Saturday, 7 August 2010

Missing in Silverlight 4: a functional GlyphTypeface class

Warning. Obscurity level: HIGH.

This note is primarily aimed at the Silverlight development team in Microsoft Redmond. Other Silverlight developers may also want to understand a limitation of Silverlight 4.

Applications that require superscripts, subscripts, and other rich text functionality need control of character/glyph positional placement. Advanced typography is also useful in applications such as e-book readers where it is often desirable to accurately represent the look and feel of the book. Specialist applications that do mathematical typography (and my ancient Egyptian work) need this kind of precision. From a developer perspective, it is the GlyphTypeface class in the .Net/WPF System.Windows.Media namespace that provides much of required functionality for WPF applications.

The problem
The Silverlight 4 documentation available from Microsoft (see states

“The GlyphTypeface object is a low-level text object that corresponds to a single face of a font family as represented by an OpenType font file, or serialized as a block of memory in a document. Each glyph defines metrics that specify how it aligns with other glyphs. The correct GlyphTypeface to use for a run of characters in a given logical font is normally determined by the Silverlight font system.
The GlyphTypeface object provides properties and methods for the following:
· Obtaining font face common metrics, such as the ratio of ascent and descent to em size.
· Obtaining metrics, outlines, and bitmaps for individual glyphs.”

Er no! The WPF 4 version of GlyphTypeface indeed does this. However Silverlight 4 only supports reading the name of the font and its version number. All the useful functionality is missing. The documentation quoted only applies to WPF.

It is therefore impossible in general to implement advanced typography in Silverlight. A big hole - typography has been possible since Windows 3.1, the first release (1992) to incorporate scalable (TrueType) fonts. [Note: sure there are clumsy workarounds in very special circumstances but I won’t go into those today].

The solution
Expand the GlyphTypeface in Silverlight 5 to provide all missing functionality except where this conflicts for some reason with the Silverlight security model. In particular, discovery of the black box for a glyph is essential, as is CharacterToGlyphMap (without which the ‘Glyphs’ class has only limited use). A fairly small amount of straightforward work in the Silverlight runtime yields a big benefit to third party developers and should also help functional enhancement to controls such as RichTextBox.

Note: Windows Phone 7 is also lacking functionality here.

No comments:

Post a Comment