Appendix C: Implementation and performance notes

Contents

Order of property value calculation
Colors
1. Gamma Correction
Fonts

3.1 Order of property value calculation

This section is under construction

Due to dependencies among properties, user agents must compute some property values in a specific order.

'font-size' must be computed before any property that may take a relative length units.
'display', 'position', and 'float' must be computed before other visual flow properties.
'line-height' must be computed before 'vertical-align'.
An element's width and height must be computed before 'background-position'.

3.2 Colors

3.2.1 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.

3.3 Fonts

3.3.1 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 often 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 generally 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 generally 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 generally 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.
URI Binding: A process of locking a particular font resource to a given Web site by embedding an encrypted URI or a digitally signed usage assertion into the font resource.

3.3.2 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 URI is inside a font description. Thus, only the relevant Accept headers need be sent (not headers related to images, HTML, etc).

3.3.3 Meaning of the Panose Digits

For further details on Panose-1, see [PANOSE].

Family

Serif Style

Any (0)
No Fit (1)
Cove (2)
Obtuse Cove (3)
Square Cove (4)
Obtuse Square Cove (5)
Square (6)
Thin (7)
Bone (8)
Exaggerated (9)
Triangle (10)
Normal Sans (11)
Obtuse Sans (12)
Perp Sans (13)
Flared (14)
Rounded (15)

Weight

Any (0)
No Fit (1)
Very Light (2)[100]
Light (3) [200]
Thin (4) [300]
Book (5) [400] same as CSS1 'normal'
Medium (6) [500]
Demi (7) [600]
Bold (8) [700] same as CSS1 'bold'
Heavy (9) [800]
Black (10) [900]
Extra Black / Nord (11) [900] force mapping to CSS1 100-900 scale

Proportion

Any (0)
No Fit (1)
Old Style (2)
Modern (3)
Even Width (4)
Expanded (5)
Condensed (6)
Very Expanded (7)
Very Condensed (8)
Monospaced (9)

Contrast

Any (0)
No Fit (1)
None (2)
Very Low (3)
Low (4)
Medium Low (5)
Medium (6)
Medium High (7)
High (8)
Very High (9)

Stroke Variation

Any (0)
No Fit (1)
No Variation (2)
Gradual/Diagonal (3)
Gradual/Transitional (4)
Gradual/Vertical (5)
Gradual/Horizontal (6)
Rapid/Vertical (7)
Rapid/Horizontal (8)
Instant/Horizontal (9)
Instant/Vertical (10)

Arm Style

Any (0)
No Fit (1)
Straight Arms/Horizontal (2)
Straight Arms/Wedge (3)
Straight Arms/Vertical (4)
Straight Arms/Single Serif (5)
Straight Arms/Double Serif (6)
Non-Straight Arms/Horizontal (7)
Non-Straight Arms/Wedge (8)
Non-Straight Arms/Vertical 90)
Non-Straight Arms/Single Serif (10)
Non-Straight Arms/Double Serif (11)

Letterform

Any (0)
No Fit (1)
Normal/Contact (2)
Normal/Weighted (3)
Normal/Boxed (4)
Normal/Flattened (5)
Normal/Rounded (6)
Normal/Off Center (7)
Normal/Square (8)
Oblique/Contact (9)
Oblique/Weighted (10)
Oblique/Boxed (11)
Oblique/Flattened (12)
Oblique/Rounded (13)
Oblique/Off Center (14)
Oblique/Square (15)

Midline

Any (0)
No Fit (1)
Standard/Trimmed (2)
Standard/Pointed (3)
Standard/Serifed (4)
High/Trimmed (5)
High/Pointed (6)
High/Serifed (7)
Constant/Trimmed (8)
Constant/Pointed (9)
Constant/Serifed (10)
Low/Trimmed (11)
Low/Pointed (12)
Low/Serifed (13)

XHeight

Any (0)
No Fit (1)
Constant/Small (2)
Constant/Standard (3)
Constant/Large (4)
Ducking/Small (5)
Ducking/Standard (6)
Ducking/Large (7)

