GEOS Font

GEOS Font files are multiple point size bitmaps stored in GEOS VLIR files;  the chain number (0-126) is  the point size. GEOS limits a font to a point size of 48. This is probably   due  to  memory  limitations  for  storing  the  font. Nonexistent point sizes have VLIR chain addresses of $00,$FF.

The header in each font file contains Font ID and point size information for the fonts in the file. This includes the FONT ID at offset  $80 (128)  within the header block, the 32 byte point size table at $82 (130) ordered from smallest to largest (padded with 0's), and the 32 byte font sizes at $61 (97) ordered from smallest to largest (padded with 0's). This leaves room for 16 fonts per font file.

Font files  are identified by a unique ID number  which  is stored in the file's info sector at offset 130. The info sector contains a word identifier for each point size in the font. These identifiers have the form: ID# * 8 + point size. In other words, bits 15-6 represent the font number while bits 0-5 hold the point size. These ID words are used by GEOwrite and GEOpaint.

0   BSW            13   Tilden 1   University     14   Evans 2   California     15   Durant 3   Roma           16   Telegraph 4   Dwinelle       17   Superb 5   Cory           18   Bowditch 6   Tolman         19   Ormond 7   Bubble         20   Elmwood 8   Fontknox       21   Hearst 9   Harmon         21   Brennens (BUG) 10  Mykonos        23   Channing 11  Boalt          24   Putnam 12  Stadium        25   LeConte

Each VLIR in a font file is organized as follows:

OffSet   Description ---  ---          $00       Number  of pixels minus 1 above the underline. This is                   the line of print. $01-$02  Number of bytes in the bit stream. $03      Point size, character height in pixels. $04-$05  Index  from  beginning of font to table of  bit  stream indices. Usually $0008. $06-$07  Index from beginning of font to first bit stream. $08-???  Table  of words which are indices into the bit streams; one for  each character from space (32) to  the  tilde (126). There is also an extra index on the  end. This extra index is needed because the difference between a                    character's index and the next character's index is the width of the character in pixels. ???-???  Pointed  to by $06-$07. The font is stored as several bit streams,  one for each line of pixels. The point size is the number of bit streams. All the character images are stored in the bit stream. The GEOS KERNAL has some very sophisticated bit manipulation  routines for accessing any given character.

(content taken from http://zimmers.net/geos/docs/fontfile.txt which is unattributed and has been floating around BBS and web sites for decades)