Package org.freedesktop.gstreamer.gst

package org.freedesktop.gstreamer.gst
Provides all the core GStreamer services, including initialization, plugin management and types, as well as the object hierarchy that defines elements and bins, along with some more specialized elements

The following native libraries are required and will be loaded: libgstreamer-1.0.0

For namespace-global declarations, refer to the Gst class documentation.

Debugutils

These utility functions help with generating dot graphs which can be rendered with [graphviz] to multiple formats.

[graphviz]: https://graphviz.org/

Gst

GStreamer is a framework for constructing graphs of various filters (termed elements here) that will handle streaming media.

Any discrete (packetizable) media type is supported, with provisions for automatically determining source type.

Formatting/framing information is provided with a powerful negotiation framework.

Plugins are heavily used to provide for all elements, allowing one to construct plugins outside of the GST library, even released binary-only if license require (please don't).

GStreamer covers a wide range of use cases including: playback, recording, editing, serving streams, voice over ip and video calls.

The GStreamer library should be initialized with gst_init() before it can be used. You should pass pointers to the main argc and argv variables so that GStreamer can process its own command line options, as shown in the following example.

Initializing the gstreamer library

int main (int argc, char *argv[])
 {
   // initialize the GStreamer library
   gst_init (&argc, &argv);
   ...
 }

It's allowed to pass two null pointers to gst_init() in case you don't want to pass the command line args to GStreamer.

You can also use GOptionContext to initialize your own parameters as shown in the next code fragment:

Initializing own parameters when initializing GStreamer

static gboolean stats = FALSE;
 ...
 int
 main (int argc, char *argv[])
 {
  GOptionEntry options[] = {
   {"tags", 't', 0, G_OPTION_ARG_NONE, &tags,
       N_("Output tags (also known as metadata)"), NULL},
   {NULL}
  };
  ctx = g_option_context_new ("[ADDITIONAL ARGUMENTS]");
  g_option_context_add_main_entries (ctx, options, GETTEXT_PACKAGE);
  g_option_context_add_group (ctx, gst_init_get_option_group ());
  if (!g_option_context_parse (ctx, &argc, &argv, &err)) {
    g_print ("Error initializing: %s\\n", GST_STR_NULL (err->message));
    exit (1);
  }
  g_option_context_free (ctx);
 ...
 }

Use gst_version() to query the library version at runtime or use the GST_VERSION_* macros to find the version at compile time. Optionally gst_version_string() returns a printable string.

The gst_deinit() call is used to clean up all internal resources used by GStreamer. It is mostly used in unit tests to check for leaks.

Gstcompat

Please do not use these in new code. These symbols are only available by defining GST_DISABLE_DEPRECATED. This can be done in CFLAGS for compiling old code.

Gsterror

GStreamer elements can throw non-fatal warnings and fatal errors. Higher-level elements and applications can programmatically filter the ones they are interested in or can recover from, and have a default handler handle the rest of them.

The rest of this section will use the term "error" to mean both (non-fatal) warnings and (fatal) errors; they are treated similarly.

Errors from elements are the combination of a GError and a debug string. The GError contains:

  • a domain type: CORE, LIBRARY, RESOURCE or STREAM
  • a code: an enum value specific to the domain
  • a translated, human-readable message
  • a non-translated additional debug string, which also contains
  • file and line information

Elements do not have the context required to decide what to do with errors. As such, they should only inform about errors, and stop their processing. In short, an element doesn't know what it is being used for.

It is the application or compound element using the given element that has more context about the use of the element. Errors can be received by listening to the GstBus of the element/pipeline for GstMessage objects with the type MessageType.ERROR or MessageType.WARNING. The thrown errors should be inspected, and filtered if appropriate.

An application is expected to, by default, present the user with a dialog box (or an equivalent) showing the error message. The dialog should also allow a way to get at the additional debug information, so the user can provide bug reporting information.

A compound element is expected to forward errors by default higher up the hierarchy; this is done by default in the same way as for other types of GstMessage.

When applications or compound elements trigger errors that they can recover from, they can filter out these errors and take appropriate action. For example, an application that gets an error from xvimagesink that indicates all XVideo ports are taken, the application can attempt to use another sink instead.

