Cookies are enabled, but for the purposes of the same-origin policy, local content does not share a domain with any other content including other local content. Any specified targets are ignored. Other targets can be specified and are supported. This rule applies to any other block element inside the balloon as well.
In Google Earth release 4. See Example below. This element takes the form of:. If you prefer not to use the CDATA element, you can use entity references to replace all the special characters. If the href is a KML file and has a. When the user clicks a link that includes a fragment URL, by default the browser flies to the Feature whose ID matches the fragment.
The behavior can be further specified by appending one of the following three strings to the fragment URL:. For example, the following code indicates to open the file CraftsFairs. Specify the following:. For example, the following URL uses the type attribute to notify Google Earth that it should attempt to load the file, even though the file extension is.
A Feature is visible only if it and all its ancestors are visible. It provides a placeholder object for all derived Geometry objects. This element draws an image overlay draped onto the terrain. This file can be either on a local file system or on a web server. Defines an image associated with an Icon style or overlay. This location can either be on a local file system or a remote web server.
Specifies how icons for point Placemarks are drawn, both in the Places panel and in the 3D viewer of Google Earth. The root element of a KML file. This element is required. It follows the xml declaration at the beginning of the file. The hint attribute is used as a signal to Google Earth to display the file as celestial data. A custom color, color mode, and scale for the label name can be specified. Specifies the coordinates of the four corner points of a quadrilateral defining the overlay area.
Exactly four coordinate tuples have to be provided, each consisting of floating point values for longitude and latitude. Insert a space between tuples. Do not include spaces within a tuple. The coordinates must be specified in counter-clockwise order with the first coordinate corresponding to the lower-left corner of the overlayed image. The shape described by these corners must be convex. If a third value is inserted into any tuple representing altitude it will be ignored.
Defines a closed line string, typically the outer boundary of a Polygon. Optionally, a LinearRing can also be used as the inner boundary of a Polygon to create holes in the Polygon. Defines a connected set of line segments. When a LineString is extruded, the line is extended to the ground, forming a polygon that looks somewhat like a wall or fence.
Specifies the drawing style color, color mode, and line width for all line geometry. Line geometry includes the outlines of outlined polygons and the extruded "tether" of Placemark icons if extrusion is enabled. Google Earth version 6. Google Earth 6. For example, a label with seven characters "example" will only display if the line is greater than m. The file is conditionally loaded and refreshed, depending on the refresh parameters supplied here. When a file is fetched, the URL that is sent to the server is composed of three pieces of information:.
In Google Earth releases 3. Specifies how a Feature is displayed in the list view. The list view is a hierarchy of containers and children; in Google Earth, this is the Places panel. Defines a virtual camera that is associated with any element derived from Feature. The LookAt element positions the "camera" in relation to the object that is being viewed. In Google Earth, the view "flies to" this LookAt viewpoint when the user double-clicks an item in the Places panel or double-clicks an icon in the 3D viewer.
Models are created in their own coordinate space and then located, positioned, and scaled in Google Earth. A multi-track element is used to combine multiple track elements into a single conceptual unit. For example, suppose you collect GPS data for a day's bike ride that includes several rest stops and a stop for lunch.
Because of the interruptions in time, one bike ride might appear as four different tracks when the times and positions are plotted. When the icon reaches the end of one segment, it moves to the beginning of the next segment. Within that element, you can define the refresh options for updating the file, based on time and camera change.
NetworkLinks can be used in combination with Regions to handle very large datasets efficiently. Without this ID, only the child object names are displayed in the List View. This is an abstract base class and cannot be used directly in a KML file. It provides the id attribute, which allows unique identification of a KML element, and the targetId attribute, which is used to reference objects that have already been loaded into Google Earth.
This element also includes specifications for stacking order of multiple overlays and for adding color and transparency values to the base image. The PhotoOverlay can be a simple 2D rectangle, a partial or full cylinder, or a sphere for spherical panoramas. The overlay is placed at the specified location and oriented toward the viewpoint. The Camera or LookAt specifies a viewpoint and a viewing direction also referred to as a view vector.
