N.B. An asterisk before a word means it has its own entry in the glossary.

on-curve and off-curve points

See *outline.

OpenType

The font format that (in some ways) unites TrueType and Type 1, jointly developed by Adobe and Microsoft. Key features of the old formats live on as the two "flavours" of OpenType, but much information is now identically formatted.
    More information is in the OpenType area of this site.

'OS/2' table

The TrueType table that holds information used by Windows. See Apple's spec or Microsoft's spec for the format in detail.

OTF

The recommended file extension for the Type 1 flavour of *OpenType. (The TrueType flavour's extension is TTF.)

outline

A digital representation of an image (such as an alphabetic character) where solid shapes are represented by the mathematical curves approximating their outlines. Circles, ellipses, *quadratic and *cubic curves have been used in different outline representations. Outlines are nicely *scalable (and transformable in other ways), unlike *bitmap representations.
    *Glyph outlines in TrueType consist of a series of points, each being either "on-curve" or "off-curve". Consecutive on-curve points define a straight line. Consecutive off-curve points have an on-curve point interpolated between them by the *scan-converter. A *quadratic *Bezier curve is defined by a sequence of on-curve, off-curve, on-curve. There's an index to where each *contour ends. Contours are self-closing.
    See the TrueType Outlines page.

OutlineThreshold

A poorly-named setting in Windows 3.1, relating to the *ppem above which the whole TrueType system becomes less precise. It's in the [TrueType] section of WIN.INI.
    There are TrueType fonts with complicated *glyphs, which, although compliant with the specification, fail to rasterize at some sizes: the "missing character" box appears instead. The makers of these complex fonts routinely advise lowering the OutlineThreshold from its default (256) down to 75 or so. This advice should only be followed in emergenices, as it affects all fonts adversely! Set it back to 256 as soon as possible after printing your complex characters.
    In full implementations of TrueType, such as the Apple Macintosh or Windows 95, no OutlineThreshold is necessary and complex glyphs rasterize just fine.