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1 <?xml version="1.0" encoding="utf-8"?> |
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2 <!-- Copyright (c) 2007-2010 Nokia Corporation and/or its subsidiary(-ies) All rights reserved. --> |
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3 <!-- This component and the accompanying materials are made available under the terms of the License |
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4 "Eclipse Public License v1.0" which accompanies this distribution, |
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5 and is available at the URL "http://www.eclipse.org/legal/epl-v10.html". --> |
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6 <!-- Initial Contributors: |
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7 Nokia Corporation - initial contribution. |
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8 Contributors: |
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9 --> |
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10 <!DOCTYPE concept |
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11 PUBLIC "-//OASIS//DTD DITA Concept//EN" "concept.dtd"> |
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12 <concept id="GUID-90644B52-69D7-595C-95E3-D6F7A30C060D" xml:lang="en"><title>Font |
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13 and Text Services Collection Overview</title><shortdesc>A font is a set of characters of matching size (height) and appearance. |
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14 In order to be displayed each character must ultimately be drawn as a series |
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15 of pixels (a bitmap). Symbian can store fonts in bitmap or vector form. A |
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16 vector font (for example, an OpenType font) must be converted to bitmaps (or |
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17 rasterized) before it can be drawn. Symbian caches and shares bitmaps for |
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18 performance and memory efficiency. </shortdesc><prolog><metadata><keywords/></metadata></prolog><conbody> |
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19 <ul> |
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20 <li id="GUID-DB17AE5F-54E9-5288-89DB-7A38473D9C94"><p> <b>Application developers</b> can |
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21 use Font and Text Services for selecting fonts for displaying text according |
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22 to various criteria (size and typeface). </p> </li> |
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23 <li id="GUID-ECFA8635-D278-5A1F-A8E8-8BADB73FE092"><p> <b> Device creators</b> can |
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24 add fonts and font rasterizers. Fonts can be included in the ROM or installed |
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25 on user drives. </p> </li> |
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26 </ul> |
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27 <section id="GUID-9F33DD34-F5E5-5EA0-8559-7A933D230129"><title>Concepts and |
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28 terms</title> <dl> |
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29 <dlentry> |
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30 <dt>Points, twips and pixels</dt> |
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31 <dd><p>Points (1/72th inch) and twips (a twentieth of a point) are absolute |
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32 measurements of distance and specify device-independent size. Pixel sizes |
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33 are only relevant for a specific graphics device. Each graphics device (screen |
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34 or printer) must provide a mechanism for converting between pixels and twips. </p> </dd> |
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35 </dlentry> |
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36 <dlentry> |
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37 <dt>Font</dt> |
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38 <dd><p>A named set of characters of matching size (height) and <b>font style</b> (normal, |
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39 bold, italic, underline). <b>Times Roman 14 point bold</b> is an example of |
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40 a font. </p> </dd> |
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41 </dlentry> |
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42 <dlentry> |
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43 <dt>Typeface</dt> |
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44 <dd><p>A group of fonts with similar appearance. The typeface does not include |
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45 the size or font style. <b>Times</b> is an example of a typeface. Typefaces |
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46 are sometimes referred to as <b>font families</b>. </p> <p>Typefaces are grouped |
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47 generically as mono-spaced, variable width, serif, sans-serif and symbol. |
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48 This allows the grouping of typefaces that have similar characteristics, so |
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49 that an alternative font can be selected when an exact match is not found. |
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50 This means that applications can display text relatively accurately when a |
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51 specific font is not available. </p> </dd> |
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52 </dlentry> |
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53 <dlentry> |
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54 <dt>OpenType font </dt> |
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55 <dd><p>A font that is stored as vector data, which can be rasterized to any |
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56 pixel size. OpenType fonts are sometimes simply called <b>open fonts</b>. |
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57 An open font can only be used with a corresponding font rasterizer. The <xref href="GUID-A03FB1BF-F67B-519D-A904-74CA3F8375D9.dita">Font and Bitmap Server</xref> caches |
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58 rasterized fonts into bitmap form. </p> <p>OpenType is an open source vector |
