This chapter of the guide introduces the basic concepts of &GStreamer;. Understanding these concepts will help you grok the issues involved in extending &GStreamer;. Many of these concepts are explained in greater detail in the &GstAppDevMan;; the basic concepts presented here serve mainly to refresh your memory. Elements are at the core of &GStreamer;. In the context of plugin development, an element is an object derived from the GstElement class. Elements provide some sort of functionality when linked with other elements: For example, a source element provides data to a stream, and a filter element acts on the data in a stream. Without elements, &GStreamer; is just a bunch of conceptual pipe fittings with nothing to link. A large number of elements ship with &GStreamer;, but extra elements can also be written. Just writing a new element is not entirely enough, however: You will need to encapsulate your element in a plugin to enable &GStreamer; to use it. A plugin is essentially a loadable block of code, usually called a shared object file or a dynamically linked library. A single plugin may contain the implementation of several elements, or just a single one. For simplicity, this guide concentrates primarily on plugins containing one element. A filter is an important type of element that processes a stream of data. Producers and consumers of data are called source and sink elements, respectively. Bin elements contain other elements. One type of bin is responsible for scheduling the elements that they contain so that data flows smoothly. Another type of bin, called autoplugger elements, automatically add other elements to the bin and links them together so that they act as a filter between two arbitary stream types. The plugin mechanism is used everywhere in &GStreamer;, even if only the standard packages are being used. A few very basic functions reside in the core library, and all others are implemented in plugins. A plugin registry is used to store the details of the plugins in an XML file. This way, a program using &GStreamer; does not have to load all plugins to determine which are needed. Plugins are only loaded when their provided elements are requested. See the &GstLibRef; for the current implementation details of GstElement and GstPlugin. Pads are used to negotiate links and data flow between elements in &GStreamer;. A pad can be viewed as a place or port on an element where links may be made with other elements, and through which data can flow to or from those elements. Pads have specific data handling capabilities: A pad can restrict the type of data that flows through it. Links are only allowed between two pads when the allowed data types of the two pads are compatible. An analogy may be helpful here. A pad is similar to a plug or jack on a physical device. Consider, for example, a home theater system consisting of an amplifier, a DVD player, and a (silent) video projector. Linking the DVD player to the amplifier is allowed because both devices have audio jacks, and linking the projector to the DVD player is allowed because both devices have compatible video jacks. Links between the projector and the amplifier may not be made because the projector and amplifier have different types of jacks. Pads in &GStreamer; serve the same purpose as the jacks in the home theater system. For the most part, all data in &GStreamer; flows one way through a link between elements. Data flows out of one element through one or more source pads, and elements accept incoming data through one or more sink pads. Source and sink elements have only source and sink pads, respectively. See the &GstLibRef; for the current implementation details of a GstPad. All streams of data in &GStreamer; are chopped up into chunks that are passed from a source pad on one element to a sink pad on another element. Data are structures used to hold these chunks of data. Data contains the following important types: An exact type indicating what type of data (control, content, ...) this Data is. A reference count indicating the number of elements currently holding a reference to the buffer. When the buffer reference count falls to zero, the buffer will be unlinked, and its memory will be freed in some sense (see below for more details). There are two types of data defined: events (control) and buffers (content). Buffers may contain any sort of data that the two linked pads know how to handle. Normally, a buffer contains a chunk of some sort of audio or video data that flows from one element to another. Buffers also contain metadata describing the buffer's contents. Some of the important types of metadata are: A pointer to the buffer's data. An integer indicating the size of the buffer's data. A timestamp indicating the preferred display timestamp of the content in the buffer. Events contain information on the state of the stream flowing between the two linked pads. Events will only be sent if the element explicitely supports them, else the core will (try to) handle the events automatically. Events are used to indicate, for example, a clock discontinuity, the end of a media stream or that the cache should be flushed. Events may contain several of the following items: A subtype indicating the type of the contained event. The other