Font -and Text Services Collection Overview

Font -and Text Services Collection OverviewA font is a set of characters of matching size (height) and appearance. -In order to be displayed each character must ultimately be drawn as a series -of pixels (a bitmap). Symbian can store fonts in bitmap or vector form. A -vector font (for example, an OpenType font) must be converted to bitmaps (or -rasterized) before it can be drawn. Symbian caches and shares bitmaps for -performance and memory efficiency. -

Application developers can -use Font and Text Services for selecting fonts for displaying text according -to various criteria (size and typeface).
Device creators can -add fonts and font rasterizers. Fonts can be included in the ROM or installed -on user drives.

Concepts and -terms

Points, twips and pixels: Points (1/72th inch) and twips (a twentieth of a point) are absolute -measurements of distance and specify device-independent size. Pixel sizes -are only relevant for a specific graphics device. Each graphics device (screen -or printer) must provide a mechanism for converting between pixels and twips.
Font: A named set of characters of matching size (height) and font style (normal, -bold, italic, underline). Times Roman 14 point bold is an example of -a font.
Typeface: A group of fonts with similar appearance. The typeface does not include -the size or font style. Times is an example of a typeface. Typefaces -are sometimes referred to as font families.

Typefaces are grouped -generically as mono-spaced, variable width, serif, sans-serif and symbol. -This allows the grouping of typefaces that have similar characteristics, so -that an alternative font can be selected when an exact match is not found. -This means that applications can display text relatively accurately when a -specific font is not available.
OpenType font: A font that is stored as vector data, which can be rasterized to any -pixel size. OpenType fonts are sometimes simply called open fonts. -An open font can only be used with a corresponding font rasterizer. The Font and Bitmap Server caches -rasterized fonts into bitmap form.

OpenType is an open source vector -font format, which is similar to TrueType. Like TrueType fonts, OpenType fonts -can contain font hinting information. However the font rasterizer's use of -the font hinting information in OpenType fonts is restricted by a software -patent.
Character: A single element (such as a letter, number, ligature, symbol or punctuation -mark) of written language. Each character has a unique number (its Unicode code point).
Glyph: A graphical representation of a character in a specific font. The mapping -between a character and a glyph is not always direct. For example, a character -may be represented by different glyphs, depending on the context.
Font metrics: Measurements (such as maximum height) that are common to all of the -characters in a font.
Character metrics: Measurements (such as character width) that apply to an individual -character within a font.
Bitmap font: A font stored as bitmap glyphs. The size of a font is described in -pixels. However, the actual size is dependent on the pixel size of the graphics -device.
Rasterizing: The process of converting drawing instructions into bitmap data. Fonts -provided in vector form (OpenType fonts) must be rasterized into fixed size -fonts before they can be used.
Linked font: A virtual font or typeface created from characters from other fonts. -The component fonts must be open fonts or other linked fonts. Linked fonts -are typically used where a preferred font does not contain all of the characters -in the character set. One of the typefaces in a linked font must be designated -as the principal typeface from which any unspecified default metrics or characters -are used.
Contextual shaping: In some scripts some characters are drawn differently depending on -the adjacent characters. This means that different glyphs are used to represent -a particular character in different contexts. Contextual shaping is the selection -of the appropriate glyph from several glyphs that represent a particular character -depending on the context.

In Latin scripts, contextual shaping is -normally used only when simulating handwriting. For example, the glyph used -for the letter p might depend on whether the letter is at the start, -middle or end of a word, as shown in the following diagram. For example, when -it is in the middle of the word, it might have a "tail" on both sides to join -it to the letters that appear before and after.
- Figure 1: Handwriting simulation might use different -glyphs for the letter p depending on its position in a word - - -
Contextual shaping is an essential feature of some scripts, including -Arabic and many Indian and East Asian scripts, such as Devanagari, Kannada, -Telugu, Tamil, Bengali, Malayalam and Gurmukhi. For this reason, support for -contextual shaping is sometimes referred to as Brahmic support.
Text shaper: A plug-in that performs the contextual shaping for scripts that require -this feature. Text shapers are font and language specific.
System font: A default font that is used when no font is specified.

