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5.7 Font Matching

 
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5.7 Font Matching

As we've seen, CSS allows for the matching of font families, weights, and variants. This is all done through font matching, which is a vaguely complicated procedure. Understanding it is important for authors who want to help user agents make good font selections when displaying their documents. I left it for the end of the chapter because it's not really necessary to understand how the font properties work, and some people will probably want to skip this part and go on to the next chapter. If you're still interested, here's how font matching works.

  1. The user agent creates, or otherwise accesses, a database of font properties. This database lists the various CSS properties of all the fonts to which the user agent has access. Typically, this will be all fonts installed on the machine, although there could be others (for example, the user agent could have its own built-in fonts). If the user agent encounters two identical fonts, it will simply ignore one of them.

  2. The user agent takes apart an element to which font properties have been applied and constructs a list of font properties necessary for the display of that element. Based on that list, the user agent makes an initial choice of a font family to use in displaying the element. If there is a complete match, then the user agent can use that font. Otherwise, it needs to do a little more work.

    1. A font is first matched against the font-style. The keyword italic is matched by any font that is labeled as either "italic" or "oblique." If neither is available, then the match fails.

    2. The next match attempt is on font-variant. Any font that is not labeled "small-caps" is assumed to be normal. A font can be matched to small-caps by any font that is labeled as "small-caps," by any font that allows the synthesis of a small-caps style, or by any font where lowercase letters are replaced by uppercase letters.

    3. The next match is to font-weight, which can never fail thanks to the way font-weight is handled in CSS (explained earlier in the chapter).

    4. Then, font-size is tackled. This must be matched within a certain tolerance, but the tolerance is left to the user agent to define. Thus, one user agent might allow matching within a 20% margin of error, whereas another might allow only 10% differences between the size specified and the size that is actually used.

  3. If there was no font match in Step 2, the user agent looks for alternate fonts within the same font family. If it finds any, then it repeats Step 2 for that font.

  4. Assuming a generic match has been found, but it doesn't contain everything needed to display a given elementthe font is missing the copyright symbol, for instancethen the user agent goes back to Step 3, which entails a search for another alternate font and another trip through Step 2.

  5. Finally, if no match has been made and all alternate fonts have been tried, then the user agent selects the default font for the given generic font family and does the best it can to display the element correctly.

The whole process is long and tedious, but it helps to understand how user agents pick the fonts they do. For example, you might specify the use of Times or any other serif font in a document:

body {font-family: Times, serif;}

For each element, the user agent should examine the characters in that element and determine whether Times can provide characters to match. In most cases, it can do so with no problem. Assume, however, that a Chinese character has been placed in the middle of a paragraph. Times has nothing that can match this character, so the user agent has to work around the character or look for another font that can fulfill the needs of displaying that element. Of course, any Western font is highly unlikely to contain Chinese characters, but should one exist (let's call it AsiaTimes), the user agent could use it in the display of that one elementor simply for the single character. Thus, the whole paragraph might be displayed using AsiaTimes, or everything in the paragraph might be in Times except for the single Chinese character, which is displayed in AsiaTimes.

5.7.1 Font Face Rules

CSS2 introduced a way to exert much greater control over font matching through an @font-face rule. Since no web browsers had fully implemented this as of spring 2003, @font-face was removed from CSS2.1. I will not spend much time on it, as the aspects of this rule are very complicated and could probably fill a chapter (or a book!) of their own.

There are four ways to arrive at a font to be used in the document. We'll look briefly at each, since future versions of CSS may use this mechanism, and most SVG renderers at least partially support the font face matching described in CSS2. If you are in a situation where you need to implement @font-face, please refer to the CSS2 specification, or whatever the latest version of CSS might be (such as the CSS3 Web Fonts module); the following descriptions are incomplete at best.

5.7.1.1 Font name matching

To match the font name, the user agent uses an available font that has the same family name as the requested font. The font's appearance and metrics might not be the same. This is the method described earlier in this section.

5.7.1.2 Intelligent font matching

In this case, the user agent uses an available font that is the closest match in appearance to the requested font. The two may not match exactly, but they should be as close as possible.

The information used to match the two fonts includes the kind of font (text or symbol), nature of serifs, weight, cap height, x-height, ascent, descent, slant, and so on. For example, an author could request that a certain font be as close as possible to a certain slant by writing:

@font-face {font-style: normal; font-family: "Times"; slope: -5;}

It would then be up to the user agent to find a serif normal (upright) font whose slope was as close to five degrees to the right as possible, if Times does not fit the bill. There are a great many font aspects described in CSS2, all of which can be used to drive the matching process in a user agent that supports them.

5.7.1.3 Font synthesis

It's also possible that a user agent would choose to actually generate, on the fly, a font whose appearance and metrics match the description given in the @font-face rule. CSS2 has this to say about the process:

In this case, the user agent creates a font that is not only a close match in appearance, but also matches the metrics of the requested font. The synthesizing information includes the matching information and typically requires more accurate values for the parameters than are used for some matching schemes. In particular, synthesis requires accurate width metrics and character to glyph substitution and position information if all the layout characteristics of the specified font are to be preserved.

If this makes sense to you, then you probably don't need my help to explain it. If not, you probably need never worry about it.

5.7.1.4 Font download

In this approach, the user agent may download a remote font for use in the document. To declare a font for downloading, you might write something like this:

@font-face {font-family: "Scarborough Fair";

   src: url(http://www.example.com/fonts/ps/scarborough.ps);}

You could then use that font throughout the document.

Even in a user agent that permits font downloading, it may take some time to retrieve the font file (such files can be quite large), which would delay the rendering of the document, or at least delay the final rendering.

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