The PhotoOverlay is positioned in relation to the viewpoint. Specifically, the plane of a 2D rectangular image is orthogonal at right angles to the view vector. The normal of this plane—that is, its front, which is the part with the photo—is oriented toward the viewpoint. A Placemark is a Feature with associated Geometry. In Google Earth, a Placemark appears as a list item in the Places panel.
A Placemark with a Point has an icon associated with it that marks a point on the Earth in the 3D viewer. Other Geometry objects do not have an icon in the 3D viewer. To give the user something to click in the 3D viewer, you would need to create a MultiGeometry object that contains both a Point and the other Geometry object.
A geographic location defined by longitude, latitude, and optional altitude. When a Point is contained by a Placemark, the point itself determines the position of the Placemark's name and icon. When a Point is extruded, it is connected to the ground with a line. This "tether" uses the current LineStyle. A Polygon is defined by an outer boundary and 0 or more inner boundaries. The boundaries, in turn, are defined by LinearRings.
When a Polygon is extruded, its boundaries are connected to the ground to form additional polygons, which gives the appearance of a building or a box. Polygons follow the "right-hand rule," which states that if you place the fingers of your right hand in the direction in which the coordinates are specified, your thumb points in the general direction of the geometric normal for the polygon.
In 3D graphics, the geometric normal is used for lighting and points away from the front face of the polygon. Since Google Earth fills only the front face of polygons, you will achieve the desired effect only when the coordinates are specified in the proper order. Otherwise, the polygon will be gray. Specifies the drawing style for all polygons, including polygon extrusions which look like the walls of buildings and line extrusions which look like solid fences.
A Region is said to be "active" when the bounding box is within the user's view and the LOD requirements are met. Objects associated with a Region are drawn only when the Region is active. In a Container or NetworkLink hierarchy, this calculation uses the Region that is the closest ancestor in the hierarchy.
Defines a square in screen space, with sides of the specified value in pixels. For example, defines a square of x pixels. The region's bounding box must be larger than this square and smaller than the maxLodPixels square in order for the Region to be active. This ramp value, expressed in screen pixels, is applied at the minimum end of the LOD visibility limits. This ramp value, expressed in screen pixels, is applied at the maximum end of the LOD visibility limits. The "id" attribute is required and must be unique within the KML file.
A Schema element contains one or more SimpleField elements. In the SimpleField, the Schema declares the type and name of the custom field. It optionally specifies a displayName the user-friendly form, with spaces and proper punctuation used for display in Google Earth for this custom field. This element draws an image overlay fixed to the screen. Sample uses for ScreenOverlays are compasses, logos, and heads-up displays. To force the image to retain its horizontal dimension, but to take up 20 percent of the vertical screen space:.
The following example places an image at the exact center of the screen, using the original width, height, and aspect ratio of the image. A Style defines an addressable style group that can be referenced by StyleMaps and Features. Styles affect how Geometry is presented in the 3D viewer and how Features appear in the Places panel of the List view.
The StyleMap element selects a style based on the current mode of the Placemark. An element derived from StyleSelector is uniquely identified by its id and its url. The value can be expressed as yyyy-mm-dd T hh:mm:ss. Additionally, the value can be expressed as a date only. The following example shows the time period representing Colorado's statehood. Represents a single moment in time. This is a simple element and contains no children.
Time values are used to control historical imagery , sunlight, and visibility of time-stamped Features. Learn more about tours. Elements extended from gx:TourPrimitive provide instructions to KML browsers during tours , including points to fly to and the duration of those flights, pauses, updates to KML features, and sound files to play.
A track describes how an object moves through the world over a given time period. This feature allows you to create one visible object in Google Earth either a Point icon or a Model that encodes multiple positions for the same object for multiple times.
In Google Earth, the time slider allows the user to move the view through time, which animates the position of the object. A gx:MultiTrack element is used to collect multiple tracks into one conceptual unit with one associated icon or Model that moves along the track.
