Guide Lines

The characters in a font are usually based on a common set of heights or widths. The upper case characters, for example, extend from the base line to one particular height, the so-called cap height. These heights are called horizontal guide lines (the lines painted at these heights are horizontal, even though they denote vertical values), and the widths are called vertical guide lines.

The letter 'E', for instance, is typically straight at its top and its bottom. It begins then exactly at the base line and ends exactly at the cap height. The letter 'O', on the other hand, is rounded at its bottom or top. Making it begin exactly at the base line, like the letter 'E', will cause the impression that the letter 'O' base is higher than the base of 'E'. An optical correction is needed to overcome this visual impression. The letter 'O' must thus begin below the base line, and, for the same reason, end above the cap height. The letter 'O' is said to have a base and a cap height overhang. Each horizontal guide line has thus a corresponding overhang zone. This overhang zone will contain the overhangs of all the characters in the font for this guide line. An example is shown below.

Guide lines and overhangs
Guide lines and overhangs

Note that, usually, the base line overhang zone will be situated below the base line, and the cap height overhang zone will be situated above the cap height.

Similarly to common heights, certain bearings are common to characters in a font, such as the left offset of a typical character from the base point. Note that there are no overhang zones for vertical guide lines.

TYPO provides the possibility to define, for each font, up to ten horizontal and up to ten vertical guide lines. The horizontal guide lines are always relative to the base point. The vertical guide lines can be chosen relative to the base point or relative to the base point of the next character (base point+width). Vertical guide lines of the second kind can, for example, be used to specify the typical right bearing in the font. The vertical guide lines can also be slanted by an angle between -45 and 45 degrees. This is, for instance, useful for the design of Italic characters.

A guide line can either be nameless, purely to assist the font designer, or it can represent a width or height meaningful to the TYPO system. The following table gives an overview of the heights and widths known to TYPO (heights are measured from the base line upward and depths from the base line downward).

Body height
The height of the character box
The height of characters with ascenders
Cap height
The height of capital letters
Digit height
The height of digits
The height of lower case characters without ascenders
The depth of the underline position
The depth of characters with descenders
Body depth
The depth of the character box
Left bearing
The horizontal offset of an average character from the base point
Right bearing
The horizontal offset of an average character from the next base point
A horizontal or vertical offset from the base point or next base point which may be marked by the user with a single letter

Guide line visibility is determined by the user with the File->Options command.

Note that the guide lines, being common to all the characters in a font, are font properties and not character properties. Creating or modifying guide lines will thus make a change to the font but not to the current character.

Creating, Modifying, and Deleting Guide Lines

Guide Lines are mainly manipulated with the guide line tools in the toolbox. There is one tool to maniplate guide lines, the Guide Line Tool, and one tool to manipulate guide line overhang zones, the Guide Overhang Tool. These are located in the same toolbox entry, one behind the other.

The Guide Line Tool

For creating new guide lines, the Guide Line Tool must be chosen from the toolbox. New horizontal guide lines are created by dragging them out of the base line. New vertical guide lines relative to the base point (i.e. to the left side of the character box) are dragged out of the vertical base line (marked with Ba). New vertical guide lines relative to the base point+width (i.e. to the right side of the character box) are dragged out of the width line (marked with Wi).

Note that a current character must be loaded, and it must belong to the current font.

Typically, the left bearing (the distance of the left end of characters from their base point) is described with a vertical line relative to the base point, whereas the right bearing (the distance of the right end of characters to the base point of the following character) is described with a vertical line relative to (base point+width). Throughout the guide line commands, each vertical guide line is shown with an an arrow indicating whether this guide line is relative to the base point or to (base point+width).

To modify a guide line, drag it as required. Note that base lines and width lines cannot be dragged, as this would create a new guide line instead. To change their values, use the Transform->Shift command and the various Metrics commands.

Guide lines may be deleted by dragging them into the line from which they were originally dragged out of at their creation, i.e., the base line for horizontal guide lines, and the vertical base line or width line for vertical guide lines.

The Guide Overhang Tool

To create a non-zero overhang of a guide line, choose the Guide Overhang Tool from the toolbox. It is in the same field as the Guide Line Tool entry.

Initially, each overhang zone is of size zero, i.e. it does not extend beyond its guide line. To define a nonzero overhang, drag it out of its guide line.

To change an overhang, drag it as required.

An overhang zone gets deleted if it is made coincident with (dragged into) its guide line.

Setting the Italics Angle

The vertical guide lines may be slanted at an angle between -45 and 45 degrees, to handle non-vertical fonts, such as Italic or oblique ones. This is achieved with the Transform->Skew command. The user is presented with a slider in the input pop-up window. An angle can be chosen on this slider, in 0.25 degree increments, or (choosing Fine), in 0.00125 degree increments. The angle is reset to zero with CONTROL-SHIFT-Left mouse button.

Guide Line Properties

To change or view the properties of a guide line, such as its type or name, use the Special->Curve Properties command in the Special menu. You must be in guide line mode, and the guide line must be selected before issuing this command. Note that this command is not applicable for the base line or the width line.