General Recommendations

Non-Standard Fonts

Non-standard fonts such as Symbol or Wingdings™ have special requirements for Microsoft platforms. These requirements affect the 'cmap' and 'name' tables; the requirements and recommendations for all other tables remain the same.

For the Macintosh, non-standard fonts can continue to use platform ID 1 (Macintosh) and encoding ID 0 (Roman character set). The 'cmap' subtable should use format 0 and follow the standard PostScript character encodings.

For non-standard fonts on Microsoft platforms, however, the 'cmap' and 'name' tables must use platform ID 3 (Microsoft) and encoding ID 0 (Unicode, non-standard character set). Remember that 'name' table encodings should agree with the 'cmap' table.

The Microsoft 'cmap' subtable (platform 3, encoding 0) must use format 4. The character codes should start at 0xF000, which is in the Private Use Area of Unicode. Microsoft suggests deriving the format 4 (Microsoft) encodings by simply adding 0xF000 to the format 0 (Macintosh) encodings.

Under both OS/2 and Windows, only the first 224 characters of non-standard fonts will be accessible: a space and up to 223 printing characters. It does not matter where in user space these start, but 0xF020 is suggested. The usFirstCharIndex and usLastCharIndex values in the 'OS/2' table would be set based on the actual minimum and maximum character indices used.

Device Resolutions

OS/2 and Windows make use of a logical device resolution. The physical resolution of a device is also available, but fonts will be rendered based on the logical resolution. The table below lists some important logical resolutions in dots per inch (Horizontal x Vertical). The most important ratios (in order) are 1:1, 1.67:1 and 1.33:1.

Device	Resolution	Aspect Ratio
CGA	96 x 48	2:1
EGA	96 x 72	1.33:1
VGA	96 x 96	1:1
8514	120 x 120	1:1
Dot Matrix	120 x 72	1.67:1
Laser Printer	300 x 300 or 600 x 600	1:1

Baseline to Baseline Distances

The suggested Baseline to Baseline Distance (BTBD) is computed differently for Windows and the Macintosh, and it is based on different TrueType metrics. However, if the recommendations below are followed, the BTBD will be the same for both Windows and the Mac.

Windows

Windows Metric	TrueType Metric
ascent	usWinAscent
descent	usWinDescent
internal leading	usWinAscent + usWinDescent - unitsPerEm
external leading	MAX( 0, LineGap - ((usWinAscent + usWinDescent) - (Ascender - Descender))

Suggested BTBD = ascent + descent + external leading
It should be clear that the "external leading" can never be less than zero. Pixels above the ascent or below the descent will be clipped from the character; this is true for all output devices.

The usWinAscent and usWinDescent are values from the 'OS/2' table. The unitsPerEm value is from the 'head' table. The LineGap, Ascender and Descender values are from the 'hhea' table.

Macintosh

Ascender and Descender are metrics defined by Apple and are not to be confused with the Windows ascent or descent, nor should they be confused with the true typographic ascender and descender that are found in AFM files.

Macintosh Metric	TrueType Metric
ascender	Ascender
descender	Descender
leading	LineGap

Suggested BTBD = ascender + descender + leading
If pixels extend above the ascent or below the descent, the character will be squashed in the vertical direction so that all pixels fit within these limitations; this is true for screen display only.

Making Them Match

If you perform some simple algebra, you will see that the suggested BTBD across both Macintosh and Windows will be identical if and only if:
LineGap >= (yMax - yMin) - (Ascender - Descender)

Style Bits

For backwards compatibility with previous versions of Windows, the macStyle bits in the 'head' table will be used to determine whether or not a font is regular, bold or italic (in the absence of an 'OS/2' table). This is completely independent of the usWeightClass and PANOSE information in the 'OS/2' table, the ItalicAngle in the 'post' table, and all other related metrics. If the 'OS/2' table is present, then the fsSelection bits are used to determine this information.

Drop-out Control

Drop-out control is needed if there is a difference in bitmaps with dropout control on and off. Two cases where drop-out control is needed are when the font is rotated or when the size of the font is at or below 8 ppem. Do not use SCANCTRL unless needed. SCANCTRL or the drop-out control rasterizer should be avoided for Roman fonts above 8 points per em (ppem) when the font is not under rotation. SCANCTRL should not be used for "stretched" fonts (e.g. fonts displayed at non-square aspect ratios, like that found on an EGA).