This feature is useful if you have multiple tracks for the same real-world object. If this value is 0, then the point or Model stops at the end of one track and jumps to the start of the next one. For example, if you want a single placemark to represent your travels on two days, and your GPS unit was turned off for four hours during this period, you would want to show a discontinuity between the points where the unit was turned off and then on again.
However, you could only associate one time element with a given Feature. Tracks are a more efficient mechanism for associating time data with visible Features, since you create only one Feature, which can be associated with multiple time elements as the object moves through space. With this new feature, Google Earth displays a graph of elevation and speed profiles plus custom data, if present for the object over time.
This behavior also applies to ExtendedData for a track. The number of elements specified should equal the number of time and position elements. Bicycle rides, for example, could include data for heart rate, cadence, and power, as shown in Example of Track with Extended Data.
In Google Earth, custom data is displayed in the Elevation Profile for the track. The number of time and position values must be equal. The boldface type in this example highlights the elements used to define and specify custom data for a bike ride. The custom data fields are internally named "heartrate," "cadence," and "power. In Google Earth, this custom data is shown with the elevation profile for the track. This example is a more realistic presentation of a track, with custom icons and separate icon and line styles for highlight and normal modes.
Note, however, that the example includes only seven sets of data values. The actual example includes tens of thousands of values. Data courtesy of Sean Broeder. This data was collected with a Garmin Edge with associated heart rate monitor and power meter. This example creates a new Placemark in a previously created Document that has an id of "region This example deletes a Placemark previously loaded into Google Earth. This Placemark may have been loaded directly by a NetworkLink with the specified URL, or it may have been loaded by a subsequent Update to the original Document.
Use this element to set the location of the link to the KML file, to define the refresh options for the server and viewer changes, and to populate a variable to return useful client information to the server. Except as otherwise noted, the content of this page is licensed under the Creative Commons Attribution 4. For details, see the Google Developers Site Policies. Keyhole Markup Language.
Home Guides Reference Samples Support. Compatibility KML versions have a double numbering system: majorVersion. About this reference Each reference entry includes a Syntax section that lists the elements contained in the main element. This section also contains the following: default values for each element or ellipses if it is a complex element or if there is no default value the type of the value see KML Fields The Syntax section can be copied and used as a template for any non-abstract element in a KML file.
The name specifies one of the following: Street View imagery "streetview" , historical imagery "historicalimagery" , and sunlight effects for a given time of day "sunlight". The enabled attribute is used to turn a given viewing mode on or off. This element has no effect on AbstractViews outside of a tour. Regular AbstractViews are assigned a value of 60 ; views within Street View are assigned a value of 85 to match the standard Street View field of view in Google Earth.
Once set, the value will be applied to subsequent views, until a new value is specified. If the KML feature is above land rather than sea, the altitude will be interpreted as being above the ground. Example The example below demonstrates a change in icon size. Color and opacity alpha values are expressed in hexadecimal notation.
The range of values for any one color is 0 to 00 to ff. For alpha, 00 is fully transparent and ff is fully opaque. The default is opaque white ffffffff. The default is black ff Google Earth looks in the current Feature for the corresponding string entity and substitutes that information in the balloon. This description balloon opens when the Placemark is loaded.
The X axis points toward the right of the camera and is called the right vector. The Y axis defines the "up" direction relative to the screen and is called the up vector. The Z axis points from the center of the screen toward the eye point. Order of Transformations The order of rotation is important. Note that each time a rotation is applied, two of the camera axes change their orientation.
Angular distance in degrees, relative to the Prime Meridian. Values east of the Meridian range from 0 to degrees. Degrees north or south of the Equator 0 degrees. See diagram. Values range from 0 to degrees. A value of 0 indicates that the view is aimed straight down toward the earth the most common case. Values greater than 90 indicate that the view is pointed up into the sky. If the point is on land rather than at sea, the Camera will be positioned on the ground. If you specify a single color component for example, a value of ffff for red , random color values for that one component red will be selected.
In this case, the values would range from 00 black to ff full red. If you specify values for two or for all three color components, a random linear scale is applied to each color component, with results ranging from black to the maximum values specified for each component. It is recommended that you use shared styles, which require the following steps: Define all Styles in a Document. Assign a unique ID to each Style.
