Editing TrueType (section)

== Hinting language ==
TrueType systems include a [[virtual machine]] that executes programs inside the font, processing the "[[Font hinting|hints]]" of the [[glyph]]s, in TrueType called “instructions”. These distort the control points which define the outline, with the intention that the rasterizer produce fewer undesirable features on the glyph. Each glyph's instruction set takes account of the size (in pixels) at which the glyph is to be displayed, as well as other less important factors of the display environment.

Although incapable of receiving input and producing output as normally understood in programming, the TrueType instruction language does offer the other prerequisites of programming languages: [[conditional branch]]ing (IF statements), [[Control flow#Loops|looping]] an arbitrary number of times (FOR- and WHILE-type statements), variables (although these are simply numbered slots in an area of memory reserved by the font), and encapsulation of code into functions. Special instructions called delta instructions are the lowest level control, moving a control point at just one pixel size.

The hallmark of effective TrueType glyph programming techniques is that it does as much as possible using variables defined just once in the whole font (e.g., stem widths, [[cap height]], [[x-height]]). This means avoiding delta instructions as much as possible. This helps the font developer to make major changes (e.g., the point at which the entire font's main stems jump from 1 to 2 pixels wide) most of the way through development.

Creating a very well-instructed TrueType font remains a significant amount of work, despite the increased user-friendliness of programs for adding instructions to fonts. Many TrueType fonts therefore have only rudimentary instructions, or have them automatically applied by the font editor, with results of various quality.