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Subject: Chapter 4 

    OS/2 Information

Subject: 4.1. OS/2 FAQ Copyright Information 

    [ ed: Except as otherwise noted, the entire OS/2 section of the
    comp.fonts FAQ List is derived from the ``Draft OS/2 Font FAQ''
    posted by David J. Birnbaum. ]

    This section if the FAQ is Copyright (C) 1993 by David J. Birnbaum.
    All Rights Reserved. Reproduced here by permission.

    [ ed: Since this section of the FAQ is wholly derived from David's
    document, some sections contain information repeated elsewhere in
    the comp.fonts FAQ. ]

    4.1.1 David Birnbaum's Introduction

    4 June 1993

    A couple of weeks ago I posted an inquiry to comp.fonts,
    comp.os.os2.misc, and the OS2-L ListServ concerning some apparent
    peculiarities in the way OS/2 handles font files. These
    "peculiarities" actually reflect regular, systematic differences in
    OS/2, Windows, and DOS font handling, which are not conveniently
    described in end-user documentation. This posting is intended to
    spare others some of the confusion I encountered as a result of
    this paradigm shift.

    This is the first (draft) distribution of this document and
    corrections and suggestions are welcome. I am grateful to Henry
    Churchyard, Marc L. Cohen, Bur Davis and Kamal Mansour for helpful
    discussions; they are not, of course, responsible for any
    misinterpretation I may have inflicted on their comments.

Subject: 4.2. Preliminaries 

    Character: an informational unit consisting of a value (usually a
    byte) and roughly corresponding to what we think of as letters,
    numbers, punctuation, etc.

    Glyph: a presentational unit corresponding roughly to what we think
    of as letters, numbers, punctuation, etc.

    Character vs glyph: Glyph and character are not necessarily the
    same; the character <a> may be mapped to a Times Roman Lower Case
    <a> glyph in one font and to a Helvetica Lower Case <a> glyph in
    another font. Change of glyphs normally means a change in style of
    presentation, while change in characters normally means a change in
    information. There are gray areas and the definitions provided
    above are general, approximate, and imprecise.

    Character set: an inventory of characters with certain assigned
    values. ASCII is a 7-bit character set that specifies which
    "character cell" (byte value) corresponds to which informational
    unit.

    Code Page: essentially synonymous with character set.

    Font: A collection of glyphs. A specific font may be isomorphic
    with a specific character set, containing only glyphs corresponding
    to characters in that set, with these glyphs mapped to the same
    byte values as the characters they are intended to represent.
    PostScript fonts often contain additional (unmapped) characters.
    Most importantly, PostScript fonts may sometimes be remapped by an
    operating environment, which is what leads to the disorienting
    cross-environment mismatch that spurred my original posting.

    Fonts may be bitmapped or outline in format; a bitmapped format
    corresponds to a particular size and weight for a particular device
    or device resolution, while a single outline font is used to
    generate multiple sizes as needed. Within an outline font system,
    different weights (bold, semibold, italic, etc.) may be encoded as
    separate font resources (separate outline files used to generate
    the glyphs) or may all be generated from a single outline (slanting
    characters to make "italics," fattening them for "bold," etc.).

Subject: 4.3. Fonts Under Dos 

    I used a large assortment of fonts under DOS for intricate
    multilingual work. My setup at that time consisted of a library of
    bitmapped fonts that could be sent to my HP LaserJet II printer, as
    well as a set of fixed-size, fixed-width screen fonts that were
    supported by my Hercules Graphics Card Plus (not the same as
    Hercules Graphics; the "Plus" included an ability to store 3072
    screen glyphs and display any of these together, while standard
    character-mode displays were normally limited to 256 or 512 such
    entities).

    Using XyWrite as a word processor, I would enter a "Mode" command
    to change fonts and character sets simultaneously; this would make
    different sets of screen glyphs available at the keyboard and would
    insert a font-change command for my printer into the text stream.
    The "Mode" and font-change commands were not displayed on the
    screen. The result was not WYSIWYG, since I was limited to
    fixed-width screen display and since I had far more printer glyphs
    available than the 3072 limit imposed by my video card; I used a
    brightness attribute to indicate bold, I used the same screen font
    for different sizes of printer fonts, etc. This worked and worked
    well, in that I could see (for example) Russian, Greek, English,
    Polish, and other characters simultaneously on the screen and I
    could print documents combining them.

    Architecturally, what was going on was that the character sets
    (code pages) and fonts were entirely isomorphic and were hard-
    coded. If I put a particular Russian letter into cell 246 of my
    screen and printer fonts, that character was always there, and any
    strategy that would let me access this cell (remapped keyboards,
    numeric keypad) was guaranteed always to find the same character.