Note that shared styles are not inherited by the Features in the Document. The following example illustrates use of a shared style. The name can have two versions: name and displayName. The name attribute is used to identify the data pair within the KML file. The Schema element identified by the schemaUrl attribute declares the custom data type. The actual data objects "instances" of the custom data are defined using the SchemaData element. The ExtendedData element is a child of the Feature that contains the custom data.
Custom data added in this manner is preserved in the KML file but is not used by Google Earth in any way. It is always saved along with the file. Specifies whether the feature is drawn in the 3D viewer when it is initially loaded. Specifies whether a Document or Folder appears closed or open when first loaded into the Places panel. This element applies only to Document, Folder, and NetworkLink. This element is used by Google Maps Mobile only. The industry standard for Java-enabled cellular phones is RFC In Google Earth, this description is displayed in the Places panel under the name of the feature.
If the style is in the same file, use a reference. If the style is defined in an external file, use a full URL along with referencing. A style defined within a Feature is called an "inline style" and applies only to the Feature that contains it.
In cases where a style element is defined both in a shared style and in an inline style for a Feature—that is, a Folder, GroundOverlay, NetworkLink, Placemark, or ScreenOverlay—the value for the Feature's inline style takes precedence over the value for the shared style. A given KML Feature can contain a combination of these types of custom data. Smooth FlyTos allow for an unbroken flight from point to point to point and on. An unbroken series of smooth FlyTos will begin and end at zero velocity, and will not slow at each point.
Bounce FlyTos each begin and end at zero velocity. Possible values are clampToGround - default Indicates to ignore the altitude specification and drape the overlay over the terrain. For example, if you set the altitude of an overlay to 10 meters with an absolute altitude mode, the overlay will appear to be at ground level if the terrain beneath is also 10 meters above sea level.
If the terrain is 3 meters above sea level, the overlay will appear elevated above the terrain by 7 meters. If the point is on land rather than at sea, the overlay will be positioned on the ground. The second group of chapters,  ,  and  are the programming guide. The chapters  and  explain how to customize the network graphics and how to write NED source code comments from which documentation can be generated. Chapter  is devoted to the support of distributed execution.
The appendices provide a reference on the NED language, configuration options, file formats, and other details. Simple modules can be grouped into compound modules and so forth; the number of hierarchy levels is unlimited. Messages can be sent either via connections that span modules or directly to other modules.
The concept of simple and compound modules is similar to DEVS atomic and coupled models. In Fig. Arrows connecting small boxes represent connections and gates. Figure: Simple and compound modules. Figure: The Node compound module. Figure: a simple module output gate, b compound module output gate, c simple module input gate, d compound module input gate. Figure: cObject is the base class for most of the simulation library. Figure: cQueue : insertion and removal.
The numbers in boxes represent the observation count values. Figure: Density estimation from the k-split cell tree. Figure: cFigure class hierarchy. For example, the unit property of parameters is not allowed to be overridden, and display is merged with special although similar rules see Chapter . In NED, a type may only extend extends keyword an element of the same component type: a simple module may extend a simple module, a channel may extend a channel, a module interface may extend a module interface, and so on.
Single inheritance is supported for modules and channels, and multiple inheritance is supported for module interfaces and channel interfaces. A network is a shorthand for a compound module with the isNetwork property set, so the same rules apply to it as to compound modules. However, a simple or compound module type may implement like keyword several module interfaces; likewise, a channel type may implement several channel interfaces.
Inheritance may: add new properties, parameters, gates, inner types, submodules, connections, as long as names do not conflict with inherited names modify inherited properties, and properties of inherited parameters and gates it may not modify inherited submodules, connections and inner types For details and examples, see the corresponding sections of this chapter simple modules [3. When a project grows, however, it sooner or later becomes necessary to introduce a directory structure, and sort the NED files into them.
