User Guide

Hinting
487
Standard Stem Widths
Typically many characters in a font use the same few standard stem widths.
As examples, let’s take the H, B, and F characters shown below. All of them
have the same width for the straight vertical stems and the same width for
the horizontal stems:
VVV V
H
H
H
HH
H
The most widely used stem widths are stored in the font header in order to
force the rasterizer to render these stems at the same width.
This information is used to control at what character size the rounded stem
width goes from one to two pixels and from two to three pixels. A step from
one to two pixels means a 100% width increase and a step from 2 to 3 pixels
a 50% increase. This means that near this value rounding errors will be
maximal and control over stem widths will be necessary.
If one stem has a width of 74 units and another a stem width of 76 units and the UPM is 1000 units,
then at a PPM of 20 pixels the first stem will be rounded to 1 pixel and the second stem to 2 pixels.
Scaled back to the original coordinates, this difference will be 50 units! That is clearly too much for
an original difference of only 2 units.
Standard widths work with stem hints. When the width of a hint is close to
one of the standard widths, the rounded width of the hint (and the real
stem outline) will be forced equal to the width of the rounded standard
stem.