Appendix C: Implementation and performance notes

Contents

Colors
1. Gamma Correction
Fonts
1. Glossary of font terms
2. Font retrieval

C.1 Colors

C.1.1 Gamma Correction

The following information is informative, not normative. See the Gamma Tutorial in the PNG specification [PNG10] if you aren't familiar with gamma issues.

In the computation, UAs displaying on a CRT may assume an ideal CRT and ignore any effects on apparent gamma caused by dithering. That means the minimal handling they need to do on current platforms is:

PC using MS-Windows: none
Unix using X11: none
Mac using QuickDraw: apply gamma 1.39 [ICC32] (ColorSync-savvy applications may simply pass the sRGB ICC profile to ColorSync to perform correct color correction)
SGI using X: apply the gamma value from /etc/config/system.glGammaVal (the default value being 1.70; applications running on Irix 6.2 or above may simply pass the sRGB ICC profile to the color management system)
NeXT using NeXTStep: apply gamma 2.22

"Applying gamma" means that each of the three R, G and B must be converted to R'=R^gamma, G'=G^gamma, B'=B^gamma, before handing to the OS.

This may rapidly be done by building a 256-element lookup table once per browser invocation thus:

  for i := 0 to 255 do
    raw := i / 255;
    corr := pow (raw, gamma);
    table[i] := trunc (0.5 + corr * 255.0)
  end

which then avoids any need to do transcendental math per color attribute, far less per pixel.

C.2 Fonts

C.2.1 Glossary of font terms

DocLock™: Bitstream's DocLock™ technology ensures that TrueDoc PFRs can only be used with the site they are published for. A TrueDoc PFR moved to a different site or referenced from another site will not work.
Digital Signature: Part of a trust management technology, used to provide signed assertions about a resource.
Font Caching: Font caching allows for a temporary copy of fonts on the client system. They are usually stored on disk with other cached items such as graphics specifically for the UA.
Font Face: A "handle" that refers to a specific face of a font, excluding the font size (? size may be needed for non-scalable fonts)
Font Matching: Font matching is a process of selecting a similar font based on using one or more attributes of the primary font. Common attribute include serif, sans serif, weight, cap height, x height, spacing, language, and posture. Font matching is dependent on the algorithm and the variety of candidate fonts.
Glyph Representation Sub-setting: Glyph Representation sub-setting is the process by which unwanted glyph representations, (together with their side bearings and kerning information) are removed from a primary font to produce a smaller subset font that covers a particular document or set of documents. This is a particular win for documents that use ideographic scripts, where the glyph complement of the base font can be very large. Glyph representation sub-setting for documents using scripts that require ligatures, such as Arabic, is difficult without knowing the ligature formation rules of the final display system.
Intellifont: Intellifont technology was developed by Agfa and is the native format for Hewlett-Packard and other printers that use the PCL5 language. It is also the native font format on the Amiga computers.
Infinifont: A font synthesis technique which, given a Panose-1 number (and, optionally, additional font description data) can generate a faux font without extrapolating from a single master outline or interpolating between two or more outlines. See [INFINIFONT].
Italic: A class of letter forms for Latin scripts, that are more cursive than roman letter forms but less so than script forms. Often, a pair of fonts are designed to be used together; one is a serifed roman and one is italic. Other terms to describe this class of letter forms include cursive and, for Cyrillic scripts, kursiv. For sans-serif faces, the companion face is often a slanted or oblique variant rather than a different class of letter form.
Kerning: Altering the spacing between selected glyph representations, which would otherwise appear to be too close or too far apart, to obtain a more even typographical color.
Multiple Master Font: A Multiple Master Font contain two primary fonts that are used with special rendering software to provide an interpolated result. Adobe Systems provides a mechanism that allows for parameters to be used to control the output or the interpolated output font. These parameters usually describe the characteristics of an original font and the multiple master result is referred to as a synthesized font.
Open Type: Open Type is an extension to the TrueType font format which contains additional information that extends the capabilities of the fonts to support high-quality international typography. Open Type can associate a single character with multiple glyph representations, and combinations of characters with a single glyph representation (ligature formation). Open Type includes two-dimensional information to support features for complex positioning and glyph attachment. TrueType Open contains explicit script and language information, so a text-processing application can adjust its behavior accordingly. See [OPENTYPE].
Server Font: A Server Font is a font resource located on the web server that is referenced by the WebFont definition. The user agent may use this resource for rendering the page.
Speedo: Speedo font technology was developed by Bitstream and is the native font format on the Atari ST and Falcon computers,. It is also used by computers running X.
TrueDoc: TrueDoc technology was developed by Bitstream for the creation, transport, and imaging of platform independent scalable font objects on the web. Creation of font objects is done by the TrueDoc character shape recorder (CSR) and the rendering of the font objects is done by TrueDoc's character shape player (CSP). The technology is intended to be used on the web for viewing and printing.
TrueDoc Portable Font Resource: A TrueDoc Portable for resource (or PFR) is a platform independent scalable font object which is produce by a character shape player. Input may be either TrueType or Type 1 of any flavor on either Windows, Mac, or Unix. TrueDoc Portable Font Resources provide good compression ratios, are platform independent, and because they are not in an native font format (TrueType or Type 1) they can not be easily installed.
TrueType: TrueType is a font format developed by Apple and licensed to Microsoft. TrueType is the native operating system font format for Windows and Macintosh. TrueType contains a hierarchical set of tables and glyph representations. Characters can be hinted on a per character and point size basis yielding excellent quality at screen resolutions. TrueType fonts for Windows and Mac have few differences, though they can be different enough to prevent cross platform usage. Font foundries provide TrueType fonts for each platform and usually include a license preventing electronic manipulation to achieve cross platform transparency.
TrueType Collection: A TrueType Collection (or TTC) is an extension to the TrueType format that includes tables that allow for multiple TrueType fonts to be contained within a single TrueType font file. TrueType collection files are relatively rare at this time.
TrueType GX Fonts: TrueType GX Fonts contain extensions to the standard TrueType format that allow for mutable fonts, similar to Multiple Master fonts. There may be several mutation axis such as weight, height, and slant. The axis can be defined to obtain almost any effect. TrueType GX can also supports alternate glyph representation substitution for ligatures, contextual forms, fractions, etc. To date, TrueType GX is available only on the Mac. See [TRUETYPEGX].
Type 1 font: Type 1 fonts, developed by Adobe Systems, were one of first scalable formats available. Type 1 fonts are usually contain 228 characters with the glyph representations described using third degree bezier curves. Mac, Windows, and X have similar but separate formats; Adobe provides Adobe Type Manager for all three platforms. Type1c is a more recent losslessly-compressed storage form for Type 1 glyph representations.
URL Binding: A process of locking a particular font resource to a given Web site by embedding an encrypted URL or a digitally signed usage assertion into the font resource.

C.2.2 Font retrieval

There are many different font formats in use by many different platforms. To select a preferred font format, transparent content negotiation is used (see [NEGOT]). It is always possible to tell when a font is being dereferenced, because the URL is inside a font description. Thus, only the relevant Accept headers need be sent (not headers related to images, HTML, etc).