Packages are also useful for reducing name conflicts, because names can be qualified with the package name. If you are familiar with Java, you'll find little surprise in this section. The simulation kernel will traverse the whole directory tree, and load all NED files from every directory. Directories in a NED source tree correspond to packages. The package name has to be explicitly declared at the top of the NED files as well, like this: package a. The only exception is the root package.
By convention, package names are all lowercase, and begin with either the project name myproject , or the reversed domain name plus the project name org. The latter convention would cause the directory tree to begin with a few levels of empty directories, but this can be eliminated with a toplevel package.
NED files called package. For example, comments in package. Also, a namespace property in a package. The toplevel package. For example, given a project where all NED types are under the org. This will cause a directory foo under the root to be interpreted as package org. Only the root package. The name that includes the package name a. Queue for a Queue module in the a. Simple names alone are not enough to unambiguously identify a type.
Here is how one can refer to an existing type: By fully qualified name. This is often cumbersome though, as names tend to be too long; Import the type, then the simple name will be enough; If the type is in the same package, then it doesn't need to be imported; it can be referred to by simple name Types can be imported with the import keyword by either fully qualified name, or by a wildcard pattern.
So, any of the following lines can be used to import a type called inet. RoutingTable : import inet. RoutingTable; import inet. RoutingTable; If an import explicitly names a type with its exact fully qualified name, then that type must exist, otherwise it is an error.
Imports containing wildcards are more permissive, it is allowed for them not to match any existing NED type although that might generate a warning. Inner types may not be referred to outside their enclosing types, so they cannot be imported either. Imports are not much use here: at the time of writing the NED file it is not yet known what NED types will be suitable for being "plugged in" there, so they cannot be imported in advance.
There is no problem with fully qualified names, but simple names need to be resolved differently. What NED does is this: it determines which interface the module or channel type must implement i. There must be exactly one such type, which is then used. If there is none or there are more than one, it will be reported as an error. RandomWalk , inet. Also suppose that there is a type called inet. MassMobility but it does not implement the interface. MassMobility will be selected; the other MassMobility doesn't interfere.
Those files are said to be in the default package. It is assumed that nothing i. This is in contrast to continuous systems where state changes are continuous. Systems that can be viewed as discrete event systems can be modeled using discrete event simulation, DES. For example, computer networks are usually viewed as discrete event systems. Some of the events are: start of a packet transmission end of a packet transmission expiry of a retransmission timeout This implies that between two events such as start of a packet transmission and end of a packet transmission , nothing interesting happens.
That is, the packet's state remains being transmitted. If we were interested in the transmission of individual bits, we would have included something like start of bit transmission and end of bit transmission among our events. Time within the model is often called simulation time , model time or virtual time as opposed to real time or CPU time which refer to how long the simulation program has been running and how much CPU time it has consumed.
The initialization step usually builds the data structures representing the simulation model, calls any user-defined initialization code, and inserts initial events into the FES to ensure that the simulation can start. Initialization strategies can differ considerably from one simulator to another. The subsequent loop consumes events from the FES and processes them.
Events are processed in strict timestamp order to maintain causality, that is, to ensure that no current event may have an effect on earlier events. Processing an event involves calls to user-supplied code. The user code may also remove events from the FES, for example when canceling timeouts. The simulation stops when there are no events left this rarely happens in practice , or when it isn't necessary for the simulation to run further because the model time or the CPU time has reached a given limit, or because the statistics have reached the desired accuracy.
At this time, before the program exits, the user will typically want to record statistics into output files. Note that there is a class called cEvent that cMessage subclasses from, but it is only used internal to the simulation kernel. Events are consumed from the FES in arrival time order, to maintain causality. More precisely, given two messages, the following rules apply: The message with the earlier arrival time is executed first. If arrival times are equal, the one with the higher scheduling priority smaller numeric value is executed first.
Scheduling priority is a user-assigned integer attribute of messages. SimTime class stores simulation time in a bit integer, using decimal fixed-point representation. The resolution is controlled by the scale exponent global configuration variable; that is, SimTime instances have the same resolution. The exponent can be chosen between attosecond resolution and 0 seconds. Some exponents with the ranges they provide are shown in the following table.
