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Microsoft Windows printer drivers provide lists of the fonts available on
the printer. Although most drivers have the text metrics for a few
device-resident fonts built in as resources, the majority of the
printer-font metrics are external to the drivers and are in the form of
Microsoft Windows printer font metrics files. This topic describes the
format of the font metrics files for Windows printer drivers.
 
Printer Font Metrics Files
 
A printer font metrics (PFM) file has the following general form:
 
PFMHEADER       Header;          /* font header */
WORD            WidthTable[];    /* character widths */
PFMEXTENSION    Extensions;      /* extensions */
char            DeviceName[];    /* printer-device name */
char            FaceName[];      /* font name */
EXTTEXTMETRIC   ExtTextMetrics;  /* extended-text metrics */
WORD            ExtentTable[];   /* unscaled-character widths */
DRIVERINFO      DriverInfo;      /* driver-specific information */
PAIRKERN        KerningPairs[];  /* pair-kerning table */
KERNTRACK       KerningTracks[]; /* track-kerning table */
 
The PFMHEADER structure, the PFMEXTENSION structure, and font name must be
present in every PFM file. Whether a PFM file contains any of the other
structures and arrays depends on the printer driver and the font vendor's
preferences.
 
The PFMHEADER and PFMEXTENSION structures must be, respectively, the first
and second structures in the file. The remaining structures and arrays may
appear in any order. Their locations are indicated by the offsets in the
PFMHEADER and PFMEXTENSION structures.
 
The following sections describe the arrays in the file, such as the width
table. For more information about the structures, see Section 2.4,
"Structure Reference," later in this topic.
 
Width Table
 
The width table in the PFM file is an array of 16-bit values containing the
width of each character in the font. The number of elements in the array is
computed as dfLastChar - dfFirst + 2. Since the last entry is extra, it is
set to zero.
 
The width table is optional. For example, fixed-pitch fonts do not require
the table since all characters have the same width. If the table is
provided, it must be placed between the PFMHEADER and PFMEXTENSION
structures.
 
Device and Font Names
 
The device and font names are null-terminated strings specifying the name of
the printer device and font face, respectively.
 
Extent Table
 
The extent table is an array of 16-bit values containing the width of each
character in the font at the default point size.
 
Printer-Cartridge Metrics Files
 
A printer-cartridge metrics (PCM) file has the following general form:
 
PCMHEADER   Header;             /* font header           */
char        CartridgeTitle[];   /* title for cartridge   */
BYTE        PFMFiles[];         /* an array of PFM files */
 
The PCMHEADER structure contains information about the PCM files, such as
its size and the location of the cartridge title. For more information about
structures, see Section 2.4, "Structure Reference" later in this topic.
 
The PCMHEADER structure must contain a nonzero offset to the cartridge
title. This string must be nonempty and null-terminated. This title is used
in the cartridge list box in the Printer Setup dialog box.
 
The PCM file must contain one PFM file for each font in the cartridge. The
files must be appended together. Since each PFM file contains its own size
in bytes, the location of each PFM file is calculated as an offset from the
previous PFM file. All file positions in a PFM file itself are offsets from
the beginning of the individual PFM file and the PCM file.
 
Font Scaling: etmMasterHeight and etmMasterUnits
 
Printer drivers use the values in the etmMasterHeight and etmMasterUnits
members of the EXTTEXTMETRIC structure to scale the character width values
in the extent table. A driver allows two methods for measuring character
widths:
 
♦  Device units or "pixels"
 
♦  Font units
 
Device units are the number of dots in the physical device (dots-per-inch);
etmMasterHeight is expressed in device units. It may be converted to points
using the formula:
 
                   etmMasterHeight device units * 72 points-per-inch
height in points = ---------------------------------------------------
                           dfVertRes device units-per-inch
 
In this case, 72 represents the number of points-per-inch. The dfVertRes
member contains the number of dots-per-inch in device units.
 
Font units may differ from device units as a convenience to the font vendor.
If the font is intended for devices that differ in resolution, there
typically is no benefit from mapping the resolution of the font to the
resolution of a particular printer. The vendor uses the units that are most
convenient for designing the font. For example, 1000 units-per-cell (or em)
is the Adobe standard for PostScript fonts; etmMasterUnits is expressed in
font units-per-em where an em equals etmMasterHeight.
 
A number may be converted from device units to font units using the
formula:
 
 
value in font units =
         value in device units * etmMasterUnits font units
         -------------------------------------------------
                   etmMasterHeight device units
 
 
The formula for the inverse operation is:
 
 
value in device units =
         value in font units * etmMasterHeight device units
         --------------------------------------------------
                  etmMasterUnits font units
 
Determining Character Widths
 
The values in the extent table are the widths of the characters when the
font height equals etmMasterHeight. When the font height does not equal
etmMasterHeight, the driver must scale the values from the extent table.
Always assume linear scaling fonts.
 
The values in the extent table are in font units. The width of a character
for a particular font height may be computed as follows:
 
 
character width      value from extent table in font units
---------------   =  -------------------------------------
  fontheight            etmMasterUnits font units
 
By multiplying font height through the equation, we have the formula for
computing character width:
 
                  font height * value from extent table in font units
character width = ---------------------------------------------------
                               etmMasterUnits font units
 
Character width takes on the units of font height. If the font height is in
device units, the resulting width will be in device units. If the font
height is in font units, the resulting width will be in font units.
 
In its normal mode of operation, the driver simply plugs the font height
into the above equation to compute the character width. Font height is
normally in device units, so the resulting width is normally in device
units.
 
For example, assume the device resolution is 300 dpi and the font units are
expressed as 1000 units-per-em. Assuming that the etmMasterHeight member
represents a 72-point font, the values would look like this:
 
dfVertRes = 300
 
etmMasterHeight = 300
 
etmMasterUnits = 1000
 
Additionally, assume the value in the width table for a capital H is 500
font units, or half the em height. Assume the height of the font for which
you want the character width is 12 points, or 50 device units. With relative
widths disabled, the character width would be computed using the first width
formula:
 
50 device units * 500 font units
--------------------------------  = 25 device units
     1000 font units
 
Using the ENABLERELATIVEWIDTHS Escape
 
When the application calls the ENABLERELATIVEWIDTHS escape, the driver
expects the application to request a font height in device units, but will
return the character width in font units. The application may want to obtain
widths in font units to alleviate any error that may result from rounding
widths to device units. It is the responsibility of the driver to correct
this error when relative widths are disabled, but the application may choose
to enable relative widths and perform its own error correction.
 
The driver converts the font height (in device units) to font units prior to
computing the character width. The resulting formula is:
 
character width in font units =
     font height in device units * character extent in font units
     ------------------------------------------------------------
                     etmMasterHeight device units
 
This formula was computed by replacing the "font height" from the first
formula for character width with the formula for converting a value from
device units to font units, and then simplifying the equation.
 
Using the same assumptions provided in the previous section, you can now
work through some more examples. You know that the width of the character is
25 device units (that is, dots or pixels) or half the height of 50 device
units. With relative widths enabled, the character width would be computed
using the second width formula:
 
50 device units * 500 font units
-------------------------------- = 83.333 font units
    300 device units
 
The width of the character is 83.333 font units. The driver will round
noninteger results.
 
To demonstrate that the width in font units is equivalent to the width in
device units, it can be converted to device units using the formula provided
in the previous section:
 
83.333 font units  * 300 device units
------------------------------------- = 25 device units
   1000 font units
 
For more information about the restrictions imposed by drivers, see Chapter
3, "The PFM Editor," and Chapter 4, "PCL/HP LaserJet Printer Driver."
 
 
                                      ♦