Subject: 4.4. Windows 

    I recently began using PostScript fonts in Windows with AmiPro as
    my word processor. These fonts came with printed cards indicating
    the glyph mappings; I could look at the card and it would tell me
    that a specific character lived in cell 246, and if I entered
    Alt-0246 at the numeric keypad that glyph would appear on the
    screen. If I loaded the font into Fontographer for Windows, these
    glyphs would be arrayed in cells according to the map provided by
    Adobe with the fonts. Fontographer also revealed that these fonts
    had other, "unmapped" glyphs assigned to cells above 255.

    Given what appeared to be a hard correspondence among what I saw in
    Fontographer, what was printed in Adobe's maps, and what was
    displayed when I entered something at the keyboard, I naively
    assumed that PostScript fonts were operating much like my bitmapped
    fonts under DOS. There were some obvious differences, the primary
    one being that glyphs of different sizes were all drawn from the
    same font resource files under PostScript, but it appeared as if a
    glyph lived in a certain cell.

Subject: 4.5. Differences between Windows and OS/2 

    This assumption was incorrect; PostScript fonts can be subdivided
    into two types, one of which observes hard and invariant encodings
    similar to those that apply to my bitmapped fonts, while the other
    represents a completely different font mapping strategy. This
    difference became apparent only when I attempted to share
    PostScript fonts between Windows and OS/2 and got some unexpected
    results.

    A PostScript font under Windows involves two files, a PFB
    (PostScript Font Binary) file, which contains the PostScript
    instructions needed to draw each glyph and some mapping
    information, and a PFM (Printer Font Metrics) file, which encodes
    width and kerning information. A PostScript font under OS/2 also
    uses the same PFB file, but instead of the PFM file it uses an AFM
    (Adobe Font Metrics) file. The AFM and PFM files contain much of
    the same basic information (although the AFM file is somewhat more
    complete); the most important differences are in format (AFM is
    plain text, PFM is binary) and use (OS/2 uses AFM, Windows uses
    PFM).

Subject: 4.6. Installation under Windows and Win-OS/2 

    The OS/2 2.0 Font Palette tool (see below for changes to be
    introduced with 2.1) by default installs fonts (both PFB and AFM
    files) into the "\os2\dll" directory. Win-OS/2 by default
    installs PFB files into "\psfonts" and PFM files into "\psfonts
    \pfm". These defaults can be changed; since OS/2 and Win-OS/2 use
    the same PFB files, the user can save disk space by allowing these
    to be shared (through installing into the same directory, e.g.,
    install OS/2 fonts into the "\psfonts" directory instead of "\
    os2\dll".) Note that fonts must be intalled and removed through
    the Font Palette; if you copy, move, or delete a font file without
    using the Font Palette, the system configuration files are not
    updated and all hell breaks loose.

    Deleting fonts from Win-OS/2 causes the system to update the
    win.ini file to remove references to the font, but does not delete
    any files physically. Deleting fonts from the OS/2 Font Palette
    updates the os2.ini configuration file _and_ physically deletes the
    AFM and PFB files from the disk. This means that if you are sharing
    PFB files between OS/2 and Win-OS/2, you can delete a Win-OS/2 font
    without hurting native OS/2 operations, since the PFB reamins
    installed where OS/2 thinks it is. But if you delete an OS/2 font
    using the Font Palette, the PFB file is erased from the disk even
    though the win.ini file is not updated, so that Win-OS/2 thinks it
    is still there.

Subject: 4.7. FontSpecific PostScript Encoding 

    Every PFB file contains an "encoding vector"; this is a plain text
    line embedded near the head of the PFB file. Encoding vectors are
    of two types: AdobeStandardEncoding and everything else. Adobe
    usually uses the label "FontSpecific" for fonts that are not
    encoded according to AdobeStandardEncoding, and I use it as a cover
    term here for any such font.

    If you look at the readable plain text information at the head of a
    FontSpecific type font, it includes a range of text that begins:

          /Encoding 256 array

    followed by a bunch of lines, each of which includes a number
    (which corresponds to a cell in the font layout) and the name of
    the glyph that lives in that cell. The unreadable binary data below
    this array specification lists the name of each glyph and the
    PostScript instructions for how the glyph is to be drawn. There may
    be PostScript code for drawing glyphs that are not included in the
    mapping array, but only glyphs mentioned in the array specification
    are available to applications.

    FontSpecific type fonts _are_ comparable to the bitmapped fonts I
    used under DOS. Each character physically is assigned to a specific
    cell within the font file and operating environments are not
    allowed to remap these. The glyph in cell 246 will be the same in
    both Windows and OS/2.

