Background

Interface fonts are designed by user interface experts for the narrow purpose of making the menus, labels and fields of a graphical user interface highly readable. They are usually finely hand-tuned bitmapped fonts, intended for use on visual displays only and not on printers, and many of the glyphs have been specially modified for this purpose. Interface fonts can be contrasted with application fonts, which are the fonts used within an application running on the XCDE desktop. Interface fonts come in a restricted set of styles and are used for short strings of text, whereas application fonts usually come in a variety of designs, styles and weights and are used for emphasis, cross-references, section headers, and so forth.

Rationale

Common font names are required to prevent XCDE clients such as dtterm(1) from needing different app-defaults files on each system. In addition, any X server or X terminal vendor may ensure that the XCDE desktop can run on their X server by mapping these standard names to fonts of the corresponding style on their individual X systems.

Interface fonts are needed because of user interface and cognitive research that has examined the readability of various fonts on the display screens in use today and found that many fine adjustments (for example, for centering, baseline, height and alignment) must be made to characters in a font to make them clear, distinguishable and consistent when used for the interface objects of a GUI. And by using hand-tuned interface fonts for the GUI objects, the desktop can achieve a very clean, crisp visual appearance.

Interface fonts are broken into 2 categories: system and user. Cognitive research has shown that this distinction is important for the usability and readability of GUIs. System fonts are those used when the system is presenting information to the user (for example, in buttons). User fonts are those used for text that a user enters into the system (for example, for a text field or terminal emulator).

XLFD Field Values for the Standard Interface Font Names

These standard names are available using the X Window System XLFD font naming scheme. There are three aspects to the standard names:

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The underlying font on each system, or X server platform, to which a standard name is mapped, typically will be different on each system.

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The standard name itself, a full XLFD name mapped to the underlying font, may be different on each system in some of the XLFD fields. However, most of the fields are the same from system to system, allowing the patterns (described next) to be the same.

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The font resource pattern containing the * wildcards, used in app-defaults files, which will match the full XLFD name of the standard name, is the same across all systems, for a given use in an app-defaults file.

Each XCDE or X server vendor implementing this specification must provide full XLFD names for the standard names, mapped to system-dependent underlying fonts, so that the XLFD patterns used in XCDE application app-defaults files will always match one of the full XLFD names provided.

The Standard Interface Font Names are identified by the presence of the following XLFD field name values:

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FOUNDRY is dt

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FAMILY_NAME is either interface system or interface user (there is a single space between the two words in each family name)

In addition, the other fields of the XLFD names defining the standard names are constrained as follows:

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WEIGHT_NAME is either medium or bold

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SLANT is always r

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SETWIDTH_NAME is always normal

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SPACING is p or m (it must be m for interface user fonts, and should be p for interface system fonts, although m is acceptable)

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ADD_STYLE_NAME contains both a nominal size value in the range xxs to xxl (see below), as well as either sans for sans serif fonts or serif for serif, if appropriate for the underlying font

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The numeric fields ( PIXEL_SIZE, POINT_SIZE, RESOLUTION_X, RESOLUTION_Y, and AVERAGE_WIDTH) must contain the same values as the underlying font.

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CHARSET_REGISTRY and CHARSET_ENCODING are not specified; the standard names may be implemented for any XCDE locale.

Although the sans and serif values in the ADD_STYLE_NAME field are not required by the XLFD font convention, they are always part of the XCDE Standard Font Names when the underlying fonts are characterized as serif or sans serif. However, this document imposes no restriction on whether the interface fonts are serif or sans serif. The relevant attribute must be coded in the ADD_STYLE_NAME field. Thus, for example, the standard names for Japanese fonts, which are not characterized as being serif or sans serif, would not include this designation in the ADD_STYLE_NAME field.

Restricted Set of Styles Available

Unlike the Standard Application Font Names, only a limited set of styles is available in the Standard Interface Font Names. The styles available represent the minimum set currently considered necessary for the desktop GUI needs:

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a medium weight of an interface system font, preferably proportionally spaced (but mono-spaced is acceptable if appropriate for the locale)

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a medium weight of an interface user font, always mono-spaced

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a bold weight of an interface user font, always mono-spaced (the standard font names for this generic typeface are recommended if available for the targeted fonts and locale, but are not required)

Named Set of Point Sizes Available

In addition, the set of seven point sizes for each of the three styles that are part of this document are ``named'' point sizes, using string values in the ADD_STYLE_NAME field. Thus, XLFD patterns matching these names match a size based on the named size, not on a numeric size, even though the latter does exist in the XLFD name. These named sizes are used because the exact size of an interface font is less important than its nominal size, and implementation differences for the hand-tuned interface fonts do not allow common numeric point sizes to be assured across systems. The seven nominal sizes are as follows:

xxs

extra extra small

xs

extra small

s

small

m

medium

l

large

xl

extra large

xxl

extra extra large

The goal of these named sizes is to provide enough fonts so that both the variety of display monitor sizes and resolutions that XCDE will run on, and the range of user preferences for comfortably reading button labels, window titles and so forth, can be accommodated in the GUI. Thus, both the smallest size, xxs, and the largest size, xxl, are meant to be reasonable sizes for displaying and viewing the XCDE desktop on common displays and X terminals; they are not meant to imply either hard-to-read fine print or headline-sized display type.

These named size values must occur first in the ADD_STYLE_NAME field, before any use of the values serif or sans (one of which is always required when the underlying font can be so characterized) and before any other additional stylistic attribute that might be appropriate. This is important when specifying wild-carded patterns in a resource specification for these fonts, since whether the underlying font these names are mapped to is serif or sans serif is not specified by XCDE, and the match must work for all XLFD names provided by XCDE system vendors or other X server vendors.