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59 font format, which is similar to TrueType. Like TrueType fonts, OpenType fonts |
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60 can contain font hinting information. However the font rasterizer's use of |
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61 the font hinting information in OpenType fonts is restricted by a software |
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62 patent. </p> </dd> |
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63 </dlentry> |
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64 <dlentry> |
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65 <dt>Character</dt> |
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66 <dd><p>A single element (such as a letter, number, ligature, symbol or punctuation |
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67 mark) of written language. Each character has a unique number (its <xref href="http://www.unicode.org/standard/WhatIsUnicode.html" scope="external">Unicode</xref> code point). </p> </dd> |
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68 </dlentry> |
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69 <dlentry> |
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70 <dt>Glyph</dt> |
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71 <dd><p>A graphical representation of a character in a specific font. The mapping |
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72 between a character and a glyph is not always direct. For example, a character |
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73 may be represented by different glyphs, depending on the context. </p> </dd> |
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74 </dlentry> |
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75 <dlentry> |
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76 <dt>Font metrics</dt> |
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77 <dd><p>Measurements (such as maximum height) that are common to all of the |
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78 characters in a font. </p> </dd> |
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79 </dlentry> |
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80 <dlentry> |
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81 <dt>Character metrics</dt> |
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82 <dd><p>Measurements (such as character width) that apply to an individual |
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83 character within a font. </p> </dd> |
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84 </dlentry> |
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85 <dlentry> |
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86 <dt>Bitmap font</dt> |
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87 <dd><p>A font stored as bitmap glyphs. The size of a font is described in |
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88 pixels. However, the actual size is dependent on the pixel size of the graphics |
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89 device. </p> </dd> |
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90 </dlentry> |
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91 <dlentry> |
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92 <dt>Rasterizing</dt> |
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93 <dd><p>The process of converting drawing instructions into bitmap data. Fonts |
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94 provided in vector form (OpenType fonts) must be rasterized into fixed size |
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95 fonts before they can be used. </p> </dd> |
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96 </dlentry> |
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97 <dlentry> |
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98 <dt>Linked font</dt> |
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99 <dd><p>A virtual font or typeface created from characters from other fonts. |
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100 The component fonts must be open fonts or other linked fonts. Linked fonts |
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101 are typically used where a preferred font does not contain all of the characters |
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102 in the character set. One of the typefaces in a linked font must be designated |
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103 as the principal typeface from which any unspecified default metrics or characters |
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104 are used. </p> </dd> |
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105 </dlentry> |
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106 <dlentry> |
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107 <dt>Contextual shaping</dt> |
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108 <dd><p>In some scripts some characters are drawn differently depending on |
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109 the adjacent characters. This means that different glyphs are used to represent |
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110 a particular character in different contexts. Contextual shaping is the selection |
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111 of the appropriate glyph from several glyphs that represent a particular character |
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112 depending on the context. </p> <p>In Latin scripts, contextual shaping is |
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113 normally used only when simulating handwriting. For example, the glyph used |
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114 for the letter <b>p</b> might depend on whether the letter is at the start, |
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115 middle or end of a word, as shown in the following diagram. For example, when |
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116 it is in the middle of the word, it might have a "tail" on both sides to join |
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117 it to the letters that appear before and after. </p> <fig id="GUID-30A9DA99-E1C5-5EE5-AEB3-0BCD6DB0A692"> |
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118 <title> Figure 1: Handwriting simulation might use different |
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119 glyphs for the letter p depending on its position in a word |
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120 </title> |
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121 <image href="GUID-8F19C257-9A41-5D38-BC8A-285D0EB16E1B_d0e402675_href.png" placement="inline"/> |
