## ZPB-TTF - TrueType font file access for Common Lisp

> ### Abstract

> TrueType fonts have a publicly documented file format. ZPB-TTF is a TrueType file parser that provides an interface for reading typographic metrics, glyph outlines, and other information from the file. It is available under a BSD-like license. The latest version is 1.0.6, released on February 12th, 2023.

> TrueType is a registered trademark of Apple Computer, Inc.

> Download shortcut: http://www.xach.com/lisp/zpb-ttf.tgz

### Contents

1. Limitations
2. Overview
3. Glyph Example
4. The ZPB-TTF Dictionary
   1. `open-font-loader`
   2. `close-font-loader`
   3. `with-font-loader`
   4. `glyph-count`
   5. `collection-font-count`
   6. `collection-font-index`
   7. `name-entry-value`
   8. `find-name-entry`
   9. `value`
   10. `italic-angle`
   11. `underline-thickness`
   12. `underline-position`
   13. `fixed-pitch-p`
   14. `units/em`
   15. `ascender`
   16. `descender`
   17. `line-gap`
   18. `postscript-name`
   19. `full-name`
   20. `family-name`
   21. `subfamily-name`
   22. `all-kerning-pairs`
   23. `glyph-exists-p`
   24. `index-glyph`
   25. `find-glyph`
   26. `bounding-box`
   27. `xmin`
   28. `ymin`
   29. `xmax`
   30. `ymax`
   31. `x`
   32. `y`
   33. `on-curve-p`
   34. `contour-count`
   35. `contour`
   36. `contours`
   37. `do-contours`
   38. `explicit-contour-points`
   39. `do-contour-segments`
   40. `do-contour-segments*`
   41. `code-point`
   42. `font-index`
   43. `advance-width`
   44. `left-side-bearing`
   45. `right-side-bearing`
   46. `kerning-offset`
   47. `string-bounding-box`
5. Feedback

### Limitations

ZPB-TTF has the following limitations:

- is not a rasterizer; it will not produce bitmaps
- is not a scaler; all units are unscaled values directly from the font file
- only works with fonts that have a Unicode character map
- has been tested on a limited number of fonts

### Overview

Font files are loaded by creating font loaders. A font loader has information that applies to the font as a whole, such as its overall bounding box and other typographic metrics, the number of glyphs it contains, and a character map that can be used to convert a character set index into a glyph index meaningful to the font loader.

A font loader can also be used to create and find glyphs. A glyph object has information about an individual glyph such as typographic metrics and its outline.

Glyph outlines are made up of arrays of control points. An array of control points is called a contour. Contours define the shape of the glyph. For example, in most fonts, the glyph for the character `#\I` has one contour defining its outline. The glyph for the character `#\O` has two contours: one for the outer edge, and one for the inner edge.

This documentation is not meant to be a substitute for the TrueType file format documentation. For file format details, see the specifications from either Apple or Microsoft.

### Glyph Example

This image is a rendering of the Greek small letter lambda (U+03BB) from the Times New Roman TrueType font. The visualization was programmed by Frederic Jolliton and rendered with CL-VECTORS.

The dark gray grid lines represent the axes of the glyph coordinate system. Light grid lines are drawn at intervals of 256 em-square units.

Black squares represent the control points returned from `CONTOUR`. Solid black squares are points on the outline (`ON-CURVE-P` returns true), and hollow black squares are not on the outline. Solid red squares represent implicit control points on the glyph outline; they are not in the original contour, but they would be returned in a new contour created with `EXPLICIT-CONTOUR-POINTS`.

The alternating blue and green portions of the glyph outline represent the separate segments whose start, control, and end points were bound with `DO-CONTOUR-SEGMENTS`.

### The ZPB-TTF Dictionary

[Function]
open-font-loader font-file-designator `&key` (collection-index 0) => font-loader

> Creates and returns a font-loader object from font-file-designator, which should be either a pathname, pathname namestring, a stream, or a font-loader object. If font-file-designator is a TrueType Collection, collection-index specifies the index of the font to load from the collection.

[Function]
close-font-loader font-loader =>

> Closes any open resources held by font-loader.

