They have since been published. 22 The concept of a transparency group in pdf specification is independent of existing notions of "group" or "layer" in applications such as Adobe Illustrator. Those groupings reflect logical relationships among objects that are meaningful when editing those objects, but they are not part of the imaging model. Interactive elements edit pdf files may contain interactive elements such as annotations, form fields, video, 3D and rich media. Rich Media pdf is a pdf file including interactive content that can be embedded or linked within the file. Interactive forms is a mechanism to add forms to the pdf file format. Pdf currently supports two different methods for integrating data and pdf forms. Both formats today coexist in pdf specification: AcroForms (also known as Acrobat forms introduced in the pdf.2 format specification and included in all later pdf specifications.
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For large fonts or fonts with non-standard glyphs, the special encodings Identity-h (for horizontal writing) and Identity-v (for vertical) are used. With such fonts it is necessary to provide a tounicode table if semantic information about the characters is to be preserved. Transparency edit The original imaging model of pdf was, like postScript's, opaque : each object drawn on the page completely replaced anything previously marked in the same location. In pdf.4 the imaging model was extended to allow transparency. When transparency is used, new objects interact with previously marked objects to produce blending effects. The addition of transparency to pdf was done by means of new extensions that were designed to be ignored in products written to the pdf.3 and earlier specifications. As a result, files that use a small amount of transparency might view acceptably in older viewers, but files making extensive use of transparency could be viewed incorrectly in an older viewer welder without warning. The transparency extensions are based on the key concepts of transparency groups, blending modes, shape, and alpha. The model is closely aligned with the features of Adobe Illustrator version. The blend modes tale were based on those used by Adobe Photoshop at the time. When the pdf.4 specification was published, the formulas for calculating blend modes were kept secret by Adobe.
The font files that may be embedded are based on widely used standard digital font formats: Type 1 (and its compressed variant cff truetype, and (beginning with pdf.6) OpenType. Additionally pdf supports the type 3 variant in which the components of the font are described by pdf graphic operators. Standard Type 1 Fonts (Standard 14 Fonts) edit fourteen typefaces, known as the standard 14 fonts, have a special significance in pdf documents: Times (v3) (in regular, italic, bold, and bold assignment italic) courier (in regular, oblique, bold and bold oblique) Helvetica (v3) (in regular, oblique. 19 These fonts, or suitable substitute fonts with the same metrics, should be available in most pdf readers, but they are not guaranteed to be available in the reader, and may only display correctly if the system has them installed. 20 Fonts may be substituted if they are not embedded in a pdf. Encodings edit within text strings, characters are shown using character codes (integers) that map to glyphs in the current font using an encoding. There are a number of predefined encodings, including WinAnsi, macRoman, and a large number of encodings for East Asian languages, and a font can have its own built-in encoding. (Although the winAnsi and MacRoman encodings are derived from the historical properties of the windows and Macintosh operating systems, fonts using these encodings work equally well on any platform.) pdf can specify a predefined encoding to use, the font's built-in encoding or provide a lookup. 21 The encoding mechanisms in pdf were designed for Type 1 fonts, and the rules for applying them to Truetype fonts are complex.
Standardized subsets of night pdf, including pdf/A and pdf/x, prohibit these features. Text edit text in pdf is represented by text elements in page content streams. A text element specifies that characters should be drawn at certain positions. The characters are specified using the encoding of a selected font resource. Fonts edit a font object in pdf is a description of a digital typeface. It may either describe the characteristics of a typeface, or it may include an embedded font file. The latter case is called an embedded font while the former is called an unembedded font.
This may be a colored tiling pattern, with the colors specified in the pattern object, or an uncolored tiling pattern, which defers color specification to the time the pattern is drawn. Beginning with pdf.3 there is also a shading pattern, which draws continuously varying colors. There are seven types of shading pattern of which the simplest are the axial shade (Type 2) and radial shade (Type 3). Raster images edit raster images in pdf (called Image xobjects ) are represented by dictionaries with an associated stream. The dictionary describes properties of the image, and the stream contains the image data. (Less commonly, a raster image may be embedded directly in a page description as an inline image.) Images are typically filtered for compression purposes. Image filters supported in pdf include the general purpose filters ascii85Decode a filter used to put the stream into 7-bit ascii asciihexDecode similar to ascii85Decode but less compact FlateDecode a commonly used filter based on the deflate algorithm defined in rfc 1951 (deflate is also. But pdf allows image data to be stored in external files by the use of external streams or Alternate Images.
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18 pdf files may be optimized using Adobe Acrobat software or qpdf. Imaging model edit The basic design of how graphics are represented in pdf is very similar to that of PostScript, except for the use of transparency, which was added in pdf.4. Pdf graphics use a device-independent Cartesian coordinate system to describe the surface of a page. A pdf page description can use a matrix to scale, rotate, or skew graphical elements. A key concept in pdf is that of the graphics state, which is a collection of graphical parameters that may be changed, saved, and restored by a page description.