Subject: 4.8. AdobeStandardEncoding 

    AdobeStandardEncoding is a specific mapping of certain glyphs to
    certain cells; in this respect it resembles FontSpecific encoding.
    Because it is standardized, the array is not spelled out in the PFB
    file; the line

          /Encoding StandardEncoding def

    tells Adobe Type Manager (ATM, either the Windows and Win-OS/2
    version or the native OS/2 version) that the encoding is
    "standard," and the environments are expected to know what this
    standard is without having the array spelled out in each font file.

    Although AdobeStandardEncoding is a real mapping, there is an
    importance difference between it and various FontSpecific mappings:
    operating environments are expected to remap AdobeStandardEncoding
    fonts according to their own requirements. That is, although
    AdobeStandardEncoding does assign glyphs to cells, no operating
    environment actually uses these assignments and any environment
    remaps the glyphs before rendering them. Confusion arises because
    Windows and OS/2 remap such fonts in different ways.

Subject: 4.9. AdobeStandardEncoding under Windows (and Win-OS/2) 

    An AdobeStandardEncoding font under Windows is remapped according
    to a character map (code page) that MicroSoft calls Windows ANSI
    (can other code pages be installed in Windows?). This determines
    which character resides in which cell and the font is remapped so
    that glyphs and characters will correspond. Since Fontographer for
    Windows is a Windows application, it displays glyphs not in the
    cells in which they live according to AdobeStandardEncoding, but in
    the cells to which they get reassigned under the remapping to
    Windows ANSI. There is nothing explicit in the PFB file that
    associates these characters with the specific cells in which they
    appear under Windows.

Subject: 4.10. AdobeStandardEncoding under OS/2 

    OS/2 operates within a set of supported code pages; two system-
    wide code pages are specified in the config.sys file and an
    application is allowed to switch the active code page to any
    supported code page (not just these two). DeScribe, for example,
    currently operates in code page (CP) 850, which includes most
    letters needed for western European Latin alphabet writing. CP 850
    does not contain typographic quotes, en- and em-dashes, and other
    useful characters. It does contain the IBM "pseudographics," which
    are useful for drawing boxes and lines with monospaced fonts.

    When the user inputs a value (through the regular keyboard or the
    numeric keypad), the application checks the active CP, looks up in
    an internal table the name of the character that lives in that cell
    within that CP, and translates it into a unique number that
    corresponds to one of the 383 glyphs supported by OS/2 (the union
    of all supported code pages). This number is passed to PM-ATM (the
    OS/2 ATM implementation), which translate the glyph number into the
    glyph name that PostScript fonts expect and searches the font for
    that name. The system never looks at where a glyph is assigned
    under the AdobeStandardEncoding array; rather, it scans the font
    looking for the character by name and gives it an assignment
    derived from the active code page. This is the remapping that OS/2
    performs on AdobeStandardEncoding type fonts.

    As a result, a situation arises where, for example, <o+diaeresis>
    is mapped to cell 246 under Windows ANSI but to cell 148 under CP
    850. Using the identical PFB file, this glyph is accessed
    differently in the two operating environments.

Subject: 4.11. Consequences for OS/2 users 

    If your font has a FontSpecific encoding, there are no unexpected
    consequences; the same glyphs will show up at the same locations in
    both Windows (Win-OS/2) and native OS/2. Regardless of what the
    active code page is, if the font has a FontSpecific encoding OS/2
    goes by cell value; a specific glyph is hard-coded to a specific
    cell and OS/2 will give you whatever it finds there, even if what
    it finds disagrees with what the active code page would normally
    predict. In other words, FontSpecific encoding means "ignore the
    mapping of the active code page and rely on the mapping hard-coded
    into the font instead."

    If your font has an AdobeStandardEncoding encoding, the following
    details obtain:

    1) The same PFB file may have glyphs that are accessible in one
    environment but not another. For example, if DeScribe thinks it is
    operating in CP 850, there is no access to typographic quotes, even
    if those do occur in the PFB file and even if Windows can find them
    in the same exact font file. DeScribe could switch code pages, but
    if the application isn't set up to do so (and DeScribe currently
    isn't), those characters are absolutely inaccessible to the user.

    2) If the active code page includes a character that isn't present
    in the font, OS/2 has to improvise. For example,
    AdobeStandardEncoding fonts do not normally include the IBM
    pseudographics, yet the user who inputs the character value for one
    of these sends the system off to look for it. As described above,
    OS/2 first checks the active font for the glyph name that
    corresponds to that character and, if it finds it, displays it. If
    the glyph isn't found, OS/2 looks to the system Symbol font. This
    is not reported back to the user in DeScribe; if I have Adobe
    Minion active (AdobeStandardEncoding, no information anywhere in
    the font files for pseudographics) and input a pseudographic
    character, DeScribe tells me it is still using Adobe Minion, even
    though it has fetched the character it displays and prints from the
    Symbol font, a different font resource file.