[Macro]
with-font-loader (font-loader font-loader-designator `&key` (collection-index 0)) `&body` body =>

> Performs body with font-loader bound to a font-loader object created as if with `(open-font-loader` font-loader-designator). Automatically closes the font-loader when finished.

[Function]
glyph-count font-loader => number

> Returns the number of glyphs available in font-loader.

[Function]
collection-font-count font-loader => number

> If font-loader was loaded from a TrueType Collection, returns the number of fonts available in the collection, otherwise returns NIL.

[Function]
collection-font-index font-loader => number

> If font-loader was loaded from a TrueType Collection, returns the index of the loaded font in the collection, otherwise returns NIL.

[Function]
name-entry-value name-designator font-loader => string

> Returns an entry from the TrueType "name" table, which contains human-readable values for copyright info, trademark notices, creator names, and the like.

> name-designator may be either an integer ID from the table of NameIDs in the TrueType "name" table specification, or a keyword designating such an ID. Valid keywords and their IDs are:

> Keyword | ID
> --- | ---
> `:COPYRIGHT-NOTICE` | 0
> `:FONT-FAMILY` | 1
> `:FONT-SUBFAMILY` | 2
> `:UNIQUE-SUBFAMILY` | 3
> `:FULL-NAME` | 4
> `:NAME-TABLE-VERSION` | 5
> `:POSTSCRIPT-NAME` | 6
> `:TRADEMARK-NOTICE` | 7
> `:MANUFACTURER-NAME` | 8
> `:DESIGNER` | 9
> `:DESCRIPTION` | 10
> `:VENDOR-URL` | 11
> `:DESIGNER-URL` | 12
> `:LICENSE-DESCRIPTION` | 13
> `:LICENCE-INFO-URL` | 14
> `:RESERVED` | 15
> `:PREFERRED-FAMILY` | 16
> `:PREFERRED-SUBFAMILY` | 17
> `:COMPATIBLE-FULL` | 18
> `:SAMPLE-TEXT` | 19

> If the font does not provide any "name" table entry for the given name-designator, returns `NIL`.

[Function]
find-name-entry platform-id language-id name-id font-loader => name-entry

> This is the low-level interface used by `NAME-ENTRY-VALUE`. platform-id, language-id, and name-id should be integer ID values from the TrueType "name" table specification. If the combination of IDs is found in the name table, a name-entry object is returned, otherwise `NIL` is returned.

[Function]
value name-entry => string

> Returns the string value of a name-entry object.

[Function]
italic-angle font-loader => number

> Returns the typographic italic angle of font-loader.

[Function]
underline-thickness font-loader => number

> Returns the typographic underline thickness of font-loader.

[Function]
underline-position font-loader => number

> Returns the typographic underline position of font-loader.

[Function]
fixed-pitch-p font-loader => boolean

> Returns true if font-loader is fixed pitch (all glyphs have the same width).

[Function]
units/em font-loader => number

> Return the number of units in the typographic em-square of font-loader.

[Function]
ascender font-loader => number

> Returns the typographic ascender value of font-loader.

[Function]
descender font-loader => number

> Returns the typographic descender value of font-loader.

[Function]
line-gap font-loader => number

> Returns the typographic line gap of font-loader.

[Generic function]
postscript-name object => string

> Returns the Postscript name of object, which may be a glyph or a font-loader.

[Function]
full-name font-loader => string

> Returns the full name of font-loader.

[Function]
family-name font-loader => string

> Returns the family name of font-loader.

[Function]
subfamily-name font-loader => string

> Returns the subfamily name of font-loader.

[Function]
all-kerning-pairs font-loader => list

> Returns a list of all the kerning pairs available in font-loader. Each element of the list is itself a list, with three elements: a left glyph, a right glyph, and a numeric kerning offset.

[Function]
glyph-exists-p character-designator font-loader => boolean

> Returns true if font-loader has glyph data for character-designator.

[Function]
index-glyph index font-loader => glyph

> Returns the glyph at index in font-loader.

