Lookup Type 4:
MarkToBase Attachment Positioning Subtable

The MarkToBase attachment (MarkBasePos) subtable is used to position combining mark glyphs with respect to base glyphs. For example, the Arabic, Hebrew, and Thai scripts combine vowels, diacritical marks, and tone marks with base glyphs.

In the MarkBasePos subtable, every mark glyph has an anchor point and is associated with a class of marks. Each base glyph then defines an anchor point for each class of marks it uses.

For example, assume two mark classes: all marks positioned above base glyphs (Class 0), and all marks positioned below base glyphs (Class 1). In this case, each base glyph that uses these marks would define two anchor points, one for attaching the mark glyphs listed in Class 0, and one for attaching the mark glyphs listed in Class 1.

To identify the base glyph that combines with a mark, the text-processing client must look backward in the glyph string from the mark to the preceding base glyph. To combine the mark and base glyph, the client aligns their attachment points, positioning the mark with respect to the final pen point (advance) position of the base glyph.

The MarkToBase Attachment subtable has one format: MarkBasePosFormat1. The subtable begins with a format identifier (PosFormat) and offsets to two Coverage tables: one that lists all the mark glyphs referenced in the subtable (MarkCoverage), and one that lists all the base glyphs referenced in the subtable (BaseCoverage).

For each mark glyph in the MarkCoverage table, a record specifies its class and an offset to the Anchor table that describes the mark's attachment point (MarkRecord). A mark class is identified by a specific integer, called a class value. ClassCount specifies the total number of distinct mark classes defined in all the MarkRecords.

The MarkBasePosFormat1 subtable also contains an offset to a MarkArray table, which contains all the MarkRecords stored in an array (MarkRecord) by MarkCoverage Index. A MarkArray table also contains a count of the defined MarkRecords (MarkCount). (For details about MarkArrays and MarkRecords, see the end of this chapter.)

The MarkBasePosFormat1 subtable also contains an offset to a BaseArray table (BaseArray).

MarkBasePosFormat1 subtable: MarkToBase attachment point

Type

Name

Description

uint16

PosFormat

Format identifier

—format = 1

Offset

Þ MarkCoverage

Offset to MarkCoverage table

—from beginning of MarkBasePos subtable

Offset

Þ BaseCoverage

Offset to BaseCoverage table

—from beginning of MarkBasePos subtable

uint16

ClassCount

Number of classes defined for marks

Offset

Þ MarkArray

Offset to MarkArray table

—from beginning of MarkBasePos subtable

Offset

Þ BaseArray

Offset to BaseArray table

—from beginning of MarkBasePos subtable


The BaseArray table consists of an array (BaseRecord) and count (BaseCount) of BaseRecords. The array stores the BaseRecords in the same order as the BaseCoverage Index. Each base glyph in the BaseCoverage table has a BaseRecord.

BaseArray table

Type

Name

Description

uint16

BaseCount

Number of BaseRecords

struct

BaseRecord

Array of BaseRecords

[BaseCount]

—in order of BaseCoverage Index


A BaseRecord declares one Anchor table for each mark class (including Class 0) identified in the MarkRecords of the MarkArray. Each Anchor table specifies one attachment point used to attach all the marks in a particular class to the base glyph.

A BaseRecord contains an array of offsets to Anchor tables (BaseAnchor). The zero-based array of offsets defines the entire set of attachment points each base glyph uses to attach marks. The offsets to Anchor tables are ordered by mark class.

Note: Anchor tables are not tagged with class value identifiers. Instead, the index value of an Anchor table in the array defines the class value represented by the Anchor table.

Example 7 at the end of this chapter defines mark positioning above and below base glyphs with a MarkBasePosFormat1 subtable.

BaseRecord

Type

Name

Description

Offset

Þ BaseAnchor

Array of offsets (one per class) to Anchor tables

[ClassCount]

—from beginning of BaseArray table

—ordered by class

—zero-based