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122 </fig> <p>Contextual shaping is an essential feature of some scripts, including |
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123 Arabic and many Indian and East Asian scripts, such as Devanagari, Kannada, |
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124 Telugu, Tamil, Bengali, Malayalam and Gurmukhi. For this reason, support for |
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125 contextual shaping is sometimes referred to as <b>Brahmic support</b>. </p> </dd> |
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126 </dlentry> |
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127 <dlentry> |
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128 <dt>Text shaper</dt> |
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129 <dd><p>A plug-in that performs the contextual shaping for scripts that require |
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130 this feature. Text shapers are font and language specific. </p> </dd> |
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131 </dlentry> |
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132 <dlentry> |
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133 <dt>System font</dt> |
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134 <dd><p>A default font that is used when no font is specified. </p> </dd> |
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135 </dlentry> |
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136 </dl> </section> |
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137 <section><title>Architectural relationships</title> <p>The Font and Text Services |
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138 components are closely integrated with the Font and Bitmap Server, GDI, BitGDI |
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139 and Window Server components in the <xref href="GUID-975CBC70-81E6-5FA2-80CE-88DD2ABE9595.dita">Graphics |
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140 package</xref>. Font size is only meaningful in the context of the current |
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141 graphics device and its pixel size. Open fonts must be rasterized in the context |
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142 of the current graphics device. The diagram below shows how BitGDI implements |
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143 Graphics Devices and Contexts. </p> <fig id="GUID-36165480-E5BC-50F3-B205-5668EA7F0306"> |
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144 <title> Figure 2: The Graphics Device and Context Architecture. |
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145 </title> |
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146 <desc><p>Note that the names of the BitGDI classes begin with <codeph>CFbs</codeph>. </p> </desc> |
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147 <image href="GUID-9D6D07D3-A93C-5290-AC08-15750AD34802_d0e402733_href.png" placement="inline"/> |
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148 </fig> <p>Applications access font services through the <xref href="GUID-71DADA82-3ABC-52D2-8360-33FAEB2E5DE9.dita">Font |
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149 and Bitmap Server (FBSERV)</xref> which caches font bitmaps in a shared heap. |
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150 The Font and Bitmap Server ensures that there is only ever a single instance |
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151 of a particular glyph bitmap in memory. However, eXecute In Place (XIP) ROM-based |
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152 bitmaps (including fonts) are used in place and are not copied into the heap. </p> <p>Use |
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153 of the Font and Bitmap Server is transparent to clients of the Window Server. |
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154 The Window Server client session (<xref href="GUID-643DDA78-C7A7-386D-AB3F-8710141DDDA9.dita"><apiname>RWsSession</apiname></xref>) automatically |
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155 creates an <xref href="GUID-E5B29AC0-4953-385F-84C5-13EE6CB77D46.dita"><apiname>RFbsSession</apiname></xref> and stores it in thread local storage |
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156 (TLS). The Font and Bitmap Server provides two client-side classes for fonts: <xref href="GUID-DE966D8B-6E00-39B6-B136-6CF29EA384D3.dita"><apiname>CFbsFont</apiname></xref> and <xref href="GUID-5E287BF6-08C1-36A2-9085-F6B61CC52550.dita"><apiname>CFbsTypefaceStore</apiname></xref>. These find and use the session in TLS automatically. Clients of the Window |
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157 Server are therefore automatically clients of the Font and Bitmap Server and |
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158 need perform no explicit session management. </p> <p>Classes representing |
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159 fonts, such as <xref href="GUID-DE966D8B-6E00-39B6-B136-6CF29EA384D3.dita"><apiname>CFbsFont</apiname></xref> (on the client side) and <xref href="GUID-25D4B920-6158-3014-8E7D-4E1C5267C5EF.dita"><apiname>CBitmapFont</apiname></xref> (on |
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160 the server side), are derived from the abstract class <xref href="GUID-2A12FE3B-47F2-3016-8161-A971CA506491.dita"><apiname>CFont</apiname></xref>. </p> <p>The <xref href="GUID-B798C19D-4030-564A-A208-D84738259D72.dita">Font Store</xref> lives in |
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161 the <xref href="GUID-71DADA82-3ABC-52D2-8360-33FAEB2E5DE9.dita">Font and Bitmap |
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162 Server</xref> and contains all of the fonts on the device and the font rasterizers |
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163 required to render them. The Font Store loads font files when the phone starts |
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164 up. It scans all of the drives on the device so that new fonts can be installed |
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165 from user drives. Font files can be loaded by the Font Store while the phone |
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166 is running. However, font rasterizers can only be installed by restarting |
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167 the phone. If a new font requires a font rasterizer to be installed, the phone |
