EBSC - Embedded Bitmap Scaling Table (OpenType 1.8)

  • Article

The 'EBSC' table provides a mechanism for describing embedded bitmaps which are created by scaling other embedded bitmaps. While this is the sort of thing that outline font technologies were invented to avoid, there are cases (small sizes of Kanji, for example) where scaling a bitmap produces a more legible font than scan-converting an outline. For this reason the 'EBSC' table allows a font to define a bitmap strike as a scaled version of another strike.

The 'EBSC' table begins with a header containing the table version and number of strikes.

ebscHeader

Type Name Description
FIXED version initially defined as 0x00020000
ULONG numSizes  

The ebscHeader is followed immediately by the bitmapScaleTable array. The numSizes in the ebscHeader indicates the number of bitmapScaleTables in the array. Each strike is defined by one bitmapScaleTable.

bitmapScaleTable

Type Name Description
sbitLineMetrics hori line metrics
sbitLineMetrics vert line metrics
BYTE ppemX target horizontal pixels per Em
BYTE ppemY target vertical pixels per Em
BYTE substitutePpemX use bitmaps of this size
BYTE substitutePpemY use bitmaps of this size

The line metrics have the same meaning as those in the bitmapSizeTable, and refer to font wide metrics after scaling. The ppemX and ppemY values describe the size of the font after scaling. The substitutePpemX and substitutePpemY values describe the size of a strike that exists as an sbit in the 'EBLC' and 'EBDT', and that will be scaled up or down to generate the new strike.

Notice that scaling in the x direction is independent of scaling in the y direction, and their scaling values may differ. A square aspect-ratio strike could be scaled to a non-square aspect ratio. Glyph metrics are scaled by the same factor as the pixels per Em (in the appropriate direction), and are rounded to the nearest integer pixel.