Draft for “4.2 Justification”
Of the four basic justification methods (flush left, flush right, justified, and centered), justified is the most challenging, as it requires changing the widths of the lines to a pre-defined measure. “Measure” refers to the width of a column of text. In a justified paragraph the width of all the lines should be the same as the paragraph’s measure (except, of course, the last line).
In Arabic there are six mechanisms for changing the width of a line of text. Each one has its limitations and considerations on when and how it can be applied. Furthermore, different typographers and calligraphers have divergent preferences for these mechanisms.
An important factor in the application of these mechanisms is their success in creating an even “color”. The color of the text refers to the amount of ink/blackness used to print or show a block of text. Color describes the density of the text against its background. Poorly justifying paragraphs can create uneven distribution of color.
These mechanisms are not exclusive. Quite the contrary, they are commonly used simultaneously to produce better justified paragraphs. Combination of these mechanisms is discussed in Combination of the Mechanisms.
This is the same mechanism widely used when justifying Latin scripts, where the width of the spaces between the words can be increased or decreased to change the width of the line.
Figure 1: Aligning lines by increasing and decreasing spaces between the words.
A minimum width is defined for how much the space can be shrunk, because putting the words too close to each other creates aesthetic and legibility problems.
Stretching the space too wide is also undesirable, but is utilized as a last resort when it is not possible to use other solutions to make fully justified paragraphs. In some applications a maximum width for the inter-word space is defined as a soft limit (compared to minimum width which is a hard limit). Reaching the maximum width makes the software to try to use other solutions for justification. If no other solution could yield the required result, the software would fall back to inter-word spacing and stretch the space past the maximum width.
Depending solely on this mechanism for aligning lines in a justified paragraph can lead to unpleasant results, such as rivers (multiple stretched spaces appearing vertically close to each other and forming a white gap inside the paragraph) and uneven distribution of color in the paragraph. Hence, typographers generally use other mechanisms as well to minimize the effect of adjusting inter-word spaces.
This solution alters the space between letters of each word to change the width of the text. Like adjusting inter-word spaces, this is used for Latin scripts as well, but using it for Arabic script involves considerations specific to Arabic. As noted in Joining and Intra-Word Spaces, the principal consideration is that gaps between characters only exist for those letters that join only to the right, such as dal and reh . Adjustment of intra-word space is not relevant where one letter is joined to its neighbors.
Figure 2: Altering intra-word spaces between unjoined letters.
Depending on the writing style and the typeface in use, different amounts of alteration to the intra-word space is acceptable for Arabic. Some writing styles allow more liberal adjustments to the closeness of the letter groups, while others can only accept small adjustments in this regard. In any case, much smaller adjustments can be used for intra-word spacing in comparison for inter-word spacing, which naturally is wider and tolerate bigger adjustments.
In addition to the four joining forms (isolated, initial, medial, and final), each Arabic letter can come with different shapes while preserving its joining form. For instance, a typeface or writing style can offer two or more shapes for the final form of a single letter.
These variant shapes usually have variant widths and hence can be used to adjust the width of the line.
Figure 3: Alternative shapes for changing the width of the text.
An advantage of using alternative letter shapes when justifying paragraphs is that it does not involve modifying default properties of the typeface (width of space or other characters). Instead, it is using shapes that are part of the typeface and are in harmony with other shapes in the lines.
But excessive use of alternative shapes, such as using multiple very wide alternatives close to each other, can create unnatural results.
It is not possible to justify paragraphs using only alternative letter shapes, because these shapes have predefined widths. For example, if a line should get 25 points wider, it is impossible to achieve that by using alternative letter shapes that are, say, 10 or 20 or 30 points wider than the default shapes. But these shapes can make the lines closer to measure, thus reducing the usage of other mechanisms.
Some Arabic fonts, following the writing styles that use special shapes when joining certain letters, provide a rich number of ligatures. These ligatures can be used in paragraph justification, since they usually reduce the widths of the words.
Figure 4: Various ligatures reducing the widths of the words
But existence of the ligatures in a font does not mean that they can be used freely. A font may provide some of its ligatures for creating an artistic style, which would be unsuitable for texts requiring optimum legibility.
For that reason, the user should be able to select which sets of ligatures can be used for justification. Fonts can offer predefined sets of ligatures to simplify this process.
Kashida refers to extending the horizontal connection between joined letters.
Figure 5: Two words extended with kashida.
This is a feature deeply related with the cursive nature of Arabic script. Kashida is an interesting tool for paragraph justification. It is more flexible than alternative letter shapes and ligatures, because it is not restricted to a limited number of predefined widths. At the same time, it has relatively less effect on the text color than spacing.
But a proper implementation of kashida involves a number of limitations and considerations.
Excessive use of kashida or applying very long kashidas results in uneven color. Also, horizontal or vertical proximity of numerous kashida creates an unnatural color.
Figure 6: Unpleasant result of excessive use of kashida.
Kashida is not always straight. Some fonts may require curvilinear kashidas, which require more advanced implementations.
Figure 7: Curvilinear kashida
Typographers can have preferred places for applying kashidas. In other words, instead of applying kashida between every joined pair of letters, they want it at certain joins.
Figure 8: There are multiple joins in this word, but only one is selected for kashida.
Another preference is avoiding multiple kashidas in a single word.
Tatweel is a dual-joining character that can be inserted between two joined letters to widen their connection. In The Unicode Standard, tatweel is represented as U+0640 ARABIC TATWEEL (ـ).
Figure 9: Tatweel
Tatweel extends letter connections in a fashion similar to kashida, but in a much more limited way. It is a character that has to be in the text or inserted like other characters. It has a predefined width, like any other character.
Yet it is much simpler to implement, since it acts like normal Arabic characters and does not require special treatment. For this reason, it can be considered useful specially in constrained implementations like fixed-width environments.
Each of the above six mechanisms have their own limitations and side effects. Utilizing only one of them for justifying paragraphs can create undesirable results. Multiple mechanisms can be used at the same time to work around their limitations and minimize their side effects.
Since Arabic provides various mechanisms that can be used for justification, an advanced implementation that supports all or most of the above features can produce exemplary justifications. More limited applications can combine what is available.
Preferences for each mechanisms can depend on the document and text and the preference of the typographers and users. Implementations can enable users to prioritize and control the mechanisms mentioned above.
- Drop “elongation” from title of this section. It’s one of the mechanisms used for justification.
- Make sure these terms have correct definitions and translations in our glossary: elongation, kashida, tatweel, measure, color, river
- Discussion at ALReq meeting on 28 June, 2016
- Justify Just or Just Justify
- Arabic text justification
- Thomas Milo’s “Arabic script and typography: A brief historical view”
- Tasmeem Manual
- Justifying Text using Cascading Style Sheets (CSS) in Internet Explorer 5.5
- TODO: Improve the images.