Pdf has (as of version.6) 24 graphics state properties, of which some of the most important are: The current transformation matrix (ctm which determines the coordinate system The clipping path The color space The alpha constant, which is a key component of transparency vector. Paths are usually composed of lines and cubic bézier curves, but can also be constructed from the outlines of text. Unlike postScript, pdf does not allow a single path to mix text outlines with lines and curves. Paths can be stroked, filled, clipping. Strokes and fills can use any color set in the graphics state, including patterns. Pdf supports several types of patterns. The simplest is the tiling pattern in which a piece of artwork is specified to be drawn repeatedly.
Comments may contain 8-bit characters. Objects may be either direct (embedded in another object) or indirect. Indirect objects are numbered with an object number and a generation number and defined between the obj and endobj keywords. An index table, also called the cross-reference table and marked with the xref keyword, follows the main body and gives the byte offset of each indirect object from the start of the file. 17 This design allows for efficient random access to the objects in the file, and also allows for small changes to be made without rewriting the entire file ( incremental update ). Beginning with pdf version.5, indirect objects may also be located in special streams known as object streams.
This technique reduces the size of files that have large numbers of small indirect objects and is especially useful for Tagged pdf. At the end of a pdf file is a trailer introduced with the trailer keyword. It contains A dictionary An offset to the start of the cross-reference table (the table starting with the xref keyword) And the eof end-of-file marker. The dictionary contains A reference to the root object of the tree structure, also known as the catalog The count of indirect objects in the cross-reference table And other optional information. There are two layouts to the pdf files: non-linear (not "optimized and linear optimized. Non-linear pdf files consume less disk space than their linear counterparts, though they are slower to access because portions of the data required to assemble pages of the document are scattered throughout the pdf file. Linear pdf files (also called "optimized" or "web optimized" pdf files) are constructed in a manner that enables them to be read in a web browser plugin without waiting for the entire file to download, since they are written to disk in a linear (as.
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Therefore, all preceding pages in a postScript document must be processed to determine the correct appearance of a given page, whereas each business page in a pdf document is unaffected by the others. As a result, pdf viewers allow the user to quickly jump to the final pages of a long document, whereas a postScript viewer needs to process all pages sequentially before being able to display the destination page (unless the optional PostScript Document Structuring Conventions have. Technical overview edit file structure edit a pdf file is a 7-bit ascii file, except for certain elements that may have binary content. A pdf file starts with a header containing the magic number and the version of the format such as pdf-1.7. The format is a subset of a cos carousel" Object Structure) format. 15 a cos tree file consists primarily of objects, of which there are eight types: 16 boolean values, representing true or false numbers Strings, enclosed within parentheses (. may contain 8-bit characters. Names, starting with a forward slash arrays, ordered collections of objects enclosed within square brackets (.) Dictionaries, collections of objects indexed by names enclosed within double pointy brackets (. ) Streams, usually containing large amounts of data, which can be compressed and binary The null object Furthermore, there may be comments, introduced with the percent sign.
The graphics commands that are output by the postScript code are collected and tokenized. Any files, graphics, or fonts to which the document refers also are collected. Then, everything is compressed to a single file. Therefore, the entire postScript world (fonts, layout, measurements) remains intact. As a document format, pdf has several advantages over PostScript: pdf contains tokenized and interpreted results of the postScript source code, for direct correspondence between changes to items in the pdf page description supplement and changes to the resulting page appearance. Pdf (from version.4) supports graphic transparency ; PostScript does not. PostScript is an interpreted programming language with an implicit global state, so instructions accompanying the description of one page can affect the appearance of any following page.
technologies: A subset of the postScript page description programming language, for generating the layout and graphics. A font-embedding /replacement system to allow fonts to travel with the documents. A structured storage system to bundle these elements and any associated content into a single file, with data compression where appropriate. PostScript edit postScript is a page description language run in an interpreter to generate an image, a process requiring many resources. It can handle graphics and standard features of programming languages such as if and loop commands. Pdf is largely based on PostScript but simplified to remove flow control features like these, while graphics commands such as lineto remain. Often, the postScript-like pdf code is generated from a source postScript file.
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For other uses, see, pDF (disambiguation). The, portable document Format pDF ) is a file format developed in the 1990s to summary present documents, including text formatting and images, in a manner independent of application software, hardware, and operating systems. Based on the, postScript language, each pdf file encapsulates a complete description of a fixed-layout flat document, including the text, fonts, vector graphics, raster images and other information needed to display. Pdf was standardized as an open format, iso 32000, in 2008, and does not require any royalties for its implementation. Today, pdf files may contain a variety of content besides flat text and graphics including logical structuring elements, interactive elements such as annotations and form-fields, layers, rich media (including video content) and three dimensional objects using. U3D or, prc, and various other data formats. Citation needed, the pdf specification also provides for encryption and digital signatures, file attachments and metadata to enable workflows requiring these features.