The Adobe Type 1 Font Format book (PDF: 445 KB) - Adobe Partners

42 Adobe Type 1 Font Format
5.9 Stem Width Information
There is a mechanism to tell Type 1 BuildChar about standard
stem widths in a font so that Type 1 BuildChar can ensure consistency
at small sizes. If a particular stem is slightly wider or
narrower than standard, either by design or as a result of a small
error in creating the font program, then at small sizes where a
single pixel difference would be very noticeable, Type 1 Build-
Char can render the stem as though it had the standard width.
However, at large sizes where a single pixel difference will produce
only a subtle visual effect, the stem will be allowed to
deviate from the standard.
When the difference between a standard stem width and a particular
stem width is small, the standard width is used. For example,
if at 10 points a standard stem width corresponds to 1.4 pixels
wide and a particular stem is 1.6 pixels wide, both can be rendered
as a 1-pixel wide stem. However, at 100 points the standard
stem would be rendered as 14 pixels wide and the particular stem
would be rendered as 16 pixels wide. The information that Type
1 BuildChar needs appears in the following Private dictionary
entries.
The entry StdHW is an array with only one real number entry
expressing the dominant width of horizontal stems (measured
vertically in character space units). For example:
/StdHW [32] def
The entry StdVW is an array with only one real number entry
expressing the dominant width of vertical stems (measured horizontally
in character space units). Typically, this will be the width
of straight stems in lower case letters. (For an italic font program,
give the width of the vertical stem measured at an angle perpendicular
to the stem direction.) For example:
/StdVW [85] def
The entry StemSnapH is an array of up to 12 real numbers of the
most common widths (including the dominant width given in
the StdHW array) for horizontal stems (measured vertically).
These widths must be sorted in increasing order. For example:
/StemSnapH [32 41] def