org.extex.font.format.xtf.tables.gps
Class OtfTableGPOS

java.lang.Object
  org.extex.font.format.xtf.tables.AbstractXtfTable
      org.extex.font.format.xtf.tables.gps.AbstractXtfSFLTable
          org.extex.font.format.xtf.tables.gps.OtfTableGPOS

All Implemented Interfaces:

LookupTableFactory, XtfGlyphName, XtfTable, XMLWriterConvertible

public class OtfTableGPOS
extends AbstractXtfSFLTable
implements XtfTable, LookupTableFactory, XMLWriterConvertible

Table gpos.

http://partners.adobe.com/public/developer/opentype/index_table_formats2.html

The Glyph Positioning table (GPOS) provides precise control over glyph placement for sophisticated text layout and rendering in each script and language system that a font supports.

Table Organization

The GPOS table begins with a header that defines offsets to a ScriptList, a FeatureList, and a LookupList:

• The ScriptList identifies all the scripts and language systems in the font that use glyph positioning.
• The FeatureList defines all the glyph positioning features required to render these scripts and language systems.
• The LookupList contains all the lookup data needed to implement each glyph positioning feature.

The GPOS table is organized so text processing clients can easily locate the features and lookups that apply to a particular script or language system. To access GPOS information, clients should use the following procedure:

1. Locate the current script in the GPOS ScriptList table.
2. If the language system is known, search the script for the correct LangSys table; otherwise, use the script's default language system (DefaultLangSys table).
3. The LangSys table provides index numbers into the GPOS FeatureList table to access a required feature and a number of additional features.
4. Inspect the FeatureTag of each feature, and select the features to apply to an input glyph string.
5. Each feature provides an array of index numbers into the GPOS LookupList table. Lookup data is defined in one or more subtables that contain information about specific glyphs and the kinds of operations to be performed on them.
6. Assemble all lookups from the set of chosen features, and apply the lookups in the order given in the LookupList table.

A lookup uses subtables to define the specific conditions, type, and results of a positioning action used to implement a feature. All subtables in a lookup must be of the same LookupType, as listed in the LookupType Enumeration table:

LookupType Enumeration table for glyph positioning

Value	Type	Description
1	Single adjustment	Adjust position of a single glyph
2	Pair adjustment	Adjust position of a pair of glyphs
3	Cursive attachment	Attach cursive glyphs
4	MarkToBase attachment	Attach a combining mark to a base glyph
5	MarkToLigature attachment	Attach a combining mark to a ligature
6	MarkToMark attachment	Attach a combining mark to another mark
7	Context positioning	Position one or more glyphs in context
8	Chained Context positioning	Position one or more glyphs in chained context
9	Extension positioning	Extension mechanism for other positionings
10+	Reserved	For future use

Each LookupType is defined by one or more subtables, whose format depends on the type of positioning operation and the resulting storage efficiency. When glyph information is best presented in more than one format, a single lookup may define more than one subtable, as long as all the subtables are of the same LookupType. For example, within a given lookup, a glyph index array format may best represent one set of target glyphs, whereas a glyph index range format may be better for another set.

A series of positioning operations on the same glyph or string requires multiple lookups, one for each separate action. The values in the ValueRecords are accumulated in these cases. Each lookup is given a different array number in the LookupList table and is applied in the LookupList order.

During text processing, a client applies a lookup to each glyph in the string before moving to the next lookup. A lookup is finished for a glyph after the client locates the target glyph or glyph context and performs a positioning, if specified. To move to the "next" glyph, the client will typically skip all the glyphs that participated in the lookup operation: glyphs that were positioned as well as any other glyphs that formed a context for the operation.

There is just one exception: the "next" glyph in a sequence may be one of those that formed a context for the operation just performed. For example, in the case of pair positioning operations (i.e., kerning), if the position value record for the second glyph is null, that glyph is treated as the "next" glyph in the sequence.

This rest of this chapter describes the GPOS header and the subtables defined for each LookupType. Several GPOS subtables share other tables: ValueRecords, Anchor tables, and MarkArrays. For easy reference, the shared tables are described at the end of this chapter.

