Xft(3)

NAME
Xft - X FreeType interface library

DESCRIPTION
Xft is a simple library which draws text and graphics: using information provided by the Fontconfig library, Xft converts font glyphs using the FreeType rasterizer, and displays the converted font data using the X Rendering Extension.

This manual page barely scratches the surface of this library.

HEADER FILE
#include <X11/Xft/Xft.h>

CONSTANTS

XFT_MAJOR
    is the major version number of Xft.

XFT_MINOR
    is the minor version number of Xft.

XFT_REVISION
    is the revision number of Xft.

XFT_VERSION
    is XFT_MAJOR times 10000 (ten thousand), plus XFT_MINOR times 100, plus XFT_REVISION.

XftVersion
    is an alias for XFT_VERSION.

The following example illustrates how Xft's version constants might be used:
    #if (XFT_VERSION >= 20107)
    (void) puts("Version 2.1.7 or later of the Xft library is in"
                " use.");
    #else
    (void) printf("Insufficient version of Xft (%d.%d.%d) installed;
                  " need at least version 2.1.7.\n", XFT_MAJOR,
                  XFT_MINOR,
                  XFT_REVISION);
    #endif

DATA TYPES
Xft datatypes follow a naming convention, prefixing all names with “Xft”

  From Xlib...
Xlib datatypes do not follow a naming convention. They are documented in Xlib - C Language Interface.

Xft uses these names: Bool, Colormap, Display, Drawable, Pixmap, Region, Visual, and XRectangle.

Some datatypes are especially relevant to Xft:

Drawable
    Declared in <X11/X.h>, this is an X resource ID, e.g., a window associated with a display. Both windows and pixmaps can be used as sources and destinations in graphics operations. These windows and pixmaps are collectively known as drawables. However, an InputOnly window cannot be used as a source or destination in a graphics operation.

Region
    Declared in <X11/Xutil.h> and <X11/Xregion.h>, a Region is an arbitrary set of pixel locations which Xlib can manipulate. Internally, a Region is represented by the union of an arbitrary number of rectangles. Xlib maintains a rectangle which tells it the extent of this union.

Visual
    Declared in <X11/Xutil.h>, this structure contains information about the possible color mapping.

  From XRender...
The X Render Extension library datatypes are inspired by Xlib, but lack the corresponding documentation. Refer to the header file for details:

    <X11/extensions/Xrender.h>

or read the source code (which provides an outline of libXrender).

Xft uses these names: Picture, XGlyphInfo, XRenderColor.

  From Fontconfig...
Fontconfig datatypes follow a naming convention, prefixing all names with “Fc” which are documented in Fontconfig Developers Reference.

Xft uses these Fontconfig names: FcBool, FcChar8, FcChar16, FcChar32, FcCharSet, FcEndian, FcFontSet, FcPattern, and FcResult.

  From FreeType 2...
FreeType 2 datatypes follow a naming convention, prefixing all names with “FT_” which are documented in FreeType API Reference.

Xft uses these names: FT_Face, FT_Library, and FT_UInt.

FreeType 2 uses FT_UInt (an unsigned integer) to represent glyphs.

  XftFont
typedef struct _XftFont {
    int     ascent;
    int     descent;
    int     height;
    int     max_advance_width;
    FcCharSet    *charset;
    FcPattern    *pattern;
} XftFont;

An XftFont is the primary data structure of interest to programmers using Xft; it contains general font metrics and pointers to the Fontconfig character set and pattern associated with the font.

XftFonts are populated with any of XftFontOpen(), XftFontOpenName(), XftFontOpenXlfd(), XftFontOpenInfo(), or XftFontOpenPattern(). XftFontCopy() is used to duplicate XftFonts, and XftFontClose() is used to mark an XftFont as unused. XftFonts are internally allocated, reference-counted, and freed by Xft; the programmer does not ordinarily need to allocate or free storage for them.

XftDrawGlyphs(), the XftDrawString*() family, XftDrawCharSpec(), and XftDrawGlyphSpec() use XftFonts to render text to an XftDraw object, which may correspond to either a core X drawable or an X Rendering Extension drawable.

XftGlyphExtents() and the XftTextExtents*() family are used to determine the extents (maximum dimensions) of an XftFont.

An XftFont's glyph or character coverage can be determined with XftFontCheckGlyph() or XftCharExists(). XftCharIndex() returns the XftFont-specific character index corresponding to a given Unicode codepoint.

