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<!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>