[Function]
find-glyph character-designator font-loader => glyph

> Returns the glyph for character-designator in font-loader. If character-designator is an integer, it is treated as a Unicode code-point and the corresponding glyph is fetched from the font. If character-designator is a character, its `char-code` is used as a Unicode code-point for lookup.
> Note: If the `char-code`s of characters the Lisp implementation do not correspond to Unicode (or a subset), this may not return the expected glyph.

> If font-loader does not contain any glyphs for character-designator, the "missing glyph" glyph is returned. To test for the existence of a glyph without returning the "missing glyph" glyph, use `GLYPH-EXISTS-P`.

[Generic function]
bounding-box object => #(xmin ymin xmax ymax)

> Returns the bounding box of object, which may be a glyph or a font-loader.

[Generic functions]
xmin object => value
ymin object => value
xmax object => value
ymax object => value

> Returns the horizontal and vertical extreme values for object, which may be a glyph or font-loader. If object is a four-element vector, each function is an accessor to the appropriate entry in the vector.

[Function]
x control-point => number
y control-point => number

> Returns the respective coordinate of control-point.

[Function]
on-curve-p control-point => boolean

> Returns true if control-point is on the contour outline.

[Function]
contour-count glyph => number

> Returns the number of contours of glyph. Some glyphs, such as the glyph for the Space character, may have zero contours.

[Function]
contour glyph index => vector

> Returns the indexth contour from the contours of glyph. A contour is represented as a vector of control point objects. Each control point may be on (touching) or off (not touching) the the contour outline.

> TrueType files may store contours that have implicit control points. When two consecutive control points are off the contour outline, there is an implied control point at the midpoint between them on the contour curve.

[Function]
contours glyph => vector

> Returns all contours of glyph as a vector.

[Macro]
do-contours (contour glyph `&optional` result) `&body` body => result

> Performs body for each contour in glyph, with the contour bound to contour. Returns result (evaluated) at the end of processing.

[Function]
explicit-contour-points contour => vector

> Returns a vector of points that is the same as contour with the implicit contour points inserted.

[Macro]
do-contour-segments (start control end) contour `&body` body =>

> For convenience, a TrueType contour may be thought of as being made up of connected segments. Each segment is either a straight line from one point on the outline to another, or a curved line that starts with a point on the outline, is controlled by a point off the outline, and ends with a point on the outline.

> `DO-CONTOUR-SEGMENTS` performs body for each segment of contour, with the start, control, and end variables (not evaluated) bound to the start, control, and end points of the segment. If the segment is a straight line, control is bound to `NIL`.

> Contours of a glyph in a TrueType file may contain implied control points; two consecutive points off the outline may be treated as if there is there is an point on the outline at the midpoint between them. `DO-CONTOUR-SEGMENTS` synthesizes and binds any implicit points as needed when going through the contour segments. (See the Glyph Example for a visual explanation.)

> Segments are not a TrueType glyph concept. The term is used for convenience by ZPB-TTF.

[Macro]
do-contour-segments* (control end) contour `&body` body =>

> `DO-CONTOUR-SEGMENTS*` is like `DO-CONTOUR-SEGMENTS`, except it does not bind a start variable.

[Function]
code-point glyph => number

> Returns the Unicode code point of a character glyph represents. Note: Since font glyphs may represent more than one character, this value may not be exactly what you expect. Note: Since glyphs in a font do not necessarily correspond to a code point, or to a single code point, this may not always return a meaningful value.

[Function]
font-index glyph => number

> Returns the integer index of glyph in its font loader.

[Function]
advance-width glyph => number

> Returns the typographic advance width of glyph.

[Function]
left-side-bearing glyph => number

> Returns the typographic left side bearing of glyph.

[Function]
right-side-bearing glyph => number

> Returns the typographic right side bearing of glyph.

[Function]
kerning-offset left-glyph right-glyph font-loader => number

> Returns the typographic kerning adjustment needed for the advance width when left-glyph and right-glyph appear next to each other.

[Function]
string-bounding-box string font-loader `&key` (kerning `t`) => #(xmin ymin xmax ymax)

> Returns the bounding box for string in the natural unit size of font-loader. If kerning is true, the inter-glyph spacing will take kerning offsets into account when calculating the bounding box.

### Feedback

Please send bug reports, patches, questions, and any other feedback to Zachary Beane.