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Acomputer font is implemented as a digitaldata file containing a set of graphically relatedglyphs. A computer font is designed and created using afont editor. A computer font specifically designed for the computer screen, and not for printing, is ascreen font.
In the terminology ofmovable metal type, atypeface is a set of characters that share common design features across styles and sizes (for example, all the varieties ofGill Sans), while afont is a set of pieces of movable type in a specific typeface, size, width, weight, slope, etc. (for example, Gill Sans bold 12 point). InHTML,CSS, and related technologies, thefont family attribute refers to the digital equivalent of a typeface. Since the 1990s, many people outside the printing industry have used the wordfont as asynonym fortypeface.
There are three basic kinds of computer font file data formats:
Bitmap fonts are faster and easier to create in computer code than other font types, but they are not scalable: a bitmap font requires a separate font for each size.[1] Outline and stroke fonts can be resized in a single font by substituting different measurements for components of each glyph, but they are more complicated to render on screen or in print than bitmap fonts because they require additional computer code to render the bitmaps to display on screen and in print. Although all font types are still in use, most fonts used on computers today are outline fonts.
Fonts can bemonospaced (i.e. every character is plotted a constant distance from the previous character that it is next to while drawing) orproportional (each character has its own width). However, the particular font-handling application can affect the spacing, particularly whenjustifying text.
A bitmap font is one that stores eachglyph as an array ofpixels (that is, abitmap). It is less commonly known as araster font or a pixel font. Bitmap fonts are simply collections ofraster images of glyphs. For each variant of the font, there is a complete set of glyph images, with each set containing an image for each character. For example, if a font has three sizes, and any combination of bold and italic, then there must be 12 complete sets of images.
Advantages of bitmap fonts include:
The primary disadvantage of bitmap fonts is that the visual quality tends to be poor when scaled or otherwise transformed, compared to outline and stroke fonts, and providing many optimized and purpose-made sizes of the same font dramatically increases memory usage. The earliest bitmap fonts were only available in certain optimized sizes such as 8, 9, 10, 12, 14, 18, 24, 36, 48, 72, and 96 points (assuming a resolution of 96 DPI), with custom fonts often available in only one specific size, such as a headline font at only 72 points.
The limited processing power and memory of early computer systems forced the exclusive use of bitmap fonts. Improvements in hardware have allowed them to be replaced with outline or stroke fonts in cases where arbitrary scaling is desirable, but bitmap fonts are still in common use in embedded systems and other places where speed and simplicity are considered important.
Bitmap fonts are used in theLinux console, theWindowsrecovery console, andembedded systems. Olderdot matrix printers used bitmap fonts; often stored in the memory of the printer and addressed by the computer'sprint driver. Bitmap fonts may be used incross-stitch.
To draw a string using a bitmap font means to successively output bitmaps of each character that the string comprises, performing per-character indentation.
Digital bitmap fonts (and thefinal rendering of vector fonts) may usemonochrome orshades of gray. The latter isanti-aliased. When displaying a text, typically an operating system properly represents the "shades of gray" as intermediate colors between the color of the font and that of the background. However, if the text is represented as animage withtransparent background, "shades of gray" require an image format allowingpartial transparency.
Bitmap fonts look best at their nativepixel size. Some systems using bitmap fonts can create some font variants algorithmically. For example, the originalApple Macintosh computer could produce bold by widening vertical strokes and oblique byshearing the image. At non-native sizes, many text rendering systems performnearest-neighbor resampling, introducing rough jagged edges. More advanced systems performanti-aliasing on bitmap fonts whose size does not match the size that the application requests. This technique works well for making the font smaller but not as well for increasing the size, as it tends to blur the edges. Some graphics systems that use bitmap fonts, especially those ofemulators, apply curve-sensitivenonlinear resampling algorithms such as2xSaI orhq3x on fonts and other bitmaps, which avoids blurring the font while introducing little objectionable distortion at moderate increases in size.
The difference between bitmap fonts and outline fonts is similar to the difference between bitmap and vector image file formats. Bitmap fonts are like image formats such asWindows Bitmap (.bmp),Portable Network Graphics (.png) andTagged Image Format (.tif or .tiff), which store the image data as a grid of pixels, in some cases with compression. Outline or stroke image formats such asWindows Metafile format (.wmf) andScalable Vector Graphics format (.svg), store instructions in the form of lines and curves of how to draw the image rather than storing the image itself.
A "trace" program can follow the outline of a high-resolution bitmap font and create an initial outline that a font designer uses to create anoutline font useful in systems such asPostScript orTrueType. Outline fonts scale easily without jagged edges or blurriness.