Regards the input values and their timestamps as a step function sample-hold style , and computes and outputs its time average integral divided by duration. Expects cPacket pointers as value, and outputs the bit length for each received one. Non- cPacket values are ignored.
Expects cPacket pointers as value, and outputs the byte length for each received one. For each value, computes the sum of values received so far, divides it by the duration, and outputs the result. Removes repeated values, i. Records the count of the input values into an output scalar; functionally equivalent to last count. Records the sum of the input values into an output scalar or zero if there was none ; functionally equivalent to last sum.
Records the minimum of the input values into an output scalar or positive infinity if there was none ; functionally equivalent to last min. Records the maximum of the input values into an output scalar or negative infinity if there was none ; functionally equivalent to last max. Records the mean of the input values into an output scalar or NaN if there was none ; functionally equivalent to last mean.
Regards the input values with their timestamps as a step function sample-hold style , and records the time average of the input values into an output scalar; functionally equivalent to last timeavg. Computes basic statistics count, mean, std. Computes a histogram and basic statistics count, mean, std.
Part of the class library's functionality has already been covered in the previous chapters, including discrete event simulation basics, the simple module programming model, module parameters and gates, scheduling events, sending and receiving messages, channel operation and programming model, finite state machines, dynamic module creation, signals, and more.
This chapter discusses the rest of the simulation library. Topics will include logging, random number generation, queues, topology discovery and routing support, and statistics and result collection. This chapter also covers some of the conventions and internal mechanisms of the simulation library to allow one extending it and using it to its full potential. Otherwise, cObject is a zero-overhead class as far as memory consumption goes: it purely defines an interface but has no data members.
Thus, having cObject a base class does not add anything to the size of a class if it already has at least one virtual member function.
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A 2-bit system uses combinations of numbers up to two place values There are four options: 00, 01, 10 and A 1-bit image can have 2 colours, a 4-bit image can have 16, an 8-bit image can have , and a bit image can have 65, These tables show how many binary combinations are available for each bit size. Bit number patterns Computer systems and files have limits that are measured in bits. Bit depth Max binary Max denary Combinations available 1 1 1 2 2 11 3 4 3 7 8 4 15 16 5 31 32 A 1-bit image can have 2 colours, a 4-bit image can have 16, an 8-bit image can have , and a bit image can have 65, Binary combinations These tables show how many binary combinations are available for each bit size.
One bit. An index table, also called the cross-reference table, is typically located near the end of the file and gives the byte offset of each indirect object from the start of the file. Before PDF version 1. Version 1. Such a stream may be used instead of the ASCII cross-reference table and contains the offsets and other information in binary format. If a cross-reference stream is not being used, the footer is preceded by the trailer keyword followed by a dictionary containing information that would otherwise be contained in the cross-reference stream object's dictionary:.
There are two layouts to the PDF files: non-linear not "optimized" and linear "optimized". Non-linear PDF files can be smaller 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 generated in a linear as in page order fashion.
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 1. 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 1. Paths can be stroked, filled, and used for 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.
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 1. There are seven types of shading patterns of which the simplest are the axial shade Type 2 and radial shade Type 3. The dictionary describes the 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 following general-purpose filters:. Normally all image content in a PDF is embedded in the file. But PDF allows image data to be stored in external files by the use of external streams or Alternate Images. 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. 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.
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 1. Fourteen typefaces, known as the standard 14 fonts , have a special significance in PDF documents:.
These fonts are sometimes called the base fourteen fonts. 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 many 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 table of differences to a predefined or built-in encoding not recommended with TrueType fonts. 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. 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 1. 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 PDF 1.
As a result, files that use a small amount of transparency might view acceptably by older viewers, but files making extensive use of transparency could be viewed incorrectly by an older viewer 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 9. The blend modes were based on those used by Adobe Photoshop at the time. When the PDF 1.
They have since been published. 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. A "tagged" PDF see clause Technically speaking, tagged PDF is a stylized use of the format that builds on the logical structure framework introduced in PDF 1.
