EBDT - Embedded Bitmap Data Table

The EBDT table is used to embed monochrome or grayscale bitmap glyph data. It is used together with the EBLC table, which provices embedded bitmap locators, and the EBSC table, which provides embedded bitmap scaling information.

OpenType embedded bitmaps are also called 'sbits' (for “scaler bitmaps”). A set of bitmaps for a face at a given size is called a strike.

The 'EBLC' table identifies the sizes and glyph ranges of the sbits, and keeps offsets to glyph bitmap data in indexSubTables. The 'EBDT' table then stores the glyph bitmap data, in a number of different possible formats. Glyph metrics information may be stored in either the 'EBLC' or 'EBDT' table, depending upon the indexSubTable and glyph bitmap data formats. The 'EBSC' table identifies sizes that will be handled by scaling up or scaling down other sbit sizes.

The 'EBDT' table is a super set of Apple's Apple Advanced Typography (AAT) 'bdat' table.

BigGlyphMetrics

Type Name
uint8 height
uint8 width
int8 horiBearingX
int8 horiBearingY
uint8 horiAdvance
int8 vertBearingX
int8 vertBearingY
uint8 vertAdvance

SmallGlyphMetrics

Type Name
uint8 height
uint8 width
int8 bearingX
int8 bearingY
uint8 advance

Glyph Bitmap Data

The nine different formats currently defined for glyph bitmap data are listed and described below. Different formats are better for different purposes. Apple 'bdat' tables support only formats 1 through 7.

In all formats, if the bitDepth is greater than 1, all of a pixel's bits are stored consecutively in memory, and all of a row's pixels are stored consecutively.

Note: Each of these formats can contain black/white or grayscale bitmaps depending on the setting of the bitDepth field in the 'EBLC' table. For performance reasons, we recommend using a byte-aligned format for embedded bitmaps with bitDepth of 8.

Format 1: small metrics, byte-aligned data

Type Name Description
SmallGlyphMetrics smallMetrics Metrics information for the glyph
VARIABLE imageData Byte-aligned bitmap data

Glyph bitmap format 1 consists of small metrics records (either horizontal or vertical depending on the bitmapSizeTable 'flag' value in the 'EBLC' table) followed by byte aligned bitmap data. The bitmap data begins with the most significant bit of the first byte corresponding to the top-left pixel of the bounding box, proceeding through succeeding bits moving left to right. The data for each row is padded to a byte boundary, so the next row begins with the most significant bit of a new byte. 1 bits correspond to black, and 0 bits to white.

Format 2: small metrics, bit-aligned data

Type Name Description
SmallGlyphMetrics smallMetrics Metrics information for the glyph
VARIABLE imageData Bit-aligned bitmap data

Glyph bitmap format 2 is the same as format 1 except that the bitmap data is bit aligned. This means that the data for a new row will begin with the bit immediately following the last bit of the previous row. The start of each glyph must be byte aligned, so the last row of a glyph may require padding. This format takes a little more time to parse, but saves file space compared to format 1.

Format 3: (obsolete)

Format 4: (not supported) metrics in EBLC, compressed data

Glyph bitmap format 4 is a compressed format used by Apple in some of their East Asian fonts. This format is not supported in OpenType.

Format 5: metrics in EBLC, bit-aligned image data only

Type Name Description
VARIABLE imageData Bit-aligned bitmap data

Glyph bitmap format 5 is similar to format 2 except that no metrics information is included, just the bit aligned data. This format is for use with 'EBLC' indexSubTable format 2 or format 5, which will contain the metrics information for all glyphs. It works well for Kanji fonts.

The rasterizer recalculates sbit metrics for Format 5 bitmap data, allowing Windows to report correct ABC widths, even if the bitmaps have white space on either side of the bitmap image. This allows fonts to store monospaced bitmap glyphs in the efficient Format 5 without breaking Windows GetABCWidths call.

Format 6: big metrics, byte-aligned data

Type Name Description
BigGlyphMetrics bigMetrics Metrics information for the glyph
VARIABLE imageData Byte-aligned bitmap data

Glyph bitmap format 6 is the same as format 1 except that is uses big glyph metrics instead of small.

Format7: big metrics, bit-aligned data

Type Name Description
BigGlyphMetrics bigMetrics Metrics information for the glyph
VARIABLE imageData Bit-aligned bitmap data

Glyph bitmap format 7 is the same as format 2 except that is uses big glyph metrics instead of small.

EbdtComponent Record

The EbdtComponent record is used in glyph bitmap data formats 8 and 9.

Type Name Description
uint16 glyphID Component glyph ID
int8 xOffset Position of component left
int8 yOffset Position of component top

The EbdtComponent record contains the glyph ID of the component, which can be used to look up the location of component glyph data in the 'EBLC' table, as well as xOffset and yOffset values, which specify where to position the top-left corner of the component in the composite. Nested composites (a composite of composites) are allowed, and the number of nesting levels is determined by implementation stack space.

Format 8: small metrics, component data

Type Name Description
SmallGlyphMetrics smallMetrics Metrics information for the glyph
uint8 pad Pad to 16-bit boundary
uint16 numComponents Number of components
EbdtComponent components[numComponents] Array of EbdtComponent records

Format 9: big metrics, component data

Type Name Description
BigGlyphMetrics bigMetrics Metrics information for the glyph
uint16 numComponents Number of components
EbdtComponent components[numComponents] Array of EbdtComponent records

Glyph bitmap formats 8 and 9 are used for composite bitmaps. For accented characters and other composite glyphs it may be more efficient to store a copy of each component separately, and then use a composite description to construct the finished glyph. The composite formats allow for any number of components, and allow the components to be positioned anywhere in the finished glyph. Format 8 uses small metrics, and format 9 uses big metrics.


This page was last updated 8 July 2017.

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