Outline fonts orvector fonts are collections ofvector images, consisting of lines and curves defining the boundary ofglyphs. Early vector fonts were used byvector monitors andvector plotters using their own internal fonts, usually with thin single strokes instead of thickly outlined glyphs. The advent of desktop publishing brought the need for a common standard to integrate thegraphical user interface of the firstMacintosh andlaser printers. The term to describe the integration technology wasWYSIWYG (What You See Is What You Get). This common standard was (and still is[when?]) AdobePostScript.[citation needed] Examples of outline fonts include: PostScriptType 1 and Type 3 fonts,TrueType,OpenType andCompugraphic.
The primary advantage of outline fonts is that, unlikebitmap fonts, they are a set of lines and curves instead of pixels; they can be scaled without causingpixelation. Therefore, outline font characters can be scaled to any size and otherwise transformed with more attractive results than bitmap fonts, but require considerably more processing and may yield undesirable rendering, depending on the font, rendering software, and output size. Even so, outline fonts can be transformed into bitmap fonts beforehand if necessary. The converse transformation is considerably harder since bitmap fonts require aheuristic algorithm to guess and approximate the corresponding curves if the pixels do not make a straight line.
Outline fonts have a major problem, in that theBézier curves used by them cannot be rendered accurately onto a raster display (such as most computer monitors and printers), and their rendering can change shape depending on the desired size and position.[2] Measures such asfont hinting have to be used to reduce the visual impact of this problem, which requires sophisticated software that is difficult to implement correctly. Many modern desktop computer systems include software to do this, but they use considerably more processing power than bitmap fonts, and there can be minor rendering defects, particularly at small font sizes. Despite this, they are frequently used because people often consider the ability to freely scale fonts, without incurring any pixelation, to be important enough to justify the defects and increasedcomputational complexity. These issues are however mostly solved by antialiasing (as described infont rasterization) and the high display resolutions that are commonly in use today.
A glyph's outline is defined by the vertices of individual stroke paths, and the corresponding stroke profiles. The stroke paths are a kind oftopological skeleton of the glyph. The advantages of stroke-based fonts over outline fonts include reducing the number of vertices needed to define a glyph, allowing the same vertices to be used to generate a font with a different weight, glyph width, or serifs using different stroke rules, and the associated size savings. For a font developer, editing a glyph by stroke is easier and less prone to error than editing outlines. A stroke-based system also allows scaling glyphs in height or width without altering stroke thickness of the base glyphs. Stroke-based fonts are heavily marketed for East Asian markets for use on embedded devices, but the technology is not limited toideograms.
Commercial developers includeAgfa Monotype (iType) and Type Solutions, Inc. (owned byBitstream Inc.) have independently developed stroke-based font types and font engines.
Although Monotype and Bitstream have claimed tremendous space saving using stroke-based fonts on East Asian character sets, most of the space saving comes from building composite glyphs, which is part of the TrueType specification and does not require a stroke-based approach.
There multiplefile formats for each file type.
Type 1 and Type 3 fonts were developed byAdobe for professional digital typesetting. UsingPostScript, the glyphs are outline fonts described with cubicBézier curves. Type 1 fonts were restricted to a subset of the PostScript language, and used Adobe's hinting system, which used to be very expensive. Type 3 allowed unrestricted use of the PostScript language, but did not include any hint information, which could lead to visible rendering artifacts on low-resolution devices (such as computer screens and dot-matrix printers).
TrueType is a font system originally developed byApple Inc. It was intended to replace Type 1 fonts, which many felt were too expensive. Unlike Type 1 fonts, TrueType glyphs are described with quadratic Bézier curves. It is currently very popular and implementations exist for all major operating systems.
OpenType is a smart font system designed byAdobe andMicrosoft. OpenType fonts contain outlines in either the TrueType orCFF format together with a wide range of metadata.
Metafont uses a different sort of glyph description. Like TrueType, it is a vector font description system. It draws glyphs using strokes produced by moving a polygonal or elliptical pen approximated by a polygon along a path made from cubiccomposite Bézier curves and straight line segments, or by filling such paths. Although when stroking a path the envelope of the stroke is never actually generated, the method causes no loss of accuracy or resolution. The method Metafont uses is more mathematically complex because theparallel curves of a Bézier can be 10th order algebraic curves.[5]
In 2004,DynaComware developed DigiType, a stroke-based font format. In 2006, the creators of theSaffron Type System announced a representation for stroke-based fonts calledStylized Stroke Fonts (SSFs) with the aim of providing the expressiveness of traditional outline-based fonts and the smallmemory footprint of uniform-width stroke-based fonts (USFs).[6]
A typical font may contain hundreds or even thousands of glyphs, often representing characters from many different languages. Oftentimes, users may only need a small subset of the glyphs that are available to them. Subsetting is the process of removing unnecessary glyphs from a font file, usually with the goal of reducing file size. This is particularly important for web fonts, since reducing file size often means reducing page load time and server load. Alternatively, fonts may be issued in different files for different regions of the world, though with the spread of the OpenType format this is now increasingly uncommon.
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