3.3.4 Deducing Unicode Ranges for TrueType

This information is available in the font by looking at the 'ulUnicodeRange' bits in the 'OS/2' table (if it has one), which holds a bitfield representation of the set. This table is defined in revision 1.66 of the TrueType specification, from Microsoft. Considering this information as a set, each element corresponds to a Unicode 1.1 character block, and the presence of that element in the set indicates that the font has one or more glyph representations to represent at least one character in that block. The set has 128 elements as described below. The order generally follows that in the Unicode 1.1 standard. This table may be used to convert the information in a TrueType font into a CSS 'unicode-range' descriptor.

Block Add Block name Unicode range
0 1 Basic Latin U+0-7F
1 2 Latin-1 Supplement U+80-FF
2 4 Latin-1 Extended-A U+100-17F
3 8 Latin Extended-B U+180-24F

4 1 IPA Extensions U+250-2AF
5 2 Spacing Modifier Letters U+2B0-2FF
6 4 Combining Diacritical Marks U+300-36F
7 8 Greek U+370-3CF

8 1 Greek Symbols and Coptic U+3D0-3EF
9 2 Cyrillic U+400-4FF
10 4 Armenian U+530-58F
11 8 Hebrew U+590-5FF

12 1 Hebrew Extended-A
Hebrew Extended-B ?? what ranges ??
13 2 Arabic U+600-69F
14 4 Arabic Extended U+670-6FF
15 8 Devanagari U+900-97F

16 1 Bengali U+980-9FF
17 2 Gurmukhi U+A00-A7F
18 4 Gujarati U+A80-AFF
19 8 Oriya U+B00-B7F

20 1 Tamil U+B80-BFF
21 2 Telugu U+C00-C7F
22 4 Kannada U+C80-CFF
23 8 Malayalam U+D00-D7F

24 1 Thai U+E00-E7F
25 2 Lao U+E80-EFF
26 4 Georgian U+10A0-10EF
27 8 Georgian Extended U+10F0-10FF ??

28 1 Hangul Jamo U+1100-11FF
29 2 Latin Extended Additional -
30 4 Greek Extended U+1F00-1FFF
31 8 General Punctuation U+2000-206F

32 1 Superscripts and Subscripts -
33 2 Currency Symbols U+20A0-20CF
34 4 Combining Marks for Symbols U+20D0-20FF
35 8 Letterlike Symbols U+2100-214F

36 1 Number Forms U+2150-218F
37 2 Arrows U+2190-21FF
38 4 Mathematical Operators U+2200-22FF
39 8 Miscellaneous Technical U+2300-23FF

40 1 Control Pictures U+2400-243F
41 2 Optical Character Recognition U+2440-245F
42 4 Enclosed Alphanumerics U+2460-24FF
43 8 Box Drawing U+2500-257F

44 1 Block Elements U+2580-259F
45 2 Geometric Shapes U+25A0-25FF
46 4 Miscellaneous Symbols U+2600-26FF
47 8 Dingbats U+2700-27BF

48 1 CJK Symbols and Punctuation U+3000-303F
49 2 Hiragana U+3040-309F
50 4 Katakana U+30A0-30FF
51 8 Bopomofo U+3100-312F

52 1 Hangul Compatibility Jamo U+3130-318F
53 2 CJK Miscellaneous ??
54 4 Enclosed CJK Letters and Months U+3200-32FF
55 8 CJK compatibility U+3300-33FF

56 1 Hangul U+AC00-D7FF
59 8 CJK Unified Ideographs U+4E00-9FFF

60 1 Private Use Area U+E000-F8FF
61 2 CJK Compatibility Ideographs U+F900-FAFF
62 4 Alphabetic Presentation Forms U+FB00-FB4F
63 8 Arabic Presentation Forms-A U+FB50-FDFF

64 1 Combining Half Marks U+FE20-FE2F
65 2 CJK compatibility Forms U+FE30-FE4F
66 4 Small Form Variants U+FE50-FE6F
67 8 Arabic Presentation Forms-B U+FE70-FEFF

