Sunday, 19 December 2021

Universe 5: Font Detection

Any Unicode codepoint browser ends up as an exercise in font wrangling. Universe is no exception.

The threat of fingerprinting means a browser script cannot typically enumerate the locally installed fonts. There is also currently no native way of determining if a font supports a particular codepoint. One would think that determining font support for codepoints would therefore be problematic, but it turns out to be relatively easy for our use-case with Chrome.

Consider the following JavaScript snippet:

var canvas = document.createElement("canvas");
var context = canvas.getContext("2d");
context.font = "10px font-a,font-b,font-c";
var measure = context.measureText(text);

If a character in the "text" string is not supported by "font-a", Chrome text rendering falls back to using "font-b". If "font-b" also doesn't support the character, "font-c" is used. If the character is not supported by "font-c" either, a system default is used.

We can take advantage of this fall back mechanism by using a "blank" font that guarantees to render any glyph as zero-width/zero-mark. Fortunately, there's just such a font already out there: Adobe Blank:

@font-face {
  font-family: "blank";
  src: url("
         adobe-blank/master/AdobeBlank.otf.woff") format("woff");

Now, we can write a function to test a font for supported characters:

function IsBlank(font, text) {
  var canvas = document.createElement("canvas");
  var context = canvas.getContext("2d");
  context.font = `10px "${font}",blank`;
  var measure = context.measureText(text);
  return (measure.width <= 0) &&
    (measure.actualBoundingBoxRight <= -measure.actualBoundingBoxLeft);

The actual code in universe.js has some optimisations and additional features; see "MeasureTextContext()" and "TextIsBlank()".

Using this technique, we can iterate around some "well-known" fonts and render them where appropriate. For our example of codepoint U+0040:

The origins of each of the glyphs above are:

  1. "notoverse" is the 32-by-32 pixel bitmap glyph described previously.
  2. "(default)" is the default font used for this codepoint by Chrome. In my case, it's "Times New Roman".
  3. "(sans-serif)" is the default sans serif font: "Arial".
  4. "(serif)" is the default serif font: "Times New Roman".
  5. "(monospace)" is the default monospace font: "Consolas".
  6. "" is the PNG from Richard Ishida's UniView online app.
  7. "" is the SVG from GlyphWiki. It looks a bit squashed because it's a half-width glyph; GlyphWiki is primarily concerned with CJK glyphs.
  8. "unifont" is the GNU Unifont font. I couldn't find a webfont-friendly source for version 14, so I had to piggyback version 12 from Terence Eden.
  9. "noto" is a composite font of several dozen Google Noto fonts. See ".noto" in universe.css.
  10. Subsequent "Noto ..." glyphs are from the corresponding, individual Noto font, in priority order.

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