# FreeType Glyph Conventions

## Version 2.1

### Copyright 1998-2000 David Turner (david@freetype.org)
Copyright 2000 The FreeType Development Team (devel@freetype.org)

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## IV. Kerning

The term kerning refers to specific information used to adjust the relative positions of coincident glyphs in a string of text. This section describes several types of kerning information, as well as the way to process them when performing text layout.

### 1. Kerning pairs

Kerning consists of modifying the spacing between two successive glyphs according to their outlines. For example, a "T" and a "y" can be easily moved closer, as the top of the "y" fits nicely under the upper right bar of the "T".

When laying out text with only their standard widths, some consecutive glyphs seem a bit too close or too distant. For example, the space between the "A" and the "V" in the following word seems a little wider than needed.

Compare this to the same word, where the distance between these two letters has been slightly reduced:

As you can see, this adjustment can make a great difference. Some font faces thus include a table containing kerning distances for a set of given glyph pairs for text layout.

- The pairs are ordered, i.e., the space for pair (A,V) isn't necessarily the space for pair (V,A). They also index glyphs, and not characters.

- Kerning distances can be expressed in horizontal or vertical directions, depending on layout and/or script. For example, some horizontal layouts like Arabic can make use of vertical kerning adjustments between successive glyphs. A vertical script can have vertical kerning distances.

- Kerning distances are expressed in grid units. They are usually oriented in the X axis, which means that a negative value indicates that two glyphs must be set closer in a horizontal layout.

### 2. Applying kerning

Applying kerning when rendering text is a rather easy process. It merely consists in adding the scaled kern distance to the pen position before writing each next glyph. However, the typographically correct renderer must take a few more details in consideration.

The "sliding dot" problem is a good example: Many font faces include a kerning distance between capital letters like "T" or "F" and a following dot ("."), in order to slide the latter glyph just right to their main leg:

This sometimes requires additional adjustments between the dot and the letter following it, depending on the shapes of the enclosing letters. When applying "standard" kerning adjustments, the previous sentence would become:

This is clearly too contracted. The solution here, as exhibited in the first example, is to only slide the dots when possible. Of course, this requires a certain knowledge of the text's meaning. The above adjustments would not necessarily be welcome if we were rendering the final dot of a given paragraph.This is only one example, and there are many others showing that a real typographer is needed to layout text properly. If not available, some kind of user interaction or tagging of the text could be used to specify some adjustments, but in all cases, this requires some support in applications and text libraries.

For more mundane and common uses, however, we can have a very simple algorithm, which avoids the sliding dot problem, and others, though not producing optimal results. It can be seen as

1. Place the first glyph on the baseline.
2. Save the location of the pen position/origin in `pen1`.
3. Adjust the pen position with the kerning distance between the first and second glyph.
4. Place the second glyph and compute the next pen position/origin in `pen2`.
5. Use `pen1` as the next pen position if it is beyond `pen2`, use `pen2` otherwise.

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