User Guide

FontLab 4
186
Baseline-to-baseline distance calculation
Windows:
Windows Metric OpenType Metric
ascent
WinAscent
descent
WinDescent
internal leading
WinAscent + WinDescent – UPM
external leading MAX(0, LineGap - ((WinAscent + WinDescent) - (Ascender -
Descender)))
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.
Macintosh:
Macintosh Metric OpenType Metric
ascender
Ascender
descender
Descender
leading
LineGap
BTBD = ascender + descender + leading