Example XLFD Patterns for the Standard Names

Using these values, the XLFD pattern

-dt-interface*-*

logically matches the full set of XCDE Standard Interface Font Names. (Note that no specific X server behavior is implied).

The full set of 21 XCDE Standard Interface Font Names can also be represented, in a more meaningful way, as follows:

-dt-interface system-medium-r-normal-*-*-*-*-*-*-*-iso8859-1
-dt-interface user-medium-r-normal-*-*-*-*-*-m-*-iso8859-1
-dt-interface user-bold-r-normal-*-*-*-*-*-m-*-iso8859-1

The full set of patterns, usable in app-defaults files, for all seven sizes for the system font, for example, is:

-dt-interface system-medium-r-normal-xxs*-*-*-*-*-*-*-iso8859-1
-dt-interface system-medium-r-normal-xs*-*-*-*-*-*-*-iso8859-1
-dt-interface system-medium-r-normal-s*-*-*-*-*-*-*-iso8859-1
-dt-interface system-medium-r-normal-m*-*-*-*-*-*-*-iso8859-1
-dt-interface system-medium-r-normal-l*-*-*-*-*-*-*-iso8859-1
-dt-interface system-medium-r-normal-xl*-*-*-*-*-*-*-iso8859-1
-dt-interface system-medium-r-normal-xxl*-*-*-*-*-*-*-iso8859-1

These patterns could be used in a resource file and will match the full XCDE Standard Interface Names for Latin-1 locales on all XCDE, or complying X server, systems.

Note in these wild-carded XLFD names that the ADD_STYLE_NAME field has a pattern, such as xxs*, and that the pattern is partly a string ( xxs) and partly the pattern-matching character *. The full XLFD name this pattern matches--the XLFD name implementing the Standard Interface name--will often contain sans or serif in the field, after the xxs and a space, and so the * is essential to match that sans or serif string (and any additional style attribute string that might be in the underlying name). Note also that the SPACING field is wild-carded in the pattern for the system font, since either p or m may appear in the standard name being matched.

Implementation of Font Names

Each XCDE system vendor and X server vendor provides mappings of its own fonts to XLFD names as described by this document. The actual XLFD names will vary from system to system, just as the fonts they are mapped to, since they contain some of the same values as the XLFD name of the underlying font. What does not vary is the behavior: the common patterns in which only specified fields are used will match each system's standard names. This is guaranteed by the field specifications given earlier.

There is no precise specification of how the named sizes xxs to xxl are mapped to sizes of underlying fonts in each system or X server product, although each size must be equal to or larger than the previous size. Nonetheless, some guidelines are appropriate.

Interface fonts have been developed because of human factors research on visual clarity of text on displays, and this has been done in the context of the display technology typically available today, mostly in the 100 dots per inch (DPI) range. That, and the use of standard point sizes (10, 12, 14, 18) in the graphics arts, have resulted in the development in the industry of hand-tuned bitmapped fonts for a set of ``pixel heights'' that are likely to be used for these standard names. However, making the XCDE desktop usable with a range of point sizes effectively means, in addition to legibility for the user, that the various XCDE applications fit ``appropriately'' on the screen using those point sizes. This means, for example, that two application windows can appear side by side on a typical display or that a certain number of buttons can appear across the screen.

Thus, these guidelines are expressed not only in pixel sizes, to reflect current usage, but also in percentage of monitor height. This allows them to remain appropriate as technological evolution improves display resolution and monitor size (for example, wall-mounted monitors). The ideal set of sizes would form a linear progression from the smallest ( xxs) to the largest ( xxl), although this is not achievable. The basic guideline is that the xxs font should be, in pixels, no less than 0.9% of the height of the display resolution, in pixels; the xxl font should be no more than 2.6% of the height.

As an approximate example that does not represent any existing mapping of fonts to a display, this table shows how the named sizes might map to real bitmapped fonts of a given pixel size, and how large those sizes are in percentage and point size terms:

Thus, the following requirements are placed on each implementation of the Standard Interface Font Names:

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The names must be fully specified XLFD names, without wild cards.

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The WEIGHT_NAME, SLANT, SETWIDTH_NAME, SPACING, CHARSET_REGISTRY and CHARSET_ENCODING fields must contain valid values as defined previously and must match those in the underlying font.

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The ADD_STYLE_NAME field must contain both a named size (for example, xxs) and, if appropriate, either the serif or sans designation, whichever matches the underlying font; any additional words about the style of the underlying font, if defined for the underlying font, must also be used. The named size must be first in the field, and must be separated from any following word in the field with a blank.

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The named sizes xxs through xxl must be mapped to fonts that are progressively larger than or equal to the previous one in the list. Thus, several standard names with adjacent sizes (for example, xxs and xs) may be mapped to the same font (for example, if there is not enough variety in sizes in the underlying fonts).

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The implemented names should attempt to meet the guidelines discussed in the previous paragraph and table.

For example, system A is assumed to be using the following sans serif font for the extra small system font:

-bitstream-swiss-medium-r-normal--11-90-85-85-p-81-iso8859-1

System B is using the following serif font for the extra small system font:

-vendorb-ersatz-medium-r-normal-Expert-8-80-75-75-m-72-iso8859-1

Their respective standard names would be implemented on their systems as:

-dt-interface system-medium-r-normal-xssans-11-90-85-85-p-81-iso8859-1
-dt-interface system-medium-r-normal-xsserif Expert-8-80-75-75-m-72-iso8859-1

Defined this way, both names will match the single XLFD pattern used in a common app-defaults file:

-dt-interface system-medium-r-normal-xs*-*-*-*-*-*-*-iso8859-1

Default XCDE Mapping of the Standard Interface Font Names

There is no default mapping of these interface names to X11R5 fonts; the mapping is implementation-specific.