Font File Extensions – TTF versus OTF
Both PostScript Type 1 and TrueType fonts are no longer made. All new fonts released are OpenType fonts, period. But why do people still refer to ttf and otf?
Because they think you can distinguish the internal outline format based on the font’s file extension. Although this is partly true, it just adds to the confusion. Let us explain it.
Concerning glyph outline formats, OpenType fonts come in these flavours:
- OpenType with TrueType based outlines (quadratic Bézier curves)
- OpenType with CFF based outlines (cubic Bézier curves, like PostScript Type 1)
- OpenType with both TrueType and CFF based outlines (rare, and not recommended)
- OpenType with only bitmaps of glyphs or with color glyphs (rare, and not recommended, better use 1. or 2. along with bitmaps or color)
Some 25 years ago Microsoft thought it would be wise, mainly for compatibility reasons, to continue to allow the original TrueType font file extension “ttf”, but only for OpenType fonts with TrueType based outlines. A new font file extensions “otf” was introduced and is allowed for both font flavors.
Since a file that ends with .otf can be either an OpenType font with TrueType based outlines or an OpenType font with CFF based outlines, the file extension should not be used to differentiate between the above OpenType font flavours. Also note, that if you take an existing font and change the file extension from .otf to .ttf it will still behave the same, so the file extension means nothing!
Here is what the official OpenType specification has to say about this:
“In all cases, software must determine the kind of outlines present in a font not from the filename extension but from the contents of the file.”
Text Layout and Rasterizer
Font vendors usually provide several font resources to choose from. This make it hard to tell which one you should use. So, what is important?
Rasterizers take the glyph outlines and output them on a device. How good the output looks mainly depends on the rasterizer, the outlines, font size, and optional hinting. Fonts usually look great on a high-resolution monitor and when printed but can look bad when viewed in an outdated web browser. So, what quality font and what outline format you need depends on your needs.
OpenType Layout Features
Another very important part of a font is the OpenType features, as those enable text-processing applications to improve text layout. Several of these advanced typesetting features are vital for complex script fonts like Arabic, while other features are used in desktop publishing and typesetting software like Adobe InDesign to activate specific character variants as available in the font. Not all software supports all OpenType layout features, so not all features are accessible in all software.
Internal Font Tables
Here is a technical summary of internal font tables that identify specific characteristics of fonts:
| |
Pure TrueType |
OpenType TT |
OpenType CFF |
|
| OS/2 version 0 or 1 (OS/2) |
Yes |
No |
No |
|
| OS/2 version 3+ (OS/2) |
No |
Yes |
Yes |
|
| TrueType based Outlines (glyf and loca) |
Yes |
Yes |
No |
|
| CFF based Outlines (CFF or CFF2) |
No |
No |
Yes |
|
| OpenType Features (BASE, GDEF, GPOS, GSUB, JSTF) |
No |
Optional |
Optional |
|
| Desktop File Extension |
.ttf |
.ttf or .otf |
.otf |
|
| Web File Extension |
n.a. |
.woff or .woff2 |
.woff or .woff2 |
|
Summary
If people tell you to use OTF over TTF, they probably don’t know what they are talking about. If they refer to file extensions, or OpenType versus TrueType, or the internal outline format (TrueType based outlines versus Compact Font Format outlines), point them to this article, as that is not what OTF and TTF is about.
If they want you to use fonts containing OpenType layout features, then they should talk about that.
Pure TrueType fonts no longer exist. Forget about them. There are basically OpenType fonts with TrueType based outlines and OpenType fonts with CFF based outlines. And both flavours can contain advanced OpenType layout features.