Architectural relationships

The Font and Text Services -components are closely integrated with the Font and Bitmap Server, GDI, BitGDI -and Window Server components in the Graphics -package. Font size is only meaningful in the context of the current -graphics device and its pixel size. Open fonts must be rasterized in the context -of the current graphics device. The diagram below shows how BitGDI implements -Graphics Devices and Contexts.

- Figure 2: The Graphics Device and Context Architecture. - -

Note that the names of the BitGDI classes begin with CFbs.

- -

Applications access font services through the Font -and Bitmap Server (FBSERV) which caches font bitmaps in a shared heap. -The Font and Bitmap Server ensures that there is only ever a single instance -of a particular glyph bitmap in memory. However, eXecute In Place (XIP) ROM-based -bitmaps (including fonts) are used in place and are not copied into the heap.

Use -of the Font and Bitmap Server is transparent to clients of the Window Server. -The Window Server client session (RWsSession) automatically -creates an RFbsSession and stores it in thread local storage -(TLS). The Font and Bitmap Server provides two client-side classes for fonts: CFbsFont and CFbsTypefaceStore. These find and use the session in TLS automatically. Clients of the Window -Server are therefore automatically clients of the Font and Bitmap Server and -need perform no explicit session management.

Classes representing -fonts, such as CFbsFont (on the client side) and CBitmapFont (on -the server side), are derived from the abstract class CFont.

The Font Store lives in -the Font and Bitmap -Server and contains all of the fonts on the device and the font rasterizers -required to render them. The Font Store loads font files when the phone starts -up. It scans all of the drives on the device so that new fonts can be installed -from user drives. Font files can be loaded by the Font Store while the phone -is running. However, font rasterizers can only be installed by restarting -the phone. If a new font requires a font rasterizer to be installed, the phone -must be restarted before the font can be used.

Open font rasterizers -are implemented as ECOM plug-ins. An open font file (represented by COpenFontFile) -can contain one or more open fonts in proprietary vector format. A corresponding -font rasterizer plug-in (COpenFontRasterizer) must convert -the vector data into bitmaps that the Font and Bitmap Server can cache and -access using the CBitmapFont and COpenFont APIs. -Applications use rasterized open fonts by using CFbsFont in -exactly the same way as native bitmap fonts. The open font framework means -that Symbian can support any font in any format. There is a performance impact -associated with open fonts, however, as they must be rasterized.

Text -shapers (CShaper) are used for contextual scripts, which -require their glyphs to be modified according to the adjacent characters. -Text shapers are font and language specific.

Components

The Font and Text Services collection -is made up of the following components:

Font -Store. This manages font matching, ownership and rasterizing.
FreeType -Font Rasterizer. This open source plug-in rasterizes OpenType fonts. -More information can be found at www.freetype.org. -This rasterizer is normally loaded by default.
ICU -Layout Engine Component. A plug-in that performs contextual shaping -for specific scripts that require this feature. When a function, such as CFbsBitGc::DrawText() draws -text in one of these scripts, internally a text shaper is used to convert -the string of characters into the appropriate series of glyphs.
Reference -Fonts. This provides a collection of reference font files including -bitmap and OpenType fonts. By default the Symbian platform supports the DejaVu -OpenType typeface (which includes variable-width, monospaced, serif and sans -serif fonts) and legacy Calculator and Agenda symbols (which are bitmap fonts).

Note: The iType -Font Rasterizer is an OpenType font rasterizer from MonoType Imaging®. -This provides additional features compared to the FreeType font rasterizer, -such as four color blending, which is required for outline and shadow fonts. -This component is not provided by the Symbian Foundation because it belongs -to Monotype. However, the Symbian platform has test cases for the four color -blending that the iType Font Rasterizer provides. These test cases can be -used by device creators who obtain this font rasterizer from MonoType.

