diff -r 43e37759235e -r 51a74ef9ed63 Symbian3/SDK/Source/GUID-4957E6BB-C41D-52F1-94CD-68CF6D61C271.dita
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/Symbian3/SDK/Source/GUID-4957E6BB-C41D-52F1-94CD-68CF6D61C271.dita	Wed Mar 31 11:11:55 2010 +0100
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+<?xml version="1.0" encoding="utf-8"?>
+<!-- Copyright (c) 2007-2010 Nokia Corporation and/or its subsidiary(-ies) All rights reserved. -->
+<!-- This component and the accompanying materials are made available under the terms of the License 
+"Eclipse Public License v1.0" which accompanies this distribution, 
+and is available at the URL "http://www.eclipse.org/legal/epl-v10.html". -->
+<!-- Initial Contributors:
+    Nokia Corporation - initial contribution.
+Contributors: 
+-->
+<!DOCTYPE concept
+  PUBLIC "-//OASIS//DTD DITA Concept//EN" "concept.dtd">
+<concept id="GUID-4957E6BB-C41D-52F1-94CD-68CF6D61C271" xml:lang="en"><title>Format
+layering</title><prolog><metadata><keywords/></metadata></prolog><conbody>
+<p>The Symbian platform uses the principle of format layering
+in which new formatting is added as a layer onto the previous layer of formatting.
+The following diagram shows a less efficient technique for applying format
+layering to text. The inefficiency arises because each format layer stores
+not only the new attribute which it introduces, but also every attribute which
+it inherits from the preceding format layer. </p>
+<fig id="GUID-DBA21807-9BC8-5FFB-962A-4CDCD9A72359">
+<image href="GUID-CFD3CD78-D99A-57B8-80EB-8C67899E5D27_d0e427545_href.png" placement="inline"/>
+</fig>
+<p>The Symbian platform avoids this inefficiency by using the system of <keyword>based-on
+links</keyword>. Here, each new layer adds attributes in a natural way without
+restating all attributes previously set. A based-on link is a pointer to the
+format layer whose attributes are inherited by another layer. For example,
+when the italic attribute is applied to a region of bold text, the new layer
+stores only the italic attribute and a pointer to the previous attribute layer.
+Chains of format layers can thus be formed, with each layer based on the previous
+one. In order to determine a character or paragraph’s <keyword>effective format</keyword>,
+a chain of format layers must typically be examined. The final layer in the
+chain of based on links is the one whose based on link is NULL and is not
+therefore based upon any other layer.</p>
+<p>This method is demonstrated in the following two diagrams, in which each
+arrow represents a based-on link.</p>
+<fig id="GUID-3BE6B9A3-D8DB-543C-AA88-DD519B991B62">
+<title>Format layering</title>
+<image href="GUID-52743192-B1E0-5B71-B85B-43487F18F412_d0e427565_href.png" placement="inline"/>
+</fig>
+<p>The following diagram demonstrates the system of based-on links when attributes
+are applied to different parts of text rather than added on top of existing
+attribute layers.</p>
+<fig id="GUID-1E0E17F0-BE93-5403-8474-ED612E118C8F">
+<image href="GUID-F99938E9-9E06-53C0-B762-2CE51BC5F1E2_d0e427573_href.png" placement="inline"/>
+</fig>
+<p>In this case, several format layers are required, all of which are based
+on the normal layer.</p>
+</conbody></concept>
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