XftGlyphRender(), XftGlyphSpecRender(), XftCharSpecRender(), and the XftTextRender*() family use XftFonts to draw into X Rendering Extension Picture structures.

Note: XftDrawGlyphs(), the XftDrawString*() family, XftDrawCharSpec(), and XftDrawGlyphSpec() provide a means of rendering fonts that is independent of the availability of the X Rendering Extension on the X server.

  XftFontInfo
is an opaque object that stores information about a font. XftFontInfo structures are created with XftFontInfoCreate(), freed with XftFontInfoDestroy(), and compared with XftFontInfoEqual(). XftFontInfo objects are internally allocated and freed by Xft; the programmer does not ordinarily need to allocate or free storage for them.

Each XftFontInfo structure in use is associated with a unique identifier, which can be retrieved with XftFontInfoHash(). An XftFont can be opened based on XftFontInfo data with XftFontOpenInfo().

  XftColor
typedef struct _XftColor {
    unsigned long     pixel;
    XRenderColor     color;
} XftColor;

An XftColor object permits text and other items to be rendered in a particular color (or the closest approximation offered by the X visual in use).

XftColorAllocName() and XftColorAllocValue() request a color allocation from the X server (if necessary) and initialize the members of XftColor. XftColorFree() instructs the X server to free the color currently allocated for an XftColor.

Once an XftColor has been initialized, XftDrawSrcPicture(), XftDrawGlyphs(), the XftDrawString*() family, XftDrawCharSpec(), XftDrawCharFontSpec(), XftDrawGlyphSpec(), XftDrawGlyphFontSpec(), and XftDrawRect() may be used to draw various objects using it.

  XftDraw
is an opaque object which holds information used to render to an X drawable using either the core protocol or the X Rendering extension.

XftDraw objects are created with any of XftDrawCreate() (which associates an XftDraw with an existing X drawable), XftDrawCreateBitmap(), or XftDrawCreateAlpha(), and destroyed with XftDrawDestroy(). The X drawable associated with an XftDraw can be changed with XftDrawChange(). XftDraw objects are internally allocated and freed by Xft; the programmer does not ordinarily need to allocate or free storage for them.

The X Display, Drawable, Colormap, and Visual properties of an XftDraw can be queried with XftDrawDisplay(), XftDrawDrawable(), XftDrawColormap(), and XftDrawVisual(), respectively.

Several functions use XftDraw objects: XftDrawCharFontSpec(), XftDrawCharSpec(), XftDrawGlyphFontSpec(), XftDrawGlyphSpec(), XftDrawGlyphs(), XftDrawRect(), XftDrawSetClip(), XftDrawSetClipRectangles(), XftDrawSetSubwindowMode(), and the XftDrawString*() family.

The X Rendering Extension Picture associated with an XftDraw is returned by XftDrawPicture(), and XftDrawSrcPicture(). It is used by XftCharFontSpecRender(), XftCharSpecRender(), XftGlyphFontSpecRender(), XftGlyphRender(), XftGlyphSpecRender(), and the XftTextRender*() family.

  XftCharSpec
typedef struct _XftCharSpec {
    FcChar32     ucs4;
    short     x;
    short     y;
} XftCharSpec;

XftCharSpec is used by XftDrawCharSpec(), and XftCharSpecRender().

  XftCharFontSpec
typedef struct _XftCharFontSpec {
    XftFont    *font;
    FcChar32     ucs4;
    short     x;
    short     y;
} XftCharFontSpec;

XftCharFontSpec is used by XftCharFontSpecRender().

  XftGlyphSpec
typedef struct _XftGlyphSpec {
    FT_UInt      glyph;
    short      x;
    short      y;
} XftGlyphSpec;

XftGlyphSpec is used by XftDrawGlyphSpec().

  XftGlyphFontSpec
typedef struct _XftGlyphFontSpec {
    XftFont    *font;
    FT_UInt     glyph;
    short     x;
    short     y;
} XftGlyphFontSpec;

XftGlyphFontSpec is used by XftDrawGlyphFontSpec(), and XftGlyphFontSpecRender().

FUNCTIONS

  Initialization
A typical application using Xft does not explicitly initialize the library. That is usually done as a side-effect of opening a font.

