
  }, 300));

This is how we get the two formulas for the width. We just need to add a listener for the resize event on the window to trigger the generation of updated fonts. We use a throttle to avoid to many calculation.