-Font and -Bitmap Server Component Overview - Font Store -Component Overview + + + + + +Font +and Text Services Collection OverviewA font is a set of characters of matching size (height) and appearance. +In order to be displayed each character must ultimately be drawn as a series +of pixels (a bitmap). Symbian can store fonts in bitmap or vector form. A +vector font (for example, an OpenType font) must be converted to bitmaps (or +rasterized) before it can be drawn. Symbian caches and shares bitmaps for +performance and memory efficiency. +

Application developers can +use Font and Text Services for selecting fonts for displaying text according +to various criteria (size and typeface).
Device creators can +add fonts and font rasterizers. Fonts can be included in the ROM or installed +on user drives.

Concepts and +terms

Points, twips and pixels: Points (1/72th inch) and twips (a twentieth of a point) are absolute +measurements of distance and specify device-independent size. Pixel sizes +are only relevant for a specific graphics device. Each graphics device (screen +or printer) must provide a mechanism for converting between pixels and twips.
Font: A named set of characters of matching size (height) and font style (normal, +bold, italic, underline). Times Roman 14 point bold is an example of +a font.
Typeface: A group of fonts with similar appearance. The typeface does not include +the size or font style. Times is an example of a typeface. Typefaces +are sometimes referred to as font families.

Typefaces are grouped +generically as mono-spaced, variable width, serif, sans-serif and symbol. +This allows the grouping of typefaces that have similar characteristics, so +that an alternative font can be selected when an exact match is not found. +This means that applications can display text relatively accurately when a +specific font is not available.
OpenType font: A font that is stored as vector data, which can be rasterized to any +pixel size. OpenType fonts are sometimes simply called open fonts. +An open font can only be used with a corresponding font rasterizer. The Font and Bitmap Server caches +rasterized fonts into bitmap form.

OpenType is an open source vector +font format, which is similar to TrueType. Like TrueType fonts, OpenType fonts +can contain font hinting information. However the font rasterizer's use of +the font hinting information in OpenType fonts is restricted by a software +patent.
Character: A single element (such as a letter, number, ligature, symbol or punctuation +mark) of written language. Each character has a unique number (its Unicode code point).
Glyph: A graphical representation of a character in a specific font. The mapping +between a character and a glyph is not always direct. For example, a character +may be represented by different glyphs, depending on the context.
Font metrics: Measurements (such as maximum height) that are common to all of the +characters in a font.
Character metrics: Measurements (such as character width) that apply to an individual +character within a font.
Bitmap font: A font stored as bitmap glyphs. The size of a font is described in +pixels. However, the actual size is dependent on the pixel size of the graphics +device.
Rasterizing: The process of converting drawing instructions into bitmap data. Fonts +provided in vector form (OpenType fonts) must be rasterized into fixed size +fonts before they can be used.
Linked font: A virtual font or typeface created from characters from other fonts. +The component fonts must be open fonts or other linked fonts. Linked fonts +are typically used where a preferred font does not contain all of the characters +in the character set. One of the typefaces in a linked font must be designated +as the principal typeface from which any unspecified default metrics or characters +are used.
Contextual shaping: In some scripts some characters are drawn differently depending on +the adjacent characters. This means that different glyphs are used to represent +a particular character in different contexts. Contextual shaping is the selection +of the appropriate glyph from several glyphs that represent a particular character +depending on the context.

In Latin scripts, contextual shaping is +normally used only when simulating handwriting. For example, the glyph used +for the letter p might depend on whether the letter is at the start, +middle or end of a word, as shown in the following diagram. For example, when +it is in the middle of the word, it might have a "tail" on both sides to join +it to the letters that appear before and after.
+ Figure 1: Handwriting simulation might use different +glyphs for the letter p depending on its position in a word + + +
Contextual shaping is an essential feature of some scripts, including +Arabic and many Indian and East Asian scripts, such as Devanagari, Kannada, +Telugu, Tamil, Bengali, Malayalam and Gurmukhi. For this reason, support for +contextual shaping is sometimes referred to as Brahmic support.
Text shaper: A plug-in that performs the contextual shaping for scripts that require +this feature. Text shapers are font and language specific.
System font: A default font that is used when no font is specified.