Tagged PDF defines a set of standard structure types and attributes that allow page content text, graphics, and images to be extracted and reused for other purposes. With the introduction of PDF version, 1. Layers, or as they are more formally known Optional Content Groups OCGs , refer to sections of content in a PDF document that can be selectively viewed or hidden by document authors or consumers.
This capability is useful in CAD drawings, layered artwork, maps, multi-language documents, etc. Basically, it consists of an Optional Content Properties Dictionary added to the document root. This dictionary contains an array of Optional Content Groups OCGs , each describing a set of information and each of which may be individually displayed or suppressed, plus a set of Optional Content Configuration Dictionaries, which give the status Displayed or Suppressed of the given OCGs.
A PDF file may be encrypted , for security, in which case a password is needed to view or edit the contents. PDF 2. PDF files may be digitally signed, to provide secure authentication; complete details on implementing digital signatures in PDF is provided in ISO PDF files may also contain embedded DRM restrictions that provide further controls that limit copying, editing or printing.
These restrictions depend on the reader software to obey them, so the security they provide is limited. The standard security provided by Acrobat PDF consists of two different methods and two different passwords: a user password , which encrypts the file and prevents opening, and an owner password , which specifies operations that should be restricted even when the document is decrypted, which can include modifying, printing, or copying text and graphics out of the document, or adding or modifying text notes and AcroForm fields.
The user password encrypts the file, while the owner password does not, instead of relying on client software to respect these restrictions. An owner password can easily be removed by software, including some free online services. Even without removing the password, most freeware or open source PDF readers ignore the permission "protections" and allow the user to print or make copy of excerpts of the text as if the document were not limited by password protection. The signature is used to validate that the permissions have been granted by a bona fide granting authority.
For example, it can be used to allow a user: . For example, Adobe Systems grants permissions to enable additional features in Adobe Reader, using public-key cryptography. Adobe Reader verifies that the signature uses a certificate from an Adobe-authorized certificate authority. Any PDF application can use this same mechanism for its own purposes. Under specific circumstances including non- patched systems of the receiver, the information the receiver of a digital signed document sees can be manipulated by the sender after the document has been signed by the signer.
PDF files can have file attachments which processors may access and open or save to a local filesystem. PDF files can contain two types of metadata. This is stored in the optional Info trailer of the file. A small set of fields is defined, and can be extended with additional text values if required. This method is deprecated in PDF 2.
This allows metadata to be attached to any stream in the document, such as information about embedded illustrations, as well as the whole document attaching to the document catalog , using an extensible schema. PDF documents can contain display settings, including the page display layout and zoom level. Adobe Reader uses these settings to override the user's default settings when opening the document. PDF files can be created specifically to be accessible for people with disabilities.
Some software can automatically produce tagged PDFs , but this feature is not always enabled by default. Adding tags to older PDFs and those that are generated from scanned documents can present some challenges. One of the significant challenges with PDF accessibility is that PDF documents have three distinct views, which, depending on the document's creation, can be inconsistent with each other.
The three views are i the physical view, ii the tags view, and iii the content view. The physical view is displayed and printed what most people consider a PDF document. The tags view is what screen readers and other assistive technologies use to deliver high-quality navigation and reading experience to users with disabilities.
The content view is based on the physical order of objects within the PDF's content stream and may be displayed by software that does not fully support the tags' view, such as the Reflow feature in Adobe's Reader. Interactive Forms is a mechanism to add forms to the PDF file format. Both formats today coexist in the PDF specification:    .
AcroForms were introduced in the PDF 1. AcroForms permit using objects e. Alongside the standard PDF action types, interactive forms AcroForms support submitting, resetting, and importing data. The "submit" action transmits the names and values of selected interactive form fields to a specified uniform resource locator URL. AcroForms can keep form field values in external stand-alone files containing key:value pairs.
Anyone may create applications that can read and write PDF files without having to pay royalties to Adobe Systems ; Adobe holds patents to PDF, but licenses them for royalty-free use in developing software complying with its PDF specification.
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