When Xft initializes, it collects information about the display, and stores some of that information in a Fontconfig pattern (essentially a collection of properties with typed values). The calling application can modify that pattern to change the library's behavior.

Not all of the collected information is stored in a pattern. The remainder is stored in internal data structures. Xft makes some of that available to the application via functions. Obtain information about the display dpy if not done already, and return true if Xft found that the display supports the X Render extension, and if it is able to find a suitable XRenderPictFormat (X Render's datatype which is analogous to Xlib's Visual) on the display. Obtain information about the display dpy if not done already, and set the Fontconfig pattern holding default properties which Xft will use for this display.

    Xft uses those properties initially to obtain these limits:

    XFT_MAX_GLYPH_MEMORY
        (maxglyphmemory). This is the maximum amount of glyph memory for all fonts used by Xft (default: 4*1024*1024).

    XFT_MAX_UNREF_FONTS
        (maxunreffonts). This is the maximum number of unreferenced fonts (default: 16).

    XFT_TRACK_MEM_USAGE
        (trackmemusage). When true, Xft tracks usage of glyph memory to improve performance when deciding which to unload when the maximum amount of glyph memory is reached (default: false).

    Xft also uses these default properties in XftDefaultSubstitute(). Xft fills in missing properties in the given pattern using default properties for the specified display dpy, e.g., as set in XftDefaultSet().

    Typical Xft applications use this function to help Fontconfig choose a suitable font. These properties are substituted before calling FcDefaultSubstitute():

    FC_ANTIALIAS
        True if FreeType should use antialiasing (default: False). (default: True).

    FC_AUTOHINT
        True if FreeType should use autohinting (default: False).

    FC_DPI
        Dots/inch used for resolution (default: computed from the display height).

    FC_EMBOLDEN
        True if FT_GlyphSlot_Embolden() should be used to embolden a font (default: False).

    FC_HINTING
        True if hinting should be used when filling in properties to open a font (default: True).

    FC_HINT_STYLE
        Hinting style used when filling in properties to open a font (default: FC_HINT_FULL).

    FC_LCD_FILTER
        Parameter passed to FT_Library_SetLcdFilter() when loading glyphs (default: FC_LCD_DEFAULT).

    FC_MINSPACE
        Minimum space value used when filling in properties to open a font (default: False).

    FC_RGBA
        RGBA value used when filling in properties to open a font (default: computed by calling XRenderQuerySubpixelOrder()).

    FC_SCALE
        Scale used in Fontconfig (default: 1.0).

    XFT_MAX_GLYPH_MEMORY
        Maximum memory for one font (default: 1024*1024).

    XFT_RENDER
        True if the display supports X Render extension (default: result from XftDefaultHasRender()).
Initializes the Fontconfig library (calling FcInit()).

    The config parameter is unused.

    Xft does not deinitialize the Fontconfig library when it is done. Initializes the FreeType library (calling FT_Init_FreeType() to create a library object) if it has not already been initialized. This is needed before using the FreeType library to read font data from a file.

    Xft calls XftInitFtLibrary() internally via XftFontInfoCreate() and XftFontOpenPattern().

    Xft does not discard the library object (e.g., using FT_Done_FreeType()) when it is done. Return XftVersion, enabling an application to determine the actual version of Xft which is in use.

  Opening and Matching Fonts
XftFontOpen takes a list of pattern element triples of the form field, type, value (terminated with a NULL), matches that pattern against the available fonts, and opens the matching font, sizing it correctly for screen number screen on display dpy. Return the matched font, or NULL if no match is found.

Example:
    font = XftFontOpen (dpy, screen,
                        XFT_FAMILY, XftTypeString, "charter",
                        XFT_SIZE, XftTypeDouble, 12.0,
                        NULL);

    This opens the “charter” font at 12 points. The point size is automatically converted to the correct pixel size based on the resolution of the monitor.

XftFontOpenName behaves as XftFontOpen does, except that it takes a Fontconfig pattern string (which is passed to the Fontconfig library's FcNameParse() function).

XftFontOpenXlfd behaves as XftFontOpen does, except that it takes a string containing an X Logical Font Description (XLFD), and uses the XftXlfdParse() function.

Also used internally by the XftFontOpen* functions, XftFontMatch can also be used directly to determine the Fontconfig font pattern resulting from an Xft font open request. XftXlfdParse parses the xlfd_orig parameter according to the X Logical Font Description Conventions document, but ignores some of the fields: setwidth_name, add_style_name, spacing, and average_width.

