Bitmap Versus Scalable Fonts

Bitmaps are series of dots or pixels that represent the image of each glyph in each typeface and size. Therefore, one bitmap font is required for each text size. For example, four fonts must be stored to support four text sizes. Bitmaps place a burden on storage requirements (Fig. 1). Bitmap fonts also present a number of limitations and challenges, including:

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  Limited type size selection: Each type size must be stored individually. Therefore, memory-constrained devices typically offer a limited number of text size selections.
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  Limited support for large character sets: Memory-constrained devices face the challenge of supporting large character sets, such as the GB 18030-2000 Chinese character set, which comprises more than 28,000 characters.
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  Inability to keep pace with handset advances: As handsets offer enhancements such as improved screen resolution, existing bitmaps often become unsuitable for newer form factors.
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  Production bottlenecks: Creating and fine-tuning bitmaps for new markets, applications, or phone models is a time-consuming process, which can easily lead to production inefficiencies.
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  Text quality issues: The display quality of bitmaps suffers when attempts are made to scale or transform them to display at non-native sizes. Bitmaps also do not support anti-aliasing techniques that soften jagged effects around the curves and edges of characters.

Fonts based on a stroke format are also scalable and are used to support the thousands of characters in East Asian scripts (Fig. 2). Stroke fonts use a series of simple lines and other information to define the size and shape of the line in a specific typeface and size, which together describe the appearance of the glyph. Points are placed along the center of each stroke or line, saving several megabytes of storage.

Scalable fonts may contain hints, which are instructions carried out by the scaling algorithm to visually improve the representation of the glyph (Fig. 3). Hinting instructions, which are created by typographic specialists, consist of small, careful adjustments that are applied to glyphs during the outline-scaling process. Without hints, certain glyphs such as those that are intricately shaped and scaled to a small text size are often illegible or unaesthetic.

Rendering Scalable Type

A scalable font rendering system is required to render outline or stroke font data. Using a font scaling engine, the mobile device can store only the points along the edges of the outline of each glyph (or in the center of each stroke or line for stroke fonts). To render the glyph, the font engine essentially “connects the dots” and fills in the outline. Scalable font solutions enable OEMs and developers to:

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  render glyphs at any size for different applications, screen sizes, resolutions, and products
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  offer display screens with different shapes, sizes, and resolutions to support many different market requirements and consumer tastes
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  use just one font file for each type design to fit more typeface designs or languages in a phone
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  take advantage of growing selections of fonts optimized for mobile phones to fit aesthetic requirements and worldwide language support
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  support customized scalable fonts for branded applications and content
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  apply anti-aliasing techniques, as well as emboldening, slanting (italic), rotation, and other effects
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  comply with mandates for font scalability as defined by industry specifications
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  more easily adapt existing software/OS designs to new hardware platforms
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  deploy products faster to offer product benefits to buyers sooner

Scalable font solutions, however, cannot be taken as is from desktop environments to work seamlessly in mobile phones. Desktop environments, where scalable font technology was first used, do not share the same constraints as mobile environments, such as smaller screen displays, lower resolutions, and memory restrictions. But technical innovations are being developed to overcome issues that might otherwise hinder text display quality, memory efficiency, and other aspects of the mobile environment.

Whether embedded in the handset or as a component of downloadable software, the font engine should include capabilities that ensure the high-quality display of scalable text on mobile phones. Several innovations are helping to maximize the benefits of scalable font technology and simplify the transition to a scalable solution from fixed-in-size bitmap fonts. For example, display fidelity ensures that text displays fully and that it predictably appears on mobile devices.

Fitting Text Into Tight Spaces

Scaled glyphs typically need to fit within a known or predictable amount of space, such as a text box in a user interface, to be fully viewable on the handset. Otherwise, character components such as accent marks or long descenders may be clipped from view in the vertical extent (Fig. 4). Such concerns are not a problem in desktop environments, where vertical space limitations are not a critical issue.

As mobile device manufacturers look to break into Asian markets or extend into other regions, there’s an increased need for customized fonts that are “height-restricted” for mobile environments. Height-restricted fonts allow all characters to fit within allotted spaces, enabling the display of characters that fit within the expected character cell height.

One advantage of height-restricted fonts is the ability to maximize the number of lines of text on screen. Developers may also find that height-restricted fonts work with existing bitmap font text layout logic, helping to smooth the transition when switching from a bitmap to a scalable font solution.

Mobile environments also can overcome character height issues effectively through intelligent scaling initiated by the font engine. The more advanced the font engine, the more likely it will provide support for technology that intelligently reshapes glyphs to fit fully within the allotted space and appear properly proportioned next to surrounding characters.