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

40 Adobe Type 1 Font Format
The BlueScale value is a number directly related to the number of
pixels tall that one character space unit will be before overshoot
suppression is turned off. The default value of BlueScale is
.039625, which corresponds to 10 points at 300 dpi. A simple formula
that relates point size as rendered on a 300-dpi device to the
BlueScale value is:
BlueScale = (pointsize − 0.49) ÷ 240
The formula provides a convenient number that font program
designers can use to determine at what integer point size overshoot
suppression should be off. However, the exact point size at
which overshoot suppression ceases is actually 0.49 points less (at
9.51 points using the default value of BlueScale) than the value
of pointsize used in the formula. Adobe recommends using the
adjustment shown in the formula so that the change in overshoot
suppression behavior occurs at an exact point size unlikely
to be used in practice.
For example, if you wish overshoot suppression to turn off at 11
points on a 300-dpi device, you should set BlueScale to
(11 − 0.49) ÷ 240 or 0.04379. With this one setting of BlueScale,
overshoot suppression will turn off at proportionately smaller
point sizes on higher resolution output devices or larger point
sizes on lower-resolution devices such as displays. A typical
BlueScale statement is:
/BlueScale .04379 def
Note There is a mandatory restriction on the BlueScale value and the
maximum height of an alignment zone that is best described in relation
to the 300-dpi point size discussed above. The product of
(pointsize − 0.49) × (maximum alignment zone height) must be less
than 240. For example, if the maximum alignment zone height is 23
in some font program, then the overshoot suppression turnoff point size
at 300 dpi can be 10 but not 11. This restriction ensures that overshoot
suppression will turn off before the overshoot reaches a full device pixel.
5.7 BlueShift
The optional BlueShift entry in the Private dictionary adds
another capability to the treatment of overshoot behavior.
The value of BlueShift is an integer that indicates a character
space distance beyond the flat position of alignment zones at
which overshoot enforcement for character features occurs. The