    XftXlfdParse creates a Fontconfig pattern, setting the property XFT_XLFD to the xlfd_orig value, and maps the collected information to Fontconfig properties. Empty or “*” fields are ignored:

    FC_FOUNDRY
        from foundry.

    FC_FAMILY
        from family.

    FC_WEIGHT
        weight_name, defaulting to FC_WEIGHT_MEDIUM.

    FC_SLANT
        from slant, defaulting to FC_SLANT_ROMAN.

    FC_SIZE
        from point_size.

    FC_PIXEL_SIZE
        from pixel_size. If point_size was set, as well as resolution_xand resolution_y, then the value is scaled convert the font's height to points.

  Families of Functions
Xft provides groups of drawing functions which differ according to the way the data is encoded, e.g., 8-bit (ISO-8859-1) versus UTF-8. Here is a summary of these related functions:
Encoding | XftDrawString* | XftTextExtents* | XftTextRender*
8 | XftDrawString8 | XftTextExtents8 | XftTextRender8
16 | XftDrawString16 | XftTextExtents16 | XftTextRender16
16BE | XftDrawString16 | XftTextExtents16 | XftTextRender16BE
16LE | XftDrawString16 | XftTextExtents16 | XftTextRender16LE
32 | XftDrawString32 | XftTextExtents32 | XftTextRender32
32BE | XftDrawString32 | XftTextExtents32 | XftTextRender32BE
32LE | XftDrawString32 | XftTextExtents32 | XftTextRender32LE
UTF-8 | XftDrawStringUtf8 | XftTextExtentsUtf8 | XftTextRenderUtf8
UTF-16 | XftDrawStringUtf16 | XftTextExtentsUtf16 | XftTextRenderUtf16

  Determining Text Extents
Xft provides several functions for determining the required height and width for displaying a text-string. After accounting for the offset, in cases where the string will be shifted up, down, left or right, these numbers are referred to as text extents. XftTextExtents8 computes the pixel extents on display dpy of no more than len glyphs of a string consisting of eight-bit characters when drawn with font, storing them in extents.

XftTextExtents16 computes the pixel extents on display dpy of no more than len glyphs of a string consisting of sixteen-bit characters when drawn with font, storing them in extents.

XftTextExtents32 computes the pixel extents on display dpy of no more than len glyphs of a string consisting of thirty-two-bit characters when drawn with font, storing them in extents.

XftTextExtentsUtf8 computes the pixel extents on display dpy of no more than len bytes of a UTF-8 encoded string when drawn with font, storing them in extents.

XftTextExtentsUtf16 computes the pixel extents on display dpy of no more than len bytes of a UTF-16LE- or UTF-16BE-encoded string when drawn with font, storing them in extents. The endianness of string must be specified in endian.

Also used internally by the XftTextExtents*() functions, XftGlyphExtents computes the pixel extents on display dpy of no more than nglyphs in the array glyphs drawn with font, storing them in extents.

    If any of the glyphs are missing (determined by a check with XftFontCheckGlyph()), the corresponding entry in extents is filled with zeroes.

  Managing XftColor
Use XAllocNamedColor() to look up the named color name for the screen associated with the colormap cmap.
    If XAllocNamedColor() returns nonzero, XftColorAllocName() fills in the resulting XftColor pixel field with the closest color supported by the screen, as well as the exact red, green and blue fields from the database, and returns True. If XAllocNamedColor() returns zero, XftColorAllocName() returns False, and does not update the XftColor referenced by result.

    The visual parameter is unused. Allocate a color value:
    If the visual class is TrueColor, XftColorAllocValue() sets the pixel field in the XftColor referenced by result using the red, green and blue fields from the color parameter. If the visual class is not TrueColor, XftColorAllocValue() calls XAllocColor() to allocate an entry in the colormap cmap. which returns the pixel value of the color closest to the specified RGB elements supported by the hardware.

        If XAllocColor() succeeds XftColorAllocValue() stores that pixel value in the XRenderColor referenced by result and returns True.

        If XAllocColor() fails, XftColorAllocValue() returns False and does not modify the result.
If the visual class is not TrueColor, Xft calls XFreeColors() to free the entry from the colormap cmap whose pixel value in the color parameter was allocated by XftColorAllocName().