contents of the event depend on the specific event type. Events will be discussed extensively in . Until then, the only event that will be used is the EOS event, which is used to indicate the end-of-stream (usually end-of-file). See the &GstLibRef; for the current implementation details of a GstMiniObject, GstBuffer and GstEvent. Buffers are able to store chunks of memory of several different types. The most generic type of buffer contains memory allocated by malloc(). Such buffers, although convenient, are not always very fast, since data often needs to be specifically copied into the buffer. Many specialized elements create buffers that point to special memory. For example, the filesrc element usually maps a file into the address space of the application (using mmap()), and creates buffers that point into that address range. These buffers created by filesrc act exactly like generic buffers, except that they are read-only. The buffer freeing code automatically determines the correct method of freeing the underlying memory. Downstream elements that recieve these kinds of buffers do not need to do anything special to handle or unreference it. Another way an element might get specialized buffers is to request them from a downstream peer. These are called downstream-allocated buffers. Elements can ask a peer connected to a source pad to create an empty buffer of a given size. If a downstream element is able to create a special buffer of the correct size, it will do so. Otherwise &GStreamer; will automatically create a generic buffer instead. The element that requested the buffer can then copy data into the buffer, and push the buffer to the source pad it was allocated from. Many sink elements have accelerated methods for copying data to hardware, or have direct access to hardware. It is common for these elements to be able to create downstream-allocated buffers for their upstream peers. One such example is ximagesink. It creates buffers that contain XImages. Thus, when an upstream peer copies data into the buffer, it is copying directly into the XImage, enabling ximagesink to draw the image directly to the screen instead of having to copy data into an XImage first. Filter elements often have the opportunity to either work on a buffer in-place, or work while copying from a source buffer to a destination buffer. It is optimal to implement both algorithms, since the &GStreamer; framework can choose the fastest algorithm as appropriate. Naturally, this only makes sense for strict filters -- elements that have exactly the same format on source and sink pads. &GStreamer; uses a type system to ensure that the data passed between elements is in a recognized format. The type system is also important for ensuring that the parameters required to fully specify a format match up correctly when linking pads between elements. Each link that is made between elements has a specified type and optionally a set of properties. &GStreamer; already supports many basic media types. Following is a table of a few of the the basic types used for buffers in &GStreamer;. The table contains the name ("mime type") and a description of the type, the properties associated with the type, and the meaning of each property. A full list of supported types is included in . Mime Type Description Property Property Type Property Values Property Description audio/* All audio types rate integer greater than 0 The sample rate of the data, in samples (per channel) per second. channels integer greater than 0 The number of channels of audio data. audio/x-raw-int Unstructured and uncompressed raw integer audio data. endianness integer G_BIG_ENDIAN (1234) or G_LITTLE_ENDIAN (4321) The order of bytes in a sample. The value G_LITTLE_ENDIAN (4321) means little-endian (byte-order is least significant byte first). The value G_BIG_ENDIAN (1234) means big-endian (byte order is most significant byte first). signed boolean TRUE or FALSE Whether the values of the integer samples are signed or not. Signed samples use one bit to indicate sign (negative or positive) of the value. Unsigned samples are always positive. width integer greater than 0 Number of bits allocated per sample. depth integer greater than 0 The number of bits used per sample. This must be less than or equal to the width: If the depth is less than the width, the low bits are assumed to be the ones used. For example, a width of 32 and a depth of 24 means that each sample is stored in a 32 bit word, but only the low 24 bits are actually used. audio/mpeg Audio data compressed using the MPEG audio encoding scheme. mpegversion integer 1, 2 or 4 The MPEG-version used for encoding the data. The value 1 refers to MPEG-1, -2 and -2.5 layer 1, 2 or 3. The values 2 and 4 refer to the MPEG-AAC audio encoding schemes. framed boolean 0 or 1 A true value indicates that each buffer contains exactly one frame. A false value indicates that frames and buffers do not necessarily match up. layer integer 1, 2, or 3 The compression scheme layer used to compress the data (only if mpegversion=1). bitrate integer greater than 0 The bitrate, in bits per second. For VBR (variable bitrate) MPEG data, this is the average bitrate. audio/x-vorbis Vorbis audio data There are currently no specific properties defined for this type.