opentype-layout

Experimental word-wrapping and layout for Opentype.js.

Install

Best used with npm and browserify. This should also work in Node.js and other environments.

npm install opentype-layout --save

Demo

Live Demo

The demo shows Canvas2D vector text (red stroke) rendered on top of DOM/CSS (black fill). The demo reads the computed CSS style and converts units into the proper EM font units. Some lines also visualize some of the available metrics.

Example

See the demo folder for a complete example.

var opentype = require('opentype.js');
var computeLayout = require('opentype-layout');

opentype.load('Font.ttf', function (err, font) {
  if (err) throw err;

  var fontSizePx = 72;
  var text = 'Hello\nWorld! This box should start word-wrapping!'
  var scale = 1 / font.unitsPerEm * fontSizePx;

  // Layout some text - notice everything is in em units!
  var result = computeLayout(font, text, {
    lineHeight: 2.5 * font.unitsPerEm, // '2.5em' in font units
    width: 500 / scale // '500px' in font units
  });

  // Array of characters after layout
  console.log(result.glyphs);

  // Computed height after word-wrap
  console.log(result.height);
});

Usage

layout = computeLayout(font, text, [opt])

Computes a new layout from the given Opentype.js Font interface and a text string.

All units should be in raw font units in the EM square, assuming a lower-left origin. For example, a lineHeight of '2em' should be passed as 2 * font.unitsPerEm. It is up to the user to scale the results to a pixel/point size after the fact.

Options:

• width the width of the box in font units, will cause word-wrapping (default Infinity)
• align string alignment of the text within its width (default 'left')
• letterSpacing the additional letter spacing in font units (default 0)
• lineHeight the line height in font units as per CSS spec, default 1.175 * font.unitsPerEm to match browsers
• start the starting character index into text to layout, default 0
• end the ending index into text to layout (exclusive), default text.length
• mode can be 'pre' (maintain spacing), or 'nowrap' (collapse whitespace but only break on newline characters), otherwise defaults to normal word-wrap behaviour

See word-wrapper for details on how word wrapping is computed.

Metrics

The returned object has the following metrics.

layout.glyphs

This provides an array of characters after layout, useful for rendering. Each element in the array has the following properties:

{
  position: [ x, y ],
  data: { ... Opentype.js Glyph object ... },
  index: charIndex,
  row: lineIndex,
  column: columnInLineIndex
}

The position is in raw font units.

layout.baseline

This is the value from pen origin to the baseline of the last line of text in the layout.

layout.leading

This is the L value in the CSS line-height spec. Divide this by two for the "half-leading", which tells you how far above the first ascender and below the last descender the text box extends to.

layout.lines

This is an array of line objects with the following properties:

{
  start: startCharIndex, // inclusive
  end: endCharIndex, // exclusive
  width: lineWidth // in font units
}

layout.lineHeight

This is the computed lineHeight in font units. If no lineHeight is specified in options, it will be equivalent to 1.175 * font.unitsPerEm.

layout.left

This is the distance from the left of the text box to the widest line of text in the box. This is zero when align is left, but changes with other alignments.

layout.right

This is the distance from the right of the box to the widest line of text in the box. This is zero when align is right, but changes with other alignments.

layout.width

The width of the text box. If no opt.width is passed, this will equal layout.maxLineWidth (i.e. length of a single line of text). If opt.width is passed, this value should equal it.

layout.height

The height of the text box, including the half leadings above the first ascender and below the last descender.

layout.maxLineWidth

This is the maximum line width in all lines. This can be used to determine the "real" width of the text box after word wrap, instead of the layout.width which may be larger.

TODOs

This module is not yet finished — below are some areas that need improvement. PRs welcome.

• 'center' and 'right' alignment do not match exactly with DOM/CSS
• Tab characters are not yet handled
• Undefined characters are not yet handled gracefully
• Word wrap algorithm is naïve and does not always match DOM/CSS
• Mainly suited for Latin left-to-right text, does not handle CTL

License

MIT, see LICENSE.md for details.