Choosing a scan conversion setting

One of the key decisions to be made in instructing a TrueType font is the choice of scan conversion mode. Font designers can choose between a fast scan conversion mode and a dropout control scan conversion mode. This choice is made by setting the value of the Graphics State variable scan_control. The interpreter considers each of three conditions in determining whether dropout control mode will be used:

Is the glyph rotated?

Is the glyph stretched?

Is the current setting for ppem less than a specified ppem value?

It is also possible to turn dropout control off completely.