UEFI HII (Part 9): Font Package Encoding

Ok, now into the gory details of how font packages are encoded. In many ways, font packages are like string packages except, instead of strings, we are talking about font glyphs. There is a font header, followed by font information and then a series of font blocks.

The glyph blocks represent the glyphs in sorted order, starting with the glyph for character value 0x0001 and incrementing from there. Each glyph block increments the character value by either 1 or N. There are five basic types of glyph blocks:

1. Glyphs. These actually contain glyph data. There are variants for a single glyph or multiple glyphs, ones which use default character cell information or their own character cell information. The current character value is incremented by either 1 (for single glyph versions) or N (for multiple glyph versions).
2. Duplicate. This block duplicates the glyph data for a character value that has already been processed.  The current character value is incremented by 1.
3. Skip. These blocks skip a specified number of character values. For example, there might be no glyphs for character values 0x2000-0x20FF, but then one more glyph for character value 0x2100. A Skip block leaves a gap. The current character value is incremented by the skip count specified.
4. Defaults. This block sets up the default character cell information for the subsequent glyph blocks which use defaults. This is useful for fonts where a lot of characters share a specific set of information, such as Courier or other fixed-format fonts.
5. End. This marks the end of the glyph block information.

The actual glyph data is packed with one bit per pixel of the glyph, but rows are rounded up to the nearest byte (8 pixel) boundary. For example, consider the letter 'A'.

In this case, the actual glyph data is 9 pixels wide and 11 pixels high. If optimally packed, this would lead to a storage size of 99 bits or 13 bytes (12 with 3 bits left over). However, when this was being discussed, some preliminary implementation data suggested that the glyphs were much easier to handle if each row was byte aligned (the blue shaded area in the diagram). Then, after testing with compression software (since we assumed that the fonts would be stored compressed) that the bit-packed data actually was larger after compression than the byte-packed data.

Conclusion

This wraps up the second on fonts. Next week, we'll dive into how HII stores and handles images.