FontBook initialization

When the application is started a global FontBook is instanced. During the FontBook's instantiation it will fast-parse each TrueType font in the operating system's font folder.

During the fast-parse the name, OS/2 and optionally other tables are read and then the font files are closed and memory is freed. This fast-parse is designed to be very fast to reduce impact on application startup time.

The following information gleamed during the fast-parse:

name Preferred Font family name; code-16 if available, otherwise code-1
name Preferred Font subfamily name: code-17 if available, otherwise code-2
OS/2 Font weight
OS/2 Regular/Italic
OS/2 Serif/Sans-serif
OS/2 Variable/Monospace
OS/2 Regular/Condensed
OS/2 Unicode ranges (as backup the cmap table may be parsed)
OS/2 The height of x and H (as back the glyf table may be parsed)

Each font family is assigned a FamilyID. Each font is assigned a FontID.

There is a list of fallback font families for well known fonts, in case certain font families are not available on the system.

Each font will be assigned a list of fallback fonts for missing glyph lookup. Priority is given for fonts that start with the same font family name. For example "Arial Arabic" will be prioritized when the current font is "Arial"

Font selection

A FontVariant consists of a FontWeight+serif-flag. This allows a user to select a font family to draw a text with and emphasize fragments of the text using italic and bold.

There are a total of maximum 20 FontVariants for each FontFamilyID.

Selecting a font to render a text is done in several steps:

A user select a font family name and a FontVariant.
The font family name is looked up in the FontBook and a FontFamilyID is returned for a font that exists on the system which closely matches the requested family.
The FontFamilyID + FontVariant is looked up in the FontBook and a FontID is returned for the variant the closely matches the requested variant that is available for that family.
A grapheme + FontID is looked up in the FontBook and a FontGlyphIDs is returned. The returned FontID may be of another font than requested if the glyphs where not available in the requested fonts.

Text shaping

Phases

Given a text and a text-style; calculate the natural size of the text
- Lookup the glyphs and their width.
Given a maximum width fold the text; shape the text.
- Line wrap the text to fit the maximum width.
- Execute the Unicode bidirectional algorithm.
- Lookup glyphs that have been replaced.
- Morph glyphs such as font ligatures
- Calculate position of each glyph
Draw the text, possibly replacing the color
Given draw coordinates find character in the original text.
Given a character in the original text find a neighboring character on the screen.

Sub-styles

Sub-styles can be selected in-line in the text using the following Unicode code-points:

Code	Name	Common Style	Description
U+FDD0	Regular	Regular	This is the default regular text
U+FDD1	Emphasis	Italic	For different but not more or less important than regular text.
U+FDD2	Strong	Bold or red	For important text like warnings.
U+FDD3	Light	Light or gray	For less important text, like long explanations.
U+FDD4	Code	Monospaced	For data, or examples of data.
U+FDD5	Link	Underlined and blue	For clickable links.

These code-points are non-character code-point and are used internally of the TTauri library.