Introduction and Overview
The FontMap module provides two functions:
- A means by which font names from other systems can be converted to RISC OS font names used by FontManager
- A means by which a font's properties can be returned and manipulated
The FontMap module allows conversion of the names os fonts from Windows (which have generally been converged upon). These font mappings are intended to be extended and configurable on a given system.
The font properties which the FontMap module manages are those that can be derived from the font name - the weight, obliqueness, and condensedness features.
Technical details
Font naming systems
The FontMap module handles two naming systems, but has scope to be expanded with more in the future. The module specifically handles the RISC OS naming system, and the Windows naming system. Although the system is called 'Windows', it is actually a more widely adopted naming scheme due to the specification of fonts on websites and the availability of fonts.
There is another naming system which is not supported by FontMap at present - the PostScript font naming system. This is partially catered for by the Printer driver system, so is not explicitly handled in the FontMap module.
The font naming systems use the following values:
| Value | Meaning |
|---|---|
| 0 | RISC OS |
| 1 | Windows |
| 2-7 | Reserved |
Font naming components
Fonts consist of four major features which can be recognised by the FontMap module:
- The font family name, for example 'Homerton' or 'Helvetica'.
- The weight of the font, for example 'Bold', or 'Light'.
- The obliqueness of the font, which is commonly given as 'Oblique' or 'Italic'.
- The stretchedness of the font, which is commonly given as 'Condensed' or 'Expanded'.
The properties recognised here take their inspiration from the SVG 'font-face' element, which is itself based on the CSS 2 '@font-face' facility.
Font family
The font family generally groups fonts which have the stylistic design. How characters are formed, and how they relate to one another is what distinguishes the families of fonts. The FontMap module can perform a mapping between these font families where they are sufficiently similar. This is necessary because RISC OS does not use the same standardised names for its fonts as are used elsewhere.
For example, the RISC OS font family has similar stylistic design to the font used elsewhere as 'Helvetica'. The FontMap module allows a translation between these font names to produce an approximation of the original intent. This translation is (commonly) bidirectional, so that as well as importing content which uses the foreign naming schemes to RISC OS, the RISC OS font names can be translated to the equivalent foreign font names.
Font weight
The weight of a font indicates the relative darkness or heaviness of the text. There is no standardised naming scheme for the weights of gonts, but there are commonly used weight names that are used in a self-consistent continuum within a family.
The FontMap module assigns these weights values which can be compared and used to understand the relationship between different fonts within the family.
| Value | Meaning |
|---|---|
| 0 | Don't care - a value that can be requested when any suitable weight is accepted |
| 100 | Thin |
| 200 | Extra light |
| 300 | Light |
| 400 | Regular |
| 500 | Medium |
| 600 | Semi-bold |
| 700 | Bold |
| 800 | Extra bold |
| 900 | Black |
Font obliqueness
The obliqueness of a font indicates whether the font is set at an angle (which is usually used for emphasis), or not. Within the FontMap module this is a binary choice - either the font is at an angle or it is not.
Stylistically, the font may differentiate the manner in which it is angled. Generally, 'Oblique' fonts are the same as the non-oblique font, but with a simple transformation applied to make the top of the font lean rightwards. 'Italic' fonts are those that have been specially crafted to look different when set at an angle, sometimes with different ornamentation or flourishes, or with different emphasis on components of the characters.
The FontMap module does not differentiate between 'Italic' and 'Oblique' variants of a font, but treats both as the same state in its values. The FontMap module has no support for the degree to which the font is angled, and has no support for reverse leaning fonts. The values used by the module for obliqueness are:
| Value | Meaning |
|---|---|
| 0 | Don't care - a value that can be requested when any suitable obliqueness is accepted |
| 1 | Plain, not at any angle |
| 2 | Oblique or italic |
Font stretch
The stretch of a font gives a relative measure of how much wider or thinner a font is than others within the same family. As with the font weight names, there is no standardised naming for these features, but a continuum within a given font is usually expected.
The values used by FontMap are:
| Value | Meaning |
|---|---|
| 0 | Don't care - a value that can be requested when any suitable stretch is accepted |
| 100 | Ultra-condensed |
| 200 | Extra-condensed |
| 300 | Condensed |
| 400 | Semi-condensed |
| 500 | Normal |
| 600 | Semi-expanded |
| 700 | Expanded |
| 800 | Extra-expanded |
| 900 | Ultra-expanded |
FontMap mappings file
The Font Mapping file describes the translation between formats. It is configured through the FontMap$Mappings variable.
