Using the entire character cell
Presentation devices that are designed for non-cursive scripts may not be adequate for cursive scripts. An English letter usually does not occupy its entire character cell, that is, there is a gap between successive letters when presented. Such a gap is not acceptable when presenting Arabic characters.
There is a marked difference in the widths of the characters in a cursive script. Arabic character width can vary from as narrow as the English letter i, to much wider than the letter M. To enhance readability, the following guideline is established.
In implementations where proportional spacing is not available, or when using a monospaced font, some Arabic characters occupy two columns or two character cells. Any shaping routine must take this situation into account.
Whenever a presentation device implements proportional spacing, a fixed pitch monospaced mode should also be available to allow easy presentation of tabular data.
Kerning is another technique to enhance the readability of cursive scripts. Kerning is the design of graphic characters so that their character cells overlap, resulting in the reduction of space between characters. This allows character boxes to be designed for cursive scripts, ligatures, and proportionally spaced fonts. An example of kerning is the printing of adjacent graphic characters so the character boxes overlap on the left or right side, as shown in the following figure.
The figure shows kerning of the English letters a and f, and suggests how this might be applied to the merging of successive cursive Arabic characters.
For more information on kerning, refer to the IBM manual, Font Object Content Architecture (FOCA) Reference (PDF, 3.2MB), S544-3285.
Arabic users can use two different presentation types for digits:
- Digits used in Latin writings (or nominal) - 1234567890
- Arabic-Indic Digits (or national) - '.
Both presentation types can be used at the same time, such as in a mixed text containing both Arabic and English data. Since digits used in Latin writings can co-exist together with Arabic-Indic digits, there should be a way to allow users to use both presentation types for digits.
Arabic numerals have different presentations - Nominal, Contextual and National:
- Nominal: All numbers are presented as digits used in Latin writings
- National: All numbers are presented as Arabic-Indic digits or other representations used for different national languages.
- Contextual: Presentation of digits follow the preceding text, that is, digits following English characters are presented as English digits and numbers following Arabic characters are presented as Arabic-Indic digits. The figure below shows an example of contextual digits presentation.
Allow users to choose the preferred Arabic digits presentation. There are different ways of viewing numbers in Arabic and users should be able to view them in different formats. For example on MS-Windows 7, the Region and Language settings control the digits representation for the Arabic Language. A guideline related to this topic is Guideline C5 that talks about allowing the user to select the cardinal number shape.