Goals of the GRUB Font Format

• Simple to read and use. Since GRUB will only be reading the font files, we are more concerned with making the code to read the font simple than we are with writing the font.
• Compact storage. The fonts will generally be stored in a small boot partition where GRUB is located, and this may be on a removable storage device such as a CD or USB flash drive where space is more limited than it is on most hard drives.
• Unicode. GRUB should not have to deal with multiple character encodings. The font should always use Unicode character codes for simple internationalization.

Why Another Font Format?

There are many existing bitmap font formats that GRUB could use. However, there are aspects of these formats that may make them less than suitable for use in GRUB at this time:

Section Types

FILE

File type ID (ASCII string). This must be the first section in the file. It has length 4 and the contents are the four bytes of the ASCII string PFF2.

NAME

Font name (ASCII string). This is the full font name including family, weight, style, and point size. For instance, "Helvetica Bold Italic 14".

FAMI

Font family name (ASCII string). For instance, "Helvetica". This should be included so that intelligent font substitution can take place.

WEIG

Font weight (ASCII string). Valid values are bold and normal. This should be included so that intelligent font substitution can take place.

SLAN

Font slant (ASCII string). Valid values are italic and normal. This should be included so that intelligent font substitution can take place.

PTSZ

Font point size (uint16be).

MAXW

Maximum character width in pixels (uint16be).

MAXH

Maximum character height in pixels (uint16be).

ASCE

Ascent in pixels (uint16be). See Font Metrics for details.

DESC

Descent in pixels (uint16be). See Font Metrics for details.

CHIX

Character index. The character index begins with a 32-bit big-endian unsigned integer indicating the total size of the section, not including this size value. For each character, there is an instance of the following entry structure:

1. Unicode code point. (32-bit big-endian integer.)
2. Storage flags. (byte.)
   • Bits 2..0:
     • If equal to 000 binary, then the character data is stored uncompressed beginning at the offset indicated by the character's offset value.
     • If equal to 001 binary, then the character data is stored within a compressed character definition block that begins at the offset within the file indicated by the character's offset value.
3. Offset. (32-bit big-endian integer.)

DATA

A marker that indicates the remainder of the file is data accessed via the character index (CHIX) section. When reading this font file, the rest of the file can be ignored when scanning the sections. The length should be set to -1 (0xFFFFFFFF).

Supported data structures:

Character definition

Each character definition consists of:

1. Width. Width of the bitmap in pixels. The bitmap's extents represent the glyph's bounding box. uint16be.

2. Height. Height of the bitmap in pixels. The bitmap's extents represent the glyph's bounding box. uint16be.

3. X offset. The number of pixels to shift the bitmap by horizontally before drawing the character. int16be.

4. Y offset. The number of pixels to shift the bitmap by vertically before drawing the character. int16be.

5. Device width. The number of pixels to advance horizontally from this character's origin to the origin of the next character. int16be.

6. Bitmap data. This is encoded as a string of bits. It is organized as a row-major, top-down, left-to-right bitmap. The most significant bit of each byte is taken to be the leftmost or uppermost bit in the byte. For the sake of compact storage, rows are not padded to byte boundaries (i.e., a single byte may contain bits belonging to multiple rows). The last byte of the bitmap is padded with zero bits in the bits positions to the right of the last used bit if the bitmap data does not fill the last byte.

   The length of the bitmap data field is (width * height + 7) / 8 using integer arithmetic, which is equivalent to ceil(width * height / 8) using real number arithmetic.

   It remains to be determined whether bitmap fonts usually make all glyph bitmaps the same height, or if smaller glyphs are stored with bitmaps having a lesser height. In the latter case, the baseline would have to be used to calculate the location the bitmap should be anchored at on screen.

Compressed character definition block (Not yet implemented.)

This contains a set of character definitions which are compressed as a single block within the file. Compressed character definitions will be implemented at a later date.

[TODO: Implement compression/decompression using LZMA.]