Perbezaan antara semakan "PostScript"

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Once the ''[[de facto]]'' standard for electronic distribution of final documents meant for publication, PostScript is steadily being supplanted in this area by one of its own descendants, the Portable Document Format or [[Portable Document Format|PDF]]. By 2001 there were fewer printer models which came with support for PostScript, largely due to the growing competition from much cheaper non-PostScript ink jet printers, and new software-based methods to render PostScript images on the computer, making them suitable for any printer (PDF provided one such method). The use of a PostScript laser printer still can, however, significantly reduce the CPU workload involved in printing documents, transferring the work of rendering PostScript images from the computer to the printer. PS is still an option on most "high end" models.
== Use in printing ==
=== Before PostScript ===
Prior to the introduction of PostScript, printers were designed to print character output given the text—typically in [[ASCII]]—as input. There were a number of technologies for this task, but most shared the property that the [[glyph]]s were physically difficult to change, as they were stamped onto [[typewriter]] keys, bands of metal, or optical plates.
This changed to some degree with the increasing popularity of [[dot matrix printer]]s. The characters on these systems were drawn as a series of dots, the proper dots to use defined as a [[typeface|font]] table inside the printer. As they grew in sophistication, dot matrix printers started including several built-in fonts from which the user could select, and some models allowed users to upload their own custom glyphs into the printer.
Dot matrix printers also introduced the ability to print [[raster graphics]]. The graphics were interpreted by the computer and sent as a series of dots to the printer using a series of [[escape sequence]]s. These [[printer control language]]s varied from printer to printer, requiring program authors to create numerous [[device driver|drivers]].
Vector graphics printing was left to special-purpose devices, called [[plotter]]s. Plotters did share a common command language, [[HPGL]], but were of limited use for anything other than printing graphics. In addition, they tended to be expensive and slow, and thus rare.
=== PostScript printing ===
Laser printers combine the best features of both printers and plotters. Like plotters, laser printers offer high quality line art, and like dot-matrix printers, they are able to generate pages of text and raster graphics. Unlike either printers or plotters, however, a laser printer makes it possible to position high-quality graphics and text on the same page. PostScript made it possible to fully exploit these characteristics, by offering a single control language that could be used on any brand of printer.
PostScript went beyond the typical printer control language and was a complete programming language of its own. Many applications can transform a document into a PostScript program whose execution will result in the original document. This program can be sent to an [[Interpreter (computer software)|interpreter]] in a printer, which results in a printed document, or to one inside another application, which will display the document on-screen. Since the document-program is the same regardless of its destination, it is called ''device-independent''.
PostScript is noteworthy for implementing on-the fly [[rasterization]]; everything, even text, is specified in terms of straight lines and cubic [[Bézier curve]]s (previously found only in [[CAD]] applications), which allows arbitrary scaling, rotating and other transformations. When the PostScript program is interpreted, the interpreter converts these instructions into the dots needed to form the output. For this reason PostScript interpreters are also sometimes called PostScript [[Raster Image Processor]]s, or RIPs.
=== Font handling ===
Almost as complex as PostScript itself was its handling of [[typeface|fonts]]. The rich font system used the PS graphics primitives to draw glyphs as [[line art]], which could then be rendered at any [[Image resolution|resolution]]. Though this sounds like a reasonably straightforward concept, there were a number of [[typographic]] issues that had to be considered.
One issue is that fonts do not actually scale linearly at small sizes; features of the glyphs will become proportionally too large or small and they start to look wrong. PostScript avoided this problem with the inclusion of [[font hinting|hint]]s which could be saved along with the font outlines. Basically they are additional information in horizontal or vertical bands that help identify the features in each letter that are important for the rasterizer to maintain. The result was significantly better-looking fonts even at low resolution; it had formerly been believed that hand-tuned bitmap fonts were required for this task.
At the time, the technology for including these hints in fonts was carefully guarded, and the hinted fonts were compressed and encrypted into what Adobe called a ''[[Type 1 Font]]'' (also known as ''PostScript Type 1 Font'', ''PS1'', ''T1'' or ''Adobe Type 1''). Type 1 was effectively a simplification of the PS system to store outline information only, as opposed to being a complete language (PDF is similar in this regard). Adobe would then sell licenses to the Type 1 technology to those wanting to add hints to their own fonts. Those who did not license the technology were left with the ''[[Type 3 Font]]'' (also known as ''PostScript Type 3 Font'', ''PS3'' or ''T3''). Type 3 fonts allowed for all the sophistication of the PostScript language, but without the standardized approach to hinting. Other differences further added to the confusion.
