User Guide

FontLab 4
478
F o n t Sc a l i ng , P P M
One of the most important features of outline fonts is that they can be used
on many different output devices – from computer monitors to image-
setters. Because character outline shapes are defined as sequences of lines
and curves it is easy to scale outlines to any size and resolution. However,
almost all output devices have discrete elements arranged in a regular
rectangular raster (grid) and the images that these devices produce are
constructed using these discrete cells. Each cell in an output image has
integer coordinates and is called a “pixel” (picture cell). On a computer
monitor these are individual fluorescent dots. On a printer they are dots of
toner or ink.
To measure scaled outlines in a resolution-independent way, it is
convenient to define a quantity called Pixels Per eM (PPM). This is the
number of pixels that can fit into the font’s height. From the Editing
Glyphs chapter you know that font height is a basic font measurement unit
equal to the Units Per eM (UPM). In TrueType fonts UPM is usually equal
to 2048 and in Type 1 fonts to 1000.
Outline character Rasterized character
UPM PPM
So, to scale a font to render at a specific point size on a device with a
specific resolution we take the resolution and point size to calculate the
PPM value. Then we scale all the outline characters by multiplying by the
PPM/UPM coefficient.