CBDT — Color Bitmap Data Table

Table structure

The CBDT table is used to embed color bitmap glyph data. It is used together with the CBLC table, which provides embedded bitmap locators. The formats of these two tables are backward compatible with the EBDT and EBLC tables used for embedded monochrome and grayscale bitmaps.

The CBDT table begins with a header containing simply the table version number.

Type Name Description
uint16 majorVersion Major version of the CBDT table, = 3.
uint16 minorVersion Minor version of the CBDT table, = 0.

Note that the first version of the CBDT table is 3.0.

The rest of the CBDT table is a collection of bitmap data. The data can be presented in three possible formats, indicated by information in the CBLC table. Some of the formats contain metric information plus image data, and other formats contain only the image data. Long word alignment is not required for these subtables; byte alignment is sufficient.

Uncompressed Color Bitmaps

A value of 32 in the bitDepth field of a BitmapSize table within the CBLC table is used to identify color bitmaps with 8-bit blue/green/red/alpha (“BGRA”) channels per pixel, encoded in that order for each pixel. The color channels represent pre-multiplied color and are encoded colors in the sRGB colorspace. For example, the color “full-green with half translucency” is encoded as \x00\x80\x00\x80, and not \x00\xFF\x00\x80.

All bitmap image data formats defined in the EBDT / EBLC tables are valid for use with the CBDT / CBLC tables.

Compressed Color Bitmaps

Images for each individual glyph are stored as straight PNG data. Only the following chunks are allowed in such PNG data: IHDR, PLTE, tRNS, sRGB, IDAT, and IEND. If other chunks are present, the behavior is undefined. The image data shall be in the sRGB colorspace, regardless of color information that may be present in other chunks in the PNG data. The individual images must have the same size as expected by the table in the bitmap metrics.

Glyph Metrics

There are also two different formats for glyph metrics: big glyph metrics and small glyph metrics. Big glyph metrics define metrics information for both horizontal and vertical layouts. This is important in fonts (such as Kanji) where both types of layout may be used. Small glyph metrics define metrics information for one layout direction only. Which direction applies, horizontal or vertical, is determined by the flags field in the BitmapSize tables within the CBLC 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 Formats

In addition to nine different formats already defined for glyph bitmap data in the EBDT table, there are three new formats described below

Format 17: small metrics, PNG image data

Type Name Description
smallGlyphMetrics glyphMetrics Metrics information for the glyph
uint32 dataLen Length of data in bytes
uint8 data[dataLen] Raw PNG data

Format 18: big metrics, PNG image data

Type Name Description
bigGlyphMetrics glyphMetrics Metrics information for the glyph
uint32 dataLen Length of data in bytes
uint8 data[dataLen] Raw PNG data

Format 19: metrics in CBLC table, PNG image data

Type Name Description
uint32 dataLen Length of data in bytes
uint8 data[dataLen] Raw PNG data

Scaling behavior

Applications using these glyphs may need to scale them to fit the raster size available for display. How this scaling takes place is application dependent. It is recommended that where possible the application downscale using the closest sized bitmap that is larger than the desired end raster size.