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168 must be restarted before the font can be used. </p> <p>Open font rasterizers |
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169 are implemented as ECOM plug-ins. An open font file (represented by <xref href="GUID-FC62EDD7-A3F7-3611-A899-CA5F17BB2EEE.dita"><apiname>COpenFontFile</apiname></xref>) |
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170 can contain one or more open fonts in proprietary vector format. A corresponding |
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171 font rasterizer plug-in (<xref href="GUID-CA16238D-478A-302A-9671-97B62B0E738A.dita"><apiname>COpenFontRasterizer</apiname></xref>) must convert |
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172 the vector data into bitmaps that the Font and Bitmap Server can cache and |
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173 access using the <xref href="GUID-25D4B920-6158-3014-8E7D-4E1C5267C5EF.dita"><apiname>CBitmapFont</apiname></xref> and <xref href="GUID-039375D3-AFD9-3E31-9C1C-ACEDC10E27CE.dita"><apiname>COpenFont</apiname></xref> APIs. |
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174 Applications use rasterized open fonts by using <xref href="GUID-DE966D8B-6E00-39B6-B136-6CF29EA384D3.dita"><apiname>CFbsFont</apiname></xref> in |
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175 exactly the same way as native bitmap fonts. The open font framework means |
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176 that Symbian can support any font in any format. There is a performance impact |
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177 associated with open fonts, however, as they must be rasterized. </p> <p>Text |
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178 shapers (<xref href="GUID-FF3DFCCC-DCFE-3DD9-B497-B96C46B4FDD1.dita"><apiname>CShaper</apiname></xref>) are used for contextual scripts, which |
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179 require their glyphs to be modified according to the adjacent characters. |
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180 Text shapers are font and language specific. </p> </section> |
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181 <section><title>Components</title> <p>The Font and Text Services collection |
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182 is made up of the following components: </p> <ul> |
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183 <li id="GUID-0C4363B1-4340-5638-8A59-ECDF2AFFA088"><p><xref href="GUID-B798C19D-4030-564A-A208-D84738259D72.dita">Font |
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184 Store</xref>. This manages font matching, ownership and rasterizing. </p> </li> |
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185 <li id="GUID-AAE1BCA4-B9B4-5AD1-AC79-3308F231FCE7"><p> <xref href="GUID-75C5A614-CEC1-5832-ACEA-E3C31B8A0DE8.dita">FreeType |
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186 Font Rasterizer</xref>. This open source plug-in rasterizes OpenType fonts. |
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187 More information can be found at <xref href="http://www.freetype.org/" scope="external">www.freetype.org</xref>. |
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188 This rasterizer is normally loaded by default. </p> </li> |
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189 <li id="GUID-7EB97752-0F44-5625-8474-FDCF64654AE6"><p><xref href="GUID-DB453F9B-55B6-50C7-984D-7154F1E84AE6.dita">ICU |
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190 Layout Engine Component</xref>. A plug-in that performs contextual shaping |
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191 for specific scripts that require this feature. When a function, such as <xref href="GUID-4A501086-7EFF-376D-8901-6D9B2EB4EFF2.dita#GUID-4A501086-7EFF-376D-8901-6D9B2EB4EFF2/GUID-176B0A61-867E-35A6-AE53-0BABA48741D9"><apiname>CFbsBitGc::DrawText()</apiname></xref> draws |
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192 text in one of these scripts, internally a text shaper is used to convert |
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193 the string of characters into the appropriate series of glyphs. </p> </li> |
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194 <li id="GUID-DED869DB-C45B-51A6-B594-BA1321BC272B"><p><xref href="GUID-F4C53728-B54B-5680-82FC-9A365B45F4B5.dita">Reference |
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195 Fonts</xref>. This provides a collection of reference font files including |
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196 bitmap and OpenType fonts. By default the Symbian platform supports the DejaVu |
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197 OpenType typeface (which includes variable-width, monospaced, serif and sans |
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198 serif fonts) and legacy Calculator and Agenda symbols (which are bitmap fonts). </p> </li> |
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199 </ul> <p> <i>Note</i>: The <xref href="GUID-771AC6EC-348D-5535-9D91-5A0232BB9576.dita"> iType |
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200 Font Rasterizer</xref> is an OpenType font rasterizer from MonoType Imaging®. |
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201 This provides additional features compared to the FreeType font rasterizer, |
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202 such as four color blending, which is required for outline and shadow fonts. |
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203 This component is not provided by the Symbian Foundation because it belongs |
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204 to Monotype. However, the Symbian platform has test cases for the four color |
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205 blending that the iType Font Rasterizer provides. These test cases can be |
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206 used by device creators who obtain this font rasterizer from MonoType. </p> </section> |
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207 </conbody><related-links> |
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208 <link href="GUID-71DADA82-3ABC-52D2-8360-33FAEB2E5DE9.dita"><linktext>Font and |
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209 Bitmap Server Component Overview </linktext></link> |
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210 <link href="GUID-416A3756-B5D5-5BCD-830E-2371C5F6B502.dita"><linktext> Font Store |
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211 Component Overview</linktext></link> |
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212 </related-links></concept> |