Architectural relationships

The Font and Text Services +components are closely integrated with the Font and Bitmap Server, GDI, BitGDI +and Window Server components in the Graphics +package. Font size is only meaningful in the context of the current +graphics device and its pixel size. Open fonts must be rasterized in the context +of the current graphics device. The diagram below shows how BitGDI implements +Graphics Devices and Contexts.

+ Figure 2: The Graphics Device and Context Architecture. + +

Note that the names of the BitGDI classes begin with CFbs.

+ +

Applications access font services through the Font +and Bitmap Server (FBSERV) which caches font bitmaps in a shared heap. +The Font and Bitmap Server ensures that there is only ever a single instance +of a particular glyph bitmap in memory. However, eXecute In Place (XIP) ROM-based +bitmaps (including fonts) are used in place and are not copied into the heap.

Use +of the Font and Bitmap Server is transparent to clients of the Window Server. +The Window Server client session (RWsSession) automatically +creates an RFbsSession and stores it in thread local storage +(TLS). The Font and Bitmap Server provides two client-side classes for fonts: CFbsFont and CFbsTypefaceStore. These find and use the session in TLS automatically. Clients of the Window +Server are therefore automatically clients of the Font and Bitmap Server and +need perform no explicit session management.

Classes representing +fonts, such as CFbsFont (on the client side) and CBitmapFont (on +the server side), are derived from the abstract class CFont.

The Font Store lives in +the Font and Bitmap +Server and contains all of the fonts on the device and the font rasterizers +required to render them. The Font Store loads font files when the phone starts +up. It scans all of the drives on the device so that new fonts can be installed +from user drives. Font files can be loaded by the Font Store while the phone +is running. However, font rasterizers can only be installed by restarting +the phone. If a new font requires a font rasterizer to be installed, the phone +must be restarted before the font can be used.

Open font rasterizers +are implemented as ECOM plug-ins. An open font file (represented by COpenFontFile) +can contain one or more open fonts in proprietary vector format. A corresponding +font rasterizer plug-in (COpenFontRasterizer) must convert +the vector data into bitmaps that the Font and Bitmap Server can cache and +access using the CBitmapFont and COpenFont APIs. +Applications use rasterized open fonts by using CFbsFont in +exactly the same way as native bitmap fonts. The open font framework means +that Symbian can support any font in any format. There is a performance impact +associated with open fonts, however, as they must be rasterized.

Text +shapers (CShaper) are used for contextual scripts, which +require their glyphs to be modified according to the adjacent characters. +Text shapers are font and language specific.

Components

The Font and Text Services collection +is made up of the following components:

Font +Store. This manages font matching, ownership and rasterizing.
FreeType +Font Rasterizer. This open source plug-in rasterizes OpenType fonts. +More information can be found at www.freetype.org. +This rasterizer is normally loaded by default.
ICU +Layout Engine Component. A plug-in that performs contextual shaping +for specific scripts that require this feature. When a function, such as CFbsBitGc::DrawText() draws +text in one of these scripts, internally a text shaper is used to convert +the string of characters into the appropriate series of glyphs.
Reference +Fonts. This provides a collection of reference font files including +bitmap and OpenType fonts. By default the Symbian platform supports the DejaVu +OpenType typeface (which includes variable-width, monospaced, serif and sans +serif fonts) and legacy Calculator and Agenda symbols (which are bitmap fonts).

Note: The iType +Font Rasterizer is an OpenType font rasterizer from MonoType Imaging®. +This provides additional features compared to the FreeType font rasterizer, +such as four color blending, which is required for outline and shadow fonts. +This component is not provided by the Symbian Foundation because it belongs +to Monotype. However, the Symbian platform has test cases for the four color +blending that the iType Font Rasterizer provides. These test cases can be +used by device creators who obtain this font rasterizer from MonoType.

+Font and +Bitmap Server Component Overview + Font Store +Component Overview

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