  Managing XftDraw
XftDrawCreate creates a structure that can be used to render text and rectangles using the specified drawable, visual, and colormap on display.

XftDrawCreateBitmap behaves as XftDrawCreate(), except that it uses an X pixmap of color depth 1 instead of an X drawable.

XftDrawCreateAlpha behaves as XftDrawCreate(), except that it uses an X pixmap of color depth depth instead of an X drawable.

XftDrawChange changes the X drawable association of the existing Xft draw object draw from its current value to drawable.

XftDrawDisplay returns a pointer to the display associated with the Xft draw object draw.

XftDrawDrawable returns the X drawable associated with the Xft draw object draw.

XftDrawColormap() returns the colormap associated with the Xft draw object draw.

XftDrawVisual returns a pointer to the visual associated with the Xft draw object draw.

XftDrawPicture returns the picture associated with the Xft draw object draw.

    If the the X server does not support the X Rendering Extension, 0 is returned.

Return an X Render Picture object, which is used for rendering glyphs, e.g., with XftGlyphRender(), XftGlyphSpecRender(), or XftGlyphFontSpecRender(), by XftDrawGlyphs(), XftDrawGlyphSpec(), XftDrawGlyphFontSpec(), respectively.

    If the X server does not support the X Render extension, those functions use XftGlyphCore(), XftGlyphSpecCore(), or XftGlyphFontSpecCore(). XftDrawDestroy destroys draw (created by one of the XftDrawCreate*() functions) and frees the memory that was allocated for it. Set up clipping for the given XftDraw parameter draw starting with a Region:
    If the Region parameter r is not null, Xft creates a new Region (to copy the parameter), Xft destroys any existing clipping region. Xft sets the clip_type for the draw parameter to XftClipTypeRegion if the r parameter was not null. Otherwise it sets the clip_type to XftClipTypeNone. Finally, Xft updates clipping for existing objects, updates the clip_mask for its X Render Picture object and sets the clipping-mask in the graphic context (GC) associated with the XftDraw parameter.

    XftDrawSetClip() returns True if no change was necessary, or if the operation succeeded. It returns False if it was unable to create the new Region(). Like XftDrawSetClip(), XftDrawSetClipRectangles() sets up clipping for the given XftDraw parameter draw but uses a set of n rectangles (the rects parameter) which could be used to construct a Region .

    Xft sets the clip_type for draw to XftClipTypeRectangles and uses XSetClipRectangles() for core (X11) clipping and XRenderSetPictureClipRectangles() for X Render clipping. Sets the subwindow-mode for the given XftDraw parameter draw. The mode can be either ClipByChildren (the default), or IncludeInferiors:
    For ClipByChildren, both source and destination windows are additionally clipped by all viewable InputOutput children. For IncludeInferiors, neither source nor destination window is clipped by inferiors. This will result in including subwindow contents in the source and drawing through subwindow boundaries of the destination.

    In addition to the subwindow-mode maintained by Xft, it updates the subwindow mode for any associated graphics context GC using XSetSubwindowMode() as well as for an X Render Picture using XRenderChangePicture().

  Drawing Strings
XftDrawString8 draws no more than len glyphs of string to Xft drawable d using font in color at position x, y.

  Drawing Other Things
XftDrawRect draws a solid rectangle of the specified color, width, and height at position x, y to Xft drawable d. XftCharFontSpecRender() converts the chars parameter from XftCharFontSpec to XftGlyphFontSpec, passing the converted array along with the other parameters unchanged to XftGlyphFontSpecRender() to render the data. Draw a list of glyphs associated with fonts at specified coordinates, passed as an array of len XftGlyphFontSpec structures via the parameter glyphs. All of the glyphs are drawn using the color specified in the color parameter.

    For each entry in the glyphs array:
    If the associated font uses the X Render extension, then Xft uses XftGlyphFontSpecRender() to draw the glyph, using a Picture obtained from calling XftDrawSrcPicture () with the draw and color parameters. Otherwise, Xft provides an analogous feature using Xlib.
This involves several steps:
    First, Xft ensures that the glyphs array is complete using XftFontCheckGlyph() and loading any missing glyphs with XftFontLoadGlyphs(). Then Xft examines the glyphs to find the maximum Unicode value. That determines the encoding size, i.e., the width. Then, for each glyph, Xft checks if its Unicode value is handled by the corresponding font. If not, the value is set to zero (0), to ignore it. While doing this, Xft also remembers which was the first font used, and computes the position at which each glyph will be drawn. Xft then constructs an equivalent array of glyphs in the format expected by the X Render library. Finally, XftGlyphFontSpecRender() uses the X Render extension to draw the glyphs, with the appropriate XRenderCompositeText*() function according to the width, and discards temporary data.