68 1 Halfwidth and Fullwidth Forms U+FF00-FFEF
69 2 Specials U+FFF0-FFFD


Block	Add	Block name	Unicode range

0	1	Basic Latin	U+0-7F
1	2	Latin-1 Supplement	U+80-FF
2	4	Latin-1 Extended-A	U+100-17F
3	8	Latin Extended-B	U+180-24F

4	1	IPA Extensions	U+250-2AF
5	2	Spacing Modifier Letters	U+2B0-2FF
6	4	Combining Diacritical Marks	U+300-36F
7	8	Greek	U+370-3CF

8	1	Greek Symbols and Coptic	U+3D0-3EF
9	2	Cyrillic	U+400-4FF
10	4	Armenian	U+530-58F
11	8	Hebrew	U+590-5FF

12	1	Hebrew Extended-A Hebrew Extended-B	?? what ranges ??
13	2	Arabic	U+600-69F
14	4	Arabic Extended	U+670-6FF
15	8	Devanagari	U+900-97F

16	1	Bengali	U+980-9FF
17	2	Gurmukhi	U+A00-A7F
18	4	Gujarati	U+A80-AFF
19	8	Oriya	U+B00-B7F

20	1	Tamil	U+B80-BFF
21	2	Telugu	U+C00-C7F
22	4	Kannada	U+C80-CFF
23	8	Malayalam	U+D00-D7F

24	1	Thai	U+E00-E7F
25	2	Lao	U+E80-EFF
26	4	Georgian	U+10A0-10EF
27	8	Georgian Extended	U+10F0-10FF ??

28	1	Hangul Jamo	U+1100-11FF
29	2	Latin Extended Additional	-
30	4	Greek Extended	U+1F00-1FFF
31	8	General Punctuation	U+2000-206F

32	1	Superscripts and Subscripts	-
33	2	Currency Symbols	U+20A0-20CF
34	4	Combining Marks for Symbols	U+20D0-20FF
35	8	Letterlike Symbols	U+2100-214F

36	1	Number Forms	U+2150-218F
37	2	Arrows	U+2190-21FF
38	4	Mathematical Operators	U+2200-22FF
39	8	Miscellaneous Technical	U+2300-23FF

40	1	Control Pictures	U+2400-243F
41	2	Optical Character Recognition	U+2440-245F
42	4	Enclosed Alphanumerics	U+2460-24FF
43	8	Box Drawing	U+2500-257F

44	1	Block Elements	U+2580-259F
45	2	Geometric Shapes	U+25A0-25FF
46	4	Miscellaneous Symbols	U+2600-26FF
47	8	Dingbats	U+2700-27BF

48	1	CJK Symbols and Punctuation	U+3000-303F
49	2	Hiragana	U+3040-309F
50	4	Katakana	U+30A0-30FF
51	8	Bopomofo	U+3100-312F

52	1	Hangul Compatibility Jamo	U+3130-318F
53	2	CJK Miscellaneous	??
54	4	Enclosed CJK Letters and Months	U+3200-32FF
55	8	CJK compatibility	U+3300-33FF

56	1	Hangul	U+AC00-D7FF
59	8	CJK Unified Ideographs	U+4E00-9FFF

60	1	Private Use Area	U+E000-F8FF
61	2	CJK Compatibility Ideographs	U+F900-FAFF
62	4	Alphabetic Presentation Forms	U+FB00-FB4F
63	8	Arabic Presentation Forms-A	U+FB50-FDFF

64	1	Combining Half Marks	U+FE20-FE2F
65	2	CJK compatibility Forms	U+FE30-FE4F
66	4	Small Form Variants	U+FE50-FE6F
67	8	Arabic Presentation Forms-B	U+FE70-FEFF

68	1	Halfwidth and Fullwidth Forms	U+FF00-FFEF
69	2	Specials	U+FFF0-FFFD

The TrueType bitfield system has the problem that it is tied to Unicode 1.1 and is unable to cope with Unicode expansion - it is unable to represent Tibetan for example.