GPOS Header

The GPOS table begins with a header that contains a version number (Version) initially set to 1.0 (0x00010000) and offsets to three tables: ScriptList, FeatureList, and LookupList.

Value	Type	Description
Fixed	Version	Version of the GPOS table-initially = 0x00010000
Offset	ScriptList	Offset to ScriptList table-from beginning of GPOS table
Offset	FeatureList	Offset to FeatureList table-from beginning of GPOS table
Offset	LookupList	Offset to LookupList table-from beginning of GPOS table

Version:

$Revision: 7274 $

Author:

Michael Niedermair

Field Summary

Fields inherited from class org.extex.font.format.xtf.tables.gps.AbstractXtfSFLTable

featureList, lookupList, scriptList

Constructor Summary

OtfTableGPOS(XtfTableMap tablemap, XtfTableDirectory.Entry de, RandomAccessR rar)
Create a new object.

Method Summary

java.lang.String	getShortcut() Returns the table shortcut.
int	getType() Get the table type, as a table directory value.
java.lang.String	lookupType(int type) Returns the name of the lookup type (The start index is 1!).
XtfLookupTable	read(RandomAccessR rar, int posOffset, int type, int offset, XtfGlyphName xtfGlyph) Read the LookupTable or null, if not found.

Methods inherited from class org.extex.font.format.xtf.tables.gps.AbstractXtfSFLTable

findLangSys, findLookup, findScript, getFeatureList, getFeatureTag, getLookupList, getScriptList, writeXML

Methods inherited from class org.extex.font.format.xtf.tables.AbstractXtfTable

getGlyphName, getInitOrder, getTableMap, init, writeStartElement

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Methods inherited from interface org.extex.font.format.xtf.tables.XtfTable

getInitOrder, getTableMap, init

Methods inherited from interface org.extex.util.xml.XMLWriterConvertible

writeXML

Constructor Detail

OtfTableGPOS

public OtfTableGPOS(XtfTableMap tablemap,
                    XtfTableDirectory.Entry de,
                    RandomAccessR rar)
             throws java.io.IOException

Create a new object.

Parameters:

tablemap - the table map

de - directory entry

rar - input

Throws:

java.io.IOException - if an IO-error occurs

Method Detail

getShortcut

public java.lang.String getShortcut()

Returns the table shortcut.

Specified by:

getShortcut in interface XtfTable

Returns:

Returns the table shortcut.

See Also:

XtfTable.getShortcut()

getType

public int getType()

Get the table type, as a table directory value.

Specified by:

getType in interface XtfTable

Returns:

Returns the table type

lookupType

public java.lang.String lookupType(int type)

Returns the name of the lookup type (The start index is 1!).

Specified by:

lookupType in interface LookupTableFactory

Parameters:

type - The type.

Returns:

Returns the name of the lookup type.

See Also:

LookupTableFactory.lookupType(int)

read

public XtfLookupTable read(RandomAccessR rar,
                           int posOffset,
                           int type,
                           int offset,
                           XtfGlyphName xtfGlyph)
                    throws java.io.IOException

Read the LookupTable or null, if not found.

Type:

1 - Single - Replace one glyph with one glyph

2 - Multiple - Replace one glyph with more than one glyph

3 - Alternate - Replace one glyph with one of many glyphs

4 - Ligature - Replace multiple glyphs with one glyph

5 - Context - Replace one or more glyphs in context

6 - Chaining - Context Replace one or more glyphs in chained context

Specified by:

read in interface LookupTableFactory

Parameters:

rar - The input.

posOffset - The offset of the pos table (GPOS, GSUB).

type - The table type.

offset - The offset.

xtfGlyph - The glyph name.

Returns:

Returns the table.

Throws:

java.io.IOException - if a io-error occurred.

See Also:

LookupTableFactory.read(org.extex.util.file.random.RandomAccessR, int, int, int, org.extex.font.format.xtf.tables.XtfGlyphName)