        The op, dst, src, srcx, and srcy parameters are used as described in the documentation for the X Render library.

  Manipulating patterns
Xft uses FcPatternVapBuild() and FcObjectSetVapBuild() to process the variable-length parameter list, and FcFontList() to obtain a list of matching Fontconfig patterns, which it returns to the caller. The caller can dispose of the return value using FcPatternDestroy(). Xft uses Fontconfig to parse the name, passing the name to FcNameParse(), returning the result. Like XfgNameParse(), Xft uses Fontconfig. In this case, it uses FcNameUnparse(), which converts the pattern pat back into a string that can be parsed. XftNameUnparse() stores the result via the caller's pointer dest, but checks first if it will fit, using the len parameter. If it fits, Xft copies the string and returns FcTrue, otherwise it returns FcFalse.

  Manipulating Font data
Using FT_Load_Glyph(), load nglyphs for the glyph indices (Unicode values) listed in the array glyphs from the given font pub.

    Loading a glyph entails more than just reading data into memory. Xft uses the Fontconfig pattern associated with the display dpy (e.g., via XftDefaultSet()) to determine whether to use a bounding box, clip the glyphs into those bounds, scale the glyphs, compute font metrics, and add it to the X Render extension using XRenderAddGlyphs(). Discards data for up to nglyph glyphs whose glyph indices (Unicode values) are listed in the array glyphs for the given font pub. If the glyphs were added to the X Render extension, Xft removes those using XRenderFreeGlyphs(). Xft keeps track of the amount of memory used for glyphs, and updates the usage associated with the display dpy. If no FreeType 2 “face” (in-memory representation of a given typeface in a given style) has been created for the pub font, create one using FT_New_Face(). Face-locks are a reference count used by Xft to ensure that only one face is created, and that it is retained until the font is no longer used.

    Face-locking is used directly in XftCharIndex() and XftFontLoadGlyphs(), which in turn are used in many functions of Xft.

    Face-locking was introduced in version 1 (October 2002). A few applications, such as Gdk/Gimp relied upon these functions. In version 2.1.9 (June 2006), face-locking was retained as part of the public API when improved shared-library configurations provided for hiding private symbols. Decrements the reference count for the FreeType 2 “face” associated with the font.

    FT_Face objects are deallocated using FT_Done_Face(). Xft does this in XftFontInfoDestroy() and when cleaning up on failure in XftFontInfoCreate() and XftFontOpenPattern().

DEBUGGING
Xft reads the environment variable XFT_DEBUG and converts that to an integer. Each bit in the resulting value tells Xft to print debugging information to the standard output:

1   font-opening

2   additional font-matching and opening (verbose)

4   shows details about the XRenderPictFormat which will be used.

8   shows the string which XftDrawString8() will draw.

16  shows which font-ids are matched.

32  shows useful information about the glyphs which will be drawn

64  shows an ASCII-art representation of the glyphs.

128 shows details about the memory-cache management

256 shows details about managing glyph cached-memory

512 shows a report on memory-usage

1024
    shows details on extended management of glyph cached-memory

COMPATIBILITY
As of version 2 (May 2002), Xft became relatively stable. It is expected to retain source and binary compatibility in future releases.

Xft provides a compatibility interface to its previous major version, Xft 1.x, described below.

  Xft 1.x Header File
#include <X11/Xft/XftCompat.h>

  Xft 1.x Data Types

XftPattern
    holds a set of names with associated value lists; each name refers to a property of a font. XftPatterns are used as inputs to the matching code as well as holding information about specific fonts.

XftFontSet
    contains a list of XftPatterns. Internally, Xft uses this data structure to hold sets of fonts. Externally, Xft returns the results of listing fonts in this format.

XftObjectSet
    holds a set of names and is used to specify which fields from fonts are placed in the the list of returned patterns when listing fonts.

AUTHOR
Keith Packard
Thomas E. Dickey (performance improvements)

SEE ALSO
Fontconfig Developers Reference
FreeType API Reference
Xlib - C Language Interface
X Logical Font Description Conventions