The translation file is a text file containing lines for mappings between the formats. The file has a simple line oriented format, which uses '#' character prefix for comments. Other lines consist of:
- The source system format, either an 'R' or a 'W' for RISC OS or Windows format of font family.
- The conversion type, either a '-' or a '>'. '-' indicates a bidirectional conversion between the source and destination system formats. '>' indicates a unidirectional conversion.
- The destination system format, either an 'R' or a 'W' for RISC OS or Windows format of font family.
- A colon (':') separator.
- The source font family name.
- A colon (':') separator.
- The destination font family name.
Described in ABNF the format is:
mapping-file = *mapping-line mapping-line = [ comment / rule ] LF comment = " #" *( SP / VCHAR )rule = src-system conversion dst-system " :" src-family ":" dst-familysrc-system = system dst-system = system conversion = " -" / ">"src-family = family dst-family = family family = 1*( SP / ALPHA / DIGIT / " -" / "_" )Syntax for FontMap mappings files
By default the FontMap module will use a font mapping file that looks similar to the following:
# Foreign font mappings # # Cut down form as fallback for ROM. # Standard mappings for RISC OS and Windows fonts R-W:Trinity:Times New Roman R-W:Homerton:Arial W>R:Helvetica:Homerton R-W:Corpus:Courier New
Translating font names
The SWI FontMap_Translate SWI provides a means of applying these properties and mappings to font names. The FontMap module will try to satisfy requests using the information available to it from the FontManager and the font mapping file.
The translation process goes through simple steps to translate the fonts:
- Identify the input font and its properties in the input font system. This means extracting the font properties from the name, and identifying the font family.
- Replace any of the properties from the input font with those supplied by the user.
- Map the font family name from the input system to the output system.
- Search the output font system for a suitable font name for the closest match to the font required.
- Return the result to the caller.
This process is expected to be inexact when translating between font systems. Different property inputs may match multiple fonts - for example, if an Extra Bold weight font is requested, but only Bold is available, the Bold will be returned, just as it would if Bold weight had been requested.
When translating RISC OS fonts, the FontMap module is able to synthesise oblique fonts by using a font transformation matrix. This is only performed when there is no suitable oblique or italic font.
System variables
The FontMap$Mappings variable contains a filename which should be used for the mappings to and from RISC OS font names. The file's format is described in the FontMap mappings file.
SWI calls
| R0 | = | Flags:
| |||||||||
| R1 | = | Pointer to a zero-terminated font name | |||||||||
| R2 | = | Requested font oblique state, or 0 for a default | |||||||||
| R3 | = | Requested font weight, or 0 for a default | |||||||||
| R4 | = | Requested font stretch state, or 0 for a default | |||||||||
| R5 | = | Pointer to output buffer for font name, or 0 to just request the length required | |||||||||
| R6 | = | Length of the font buffer | |||||||||
| R0 | preserved | |
| R1 | preserved | |
| R2 | = | Returned font oblique state |
| R3 | = | Returned font weight |
| R4 | = | Returned font stretch state |
| R5 | preserved | |
| R6 | = | Number of bytes written to the output buffer, including the terminator |
The Font_Translate SWI is used to either convert between RISC OS and foreign font naming systems, or to apply styling to a font name. The requested properties will be matched as closely as possible for a given font, and the actual matched font's properties will be returned. Because the closest match for a given font may not be suitable, callers may wish to check the returned properties.
If no translation is required, and only the font properties are desired, R5 may be set to 0. This will perform the translation and update the properties in the return registers, but no font name will be returned.
If the font name will not fit into the output buffer, the value of R6 will be updated to the size required, and the Buffer overflow (error number &1E5) error will be returned.
Error messages
This error is returned by the FontMap module when the flags supplied in R0 are not recognised.
This error is returned by the FontMap module when the input or output font system supplied in R0 was not recognised.
This error is returned by the FontMap module when the font name, or the font family name, was nor recognised, or when the equivalent font is not available in the output font system.
Examples
Reading the properties of a font:
>DIM buf% 256
>SYS "FontMap_Translate",(0<<0) OR (0<<0), "Homerton.Medium", 0,0,0 TO ,,oblique,weight,stretch
>PRINT oblique, weight, stretch
1 500 500
Making a font bold:
>DIM buf% 256
>SYS "FontMap_Translate",(0<<0) OR (0<<0), "Homerton.Medium", 0,900,0,buf%,256 TO ,,oblique,weight,stretch,font$
PRINT oblique, weight, stretch; " - ";font$
1 900 500 - Homerton.Bold