[[Type 2 font|Type 2]] was designed to be used with the [[Compact Font Format]] (CFF), and were implemented for a compact representation of the glyph description procedures to reduce the overall font file size. The [[CFF/Type 2 font|CFF/Type2]] format later became the basis for Type 1 [[OpenType]] fonts.
[[CID-keyed font]] format was also designed, to solve the problems in the [[OCF/Type 0 font]]s, for addressing the complex Asian-language ([[CJK]]) encoding and very large character set issues. CID-keyed font format can be used with the Type 1 font format for standard CID-keyed fonts, or Type 2 for CID-keyed OpenType fonts.
Adobe's rates were widely considered to be prohibitively high, and it was this issue{{Fact|date=February 2007}} that led Apple to design their own system, [[TrueType]], around 1991. Immediately following the announcement of TrueType, Adobe published the specification for the Type 1 font format. Retail tools such as Altsys [[Fontographer]] (acquired by [[Macromedia]] in January 1995, owned by [[FontLab]] since May 2005) added the ability to create Type 1 fonts. Since then, many free Type 1 fonts have been released; for instance, the fonts used with the [[TeX]] typesetting system are available in this format.
In the early 1990s there were several other systems for storing outline-based fonts, developed by [[Bitstream Inc.|Bitstream]] and [[METAFONT]] for instance, but none included a general-purpose printing solution and they were therefore not widely used as a result.
In the late 1990s, Adobe joined Microsoft in developing [[OpenType]], essentially a functional superset of the Type 1 and TrueType formats. When printed to a PostScript output device, the unneeded parts of the OpenType font are omitted, and what is sent to the device by the driver is the same as it would be for a TrueType or Type 1 font, depending on which kind of outlines were present in the OpenType font.
=== Other implementations ===
In the 1980s, Adobe drew most of its revenue from the licensing fees for their implementation of PostScript for printers, known as a [[raster image processor]] or ''RIP''. As a number of new [[RISC]]-based platforms became available in the mid 1980s, some found Adobe's support of the new machines to be lacking.
This and issues of cost led to third-party implementations of PostScript becoming common, particularly in low-cost printers (where the licensing fee was the sticking point) or in high-end typesetting equipment (where the quest for speed demanded support for new platforms faster than Adobe could provide). At one point, Microsoft and Apple teamed up to try to unseat Adobe's laser printer monopoly, Microsoft licensing to Apple a PostScript-compatible interpreter it had bought called [[TrueImage]], and Apple licensing to Microsoft its new font format, [[TrueType]] (Apple ended up reaching an accord with Adobe and licensed genuine PostScript for its printers, but TrueType became the standard [[outline font]] technology for both Windows and the Macintosh).
Today, third-party PostScript-compatible interpreters are widely used in printers and multifunction peripherals (MFPs). For example, [[Zoran Corporation]]'s IPS PS3<ref>[ IPS PS3]</ref> interpreter, formerly known as PhoenixPage, is standard in many printers and MFPs, including those developed by [[Hewlett-Packard]] and sold under the [[LaserJet]] and Color LaserJet lines. Other third-party PostScript solutions used by print and MFP manufacturers include Jaws<ref>[ Jaws]</ref> and Harlequin<ref>[ Harlequin]</ref>, both provided by [[Global Graphics]].
Still, some basic, inexpensive laser printers don't support PostScript, instead coming with drivers that simply rasterize the platform's native graphics formats rather than converting them to PostScript first. When PostScript support is needed for such a printer, a [[Free software|free]] PostScript-compatible interpreter called [[Ghostscript]] can be used. Ghostscript prints PostScript documents on non-PostScript printers using the [[Central processing unit|CPU]] of the host computer to do the rasterization, sending the result as a single large bitmap to the printer. Ghostscript can also be used to preview PostScript documents on a computer monitor and to convert PostScript pages into [[raster graphics]] such as [[TIFF]] and [[Portable Network Graphics|PNG]], and vector formats such as [[Portable Document Format|PDF]].
Very high-resolution devices, such as [[imagesetter]]s or [[Computer to plate|CTP]] [[platesetter]]s, in which resolutions exceeding 2500 dpi are common, still require external RIPs with large amounts of memory and hard drive space. Very high-end laser printer systems (known as digital presses) also use an external RIP to separate the more readily-upgradable computer from the specialized printing hardware. Companies such as [[EFI]] and [[Xitron]] specialize in such RIP software.
== Use as a display system ==