Subject: 4.12. Advice to the User 

    OS/2's code page orientation provides some advantages, in that it
    separates the character set (code page) mapping from the encoded
    font mapping. The main inconvenience isn't a loss of function, but
    a disorientation as users become accustomed to the new paradigm.

    If you need a glyph that you know is in your PFB file but that
    isn't in the active code page (and if you can't change code pages
    within your application), you can't get at it in OS/2 without
    tampering with the font files. To tamper, you can use font
    manipulation tools to redesignate the PFB file as FontSpecific
    ("Symbol" character set to Fontographer). If you then map the
    glyphs you need into one of the lower 256 cells (with some
    limitations), they will be accessible in all environments. The
    Fontographer manual does not explain what the "Symbol" character
    encoding label really does, it just tells you not to use it except
    for real symbol fonts. In fact you should use it for any font that
    will not correspond in inventory to the code page supported by your
    application, which means any non-Latin fonts.

    You do not have to recode all your fonts, and you wouldn't normally
    want to do so, since Fontographer hinting is not nearly as good as
    Adobe's own hand-tuning and regenerating a font regenerates the
    hints. All you have to do is make sure you have one FontSpecific
    type font installed that includes your typographic quotes, etc. for
    each typeface you need. Within DeScribe, you can then write a macro
    that will let you switch fonts, fetch a character, and switch back,
    thereby allowing you to augment any group of fonts with a single,
    shared set of typographic quotes (or whatever) that you put in a
    single FontSpecific font. Alternatively, OS/2 also supports CP
    1004, which does contain typographic quotes and other characters
    used for high-quality typography, but the user may not be able to
    convince an application to invoke this code page if it was not
    designed to do so.

    You can have any number of FontSpecific fonts installed, which
    means that there is a mechanism for dealing with unsupported
    character sets (code pages).

    You can also tinker with the font files to try to trick the
    operating system. For example, using Fontographer or other
    utilities, you can change the name assigned to a glyph description
    within the PFB file. If you want to use AdobeStandardEncoding and
    you want to see a specific glyph at a specific cell when DeScribe
    thinks it's using CP 850, you have to make sure that the name
    assigned to the description of that glyph is what DeScribe expects
    to find. OS/2 doesn't care whether, say, <o+diaeresis> really looks
    like <o> with two dots over it, as long as it bears the right name.

    This second approach is obviously far more complex and provides
    much more opportunity for error. Its advantage is that OS/2 does
    not support case conversion and sorting (other than in machine
    order) for unsupported code pages, since these operations depend on
    character names. Keeping supported names from supported code pages
    while changing the artwork is one way to maintain order and case
    correspondences while increasing the range of glyphs actually
    supported. I have not experimented with this approach, since the
    use I would get out of the adding functionality (over the
    FontSpecific encoding approach) is not worth the amount of effort
    required.

Subject: 4.13. OS/2 2.1 and beyond 

    OS/2 2.1 will change some aspects of font handling. First, OS/2 2.0
    GA+SP has a bug that can cause OS/2 to crash when an AFM file with
    more than 512 kern pairs is read. This is fixed in 2.1. (This bug
    is separate from a design limitation in MicroSoft Windows that
    causes large kern tables to be read incorrectly. This problem is
    still under investigation; watch this space for a report.)

    Fonts in 2.1 will be installed by default into the "\psfonts"
    directory, so that they will normally be shared with Win-OS/2
    fonts. (The user will still be able to specify a directory; all
    that will change is the default). The user will also be able to
    instruct the Font Palette not to delete font files when fonts are
    uninstalled, so as to avoid clobbering a Win-OS/2 font by removing
    it from native OS/2 use through the Font Palette (although the
    default will still be to delete the physical font files).

    OS/2 will stop using AFM files and will replace these with OFM
    files, a binary metrics file (different from PFM) that OS/2 will
    compile from the AFM file during font installation. This will speed
    font loading, since the system will not have to parse a plain text
    metrics file. Additionally, the OS/2 PostScript printer driver used
    to install its own, large font files, but will now use the OFM and
    PFB files, thereby saving 50k-200k of disk space per installed font
    outline.

    IBM's long-term goal is to replace the 383-entity inventory of
    supported glyphs with Unicode. This is very much a long-term goal
    and there is not even a hint of when it might become available. It
    has its own problems, stemming from the fact that Unicode is
    essentially a character standard and glyph and character
    inventories may differ is assorted ways, but it will be a
    significant step in the proverbial right direction.