I’ve been trying to understand how web pages need to support justification of Arabic text, so that there are straight lines down both left and right margins.

The following is an extract from a talk I gave at the MultilingualWeb workshop in Madrid at the beginning of May. (See the whole talk.) It’s very high level, and basically just draws out some of the uncertainties that seem to surround the topic.

Let’s suppose that we want to justify the following Arabic text, so that there are straight lines at both left and right margins.

Arabic justification #1

Unjustified Arabic text

Generally speaking, received wisdom says that Arabic does this by stretching the baseline inside words, rather than stretching the inter-word spacing (as would be the case in English text).

To keep it simple, lets just focus on the top two lines.

One way you may hear that this can be done is by using a special baseline extension character in Unicode, U+0640 ARABIC TATWEEL.

Arabic justification #2

Justification using tatweels

The picture above shows Arabic text from a newspaper where we have justified the first two lines using tatweels in exactly the same way it was done in the newspaper.

Apart from the fact that this looks ugly, one of the big problems with this approach is that there are complex rules for the placement of baseline extensions. These include:

  • extensions can only appear between certain characters, and are forbidden around other characters
  • the number of allowable extensions per word and per line is usually kept to a minimum
  • words vary in appropriateness for extension, depending on word length
  • there are rules about where in the line extensions can appear – usually not at the beginning
  • different font styles have different rules

An ordinary web author who is trying to add tatweels to manually justify the text may not know how to apply these rules.

A fundamental problem on the Web is that when text size or font is changed, or a window is stretched, etc, the tatweels will end up in the wrong place and cause problems. The tatweel approach is of no use for paragraphs of text that will be resized as the user stretches the window of a web page.

In the next picture we have simply switched to a font in the Naskh style. You can see that the tatweels applied to the word that was previously at the end of the first line now make the word to long to fit there. The word has wrapped to the beginning of the next line, and we have a large gap at the end of the first line.

Arabic justification #3

Tatweels in the wrong place due to just a font change

To further compound the difficulties mentioned above regarding the rules of placement for extensions, each different style of Arabic font has different rules. For example, the rules for where and how words are elongated are different in the Nastaliq version of the same text which you can see below. (All the characters are exactly the same, only the font has changed.) (See a description of how to justify Urdu text in the Nastaliq style.)

Arabic justification #4: Nastaliq
Same text in the Nastaliq font style

And fonts in the Ruqah style never use elongation at all. (We’ll come back to how you justify text using Ruqah-style fonts in a moment.)

Arabic justification #5: Ruqah
Same text in the Ruqah font style

In the next picture we have removed all the tatweel characters, and we are showing the text using a Naskh-style font. Note that this text has more ligatures on the first line, so it is able to fit in more of the text on that line than the first font we saw. We’ll again focus on the first two lines, and consider how to justify them.

Arabic justification #6: Naskh
Same text in the Naskh font style

High end systems have the ability to allow relevant characters to be elongated by working with the font glyphs themselves, rather than requiring additional baseline extension characters.

Arabic justification #7: kashida elongation
Justification using letter elongation (kashida)

In principle, if you are going to elongate words, this is a better solution for a dynamic environment. It means, however, that:

  1. the rules for applying the right-sized elongations to the right characters has to be applied at runtime by the application and font working together, and as the user or author stretches the window, changes font size, adds text, etc, the location and size of elongations needs to be reconfigured
  2. there needs to be some agreement about what those rules are, or at least a workable set of rules for an off-the-shelf, one-size-fits-all solution.

The latter is the fundamental issue we face. There is very little, high-quality information available about how to do this, and a lack of consensus about, not only what the rules are, but how justification should be done.

Some experts will tell you that text elongation is the primary method for justifying Arabic text (for example), while others will tell you that inter-word and intra-word spacing (where there are gaps in the letter-joins within a single word) should be the primary approach, and kashida elongation may or may not be used in addition where the space method is strained.

Arabic justification #8: space based

Justification using inter-word spacing

The space-based approach, of course, makes a lot of sense if you are dealing with fonts of the Ruqah style, which do not accept elongation. However, the fact that the rules for justification need to change according to the font that is used presents a new challenge for a browser that wants to implement justification for Arabic. How does the browser know the characteristics of the font being used and apply different rules as the font is changed? Fonts don’t currently indicate this information.

Looking at magazines and books on a recent trip to Oman I found lots of justification. Sometimes the justification was done using spaces, other times using elongations, and sometimes there was a mixture of both. In a later post I’ll show some examples.

By the way, for all the complexity so far described this is all quite a simplistic overview of what’s involved in Arabic justification. For example, high end systems that justify Arabic text also allow the typesetter to adjust the length of a line of text by manual adjustments that tweak such things as alternate letter shapes, various joining styles, different lengths of elongation, and discretionary ligation forms.

The key messages:

  1. We need an Arabic Layout Requirements document to capture the script needs.
  2. Then we need to figure out how to adapt Open Web Platform technologies to implement the requirements.
  3. To start all this, we need experts to provide information and develop consensus.

Any volunteers to create an Arabic Layout Requirements document? The W3C would like to hear from you!