Elements throw errors using the GST_ELEMENT_ERROR convenience macro:

Throwing an error


     GST_ELEMENT_ERROR (src, RESOURCE, NOT_FOUND,
       (_("No file name specified for reading.")), (NULL));

Things to keep in mind:

  • Don't go off inventing new error codes. The ones currently provided should be enough. If you find your type of error does not fit the current codes, you should use FAILED.
  • Don't provide a message if the default one suffices. this keeps messages more uniform. Use (null) - not forgetting the parentheses.
  • If you do supply a custom message, it should be marked for translation. The message should start with a capital and end with a period. The message should describe the error in short, in a human-readable form, and without any complex technical terms. A user interface will present this message as the first thing a user sees. Details, technical info, ... should go in the debug string.
  • The debug string can be as you like. Again, use (null) if there's nothing to add - file and line number will still be passed. GST_ERROR_SYSTEM can be used as a shortcut to give debug information on a system call error.

Gstformat

GstFormats functions are used to register a new format to the gstreamer core. Formats can be used to perform seeking or conversions/query operations.

Gstinfo

GStreamer's debugging subsystem is an easy way to get information about what the application is doing. It is not meant for programming errors. Use GLib methods (g_warning and friends) for that.

The debugging subsystem works only after GStreamer has been initialized

  • for example by calling gst_init().

The debugging subsystem is used to log informational messages while the application runs. Each messages has some properties attached to it. Among these properties are the debugging category, the severity (called "level" here) and an optional GObject it belongs to. Each of these messages is sent to all registered debugging handlers, which then handle the messages. GStreamer attaches a default handler on startup, which outputs requested messages to stderr.

Messages are output by using shortcut macros like GST_DEBUG, GST_CAT_ERROR_OBJECT or similar. These all expand to calling gst_debug_log() with the right parameters. The only thing a developer will probably want to do is define his own categories. This is easily done with 3 lines. At the top of your code, declare the variables and set the default category. 

  GST_DEBUG_CATEGORY_STATIC (my_category);  // define category (statically)
   #define GST_CAT_DEFAULT my_category       // set as default
After that you only need to initialize the category. 
  GST_DEBUG_CATEGORY_INIT (my_category, "my category",
                            0, "This is my very own");
Initialization must be done before the category is used first. Plugins do this in their plugin_init function, libraries and applications should do that during their initialization.

The whole debugging subsystem can be disabled at build time with passing the --disable-gst-debug switch to configure. If this is done, every function, macro and even structs described in this file evaluate to default values or nothing at all. So don't take addresses of these functions or use other tricks. If you must do that for some reason, there is still an option. If the debugging subsystem was compiled out, GST_DISABLE_GST_DEBUG is defined in <gst/gst.h>, so you can check that before doing your trick. Disabling the debugging subsystem will give you a slight (read: unnoticeable) speed increase and will reduce the size of your compiled code. The GStreamer library itself becomes around 10% smaller.

Please note that there are naming conventions for the names of debugging categories. These are explained at GST_DEBUG_CATEGORY_INIT().

Gstparamspec

GParamSpec implementations specific to GStreamer.

Gstparse

These function allow to create a pipeline based on the syntax used in the gst-launch-1.0 utility (see man-page for syntax documentation).

Please note that these functions take several measures to create somewhat dynamic pipelines. Due to that such pipelines are not always reusable (set the state to NULL and back to PLAYING).

Gstprotection

The GstProtectionMeta class enables the information needed to decrypt a GstBuffer to be attached to that buffer.

Typically, a demuxer element would attach GstProtectionMeta objects to the buffers that it pushes downstream. The demuxer would parse the protection information for a video/audio frame from its input data and use this information to populate the GstStructure info field, which is then encapsulated in a GstProtectionMeta object and attached to the corresponding output buffer using the gst_buffer_add_protection_meta() function. The information in this attached GstProtectionMeta would be used by a downstream decrypter element to recover the original unencrypted frame.

In addition to the GstProtectionMeta demuxers signal encrypted streams with specific caps. The caps GstStructure name will usually depend on the encryption scheme, for instance Common Encryption will be signaled with application/x-cenc. To prevent loss of information the media type of the decrypted stream will be stored in a original-media-type string field. Downstream elements can re-use that information, for instance decryptors can set their source pad caps according to the original-media-type received on their sink pad.

Gststreams

A GstStream is a high-level object defining a stream of data which is, or can be, present in a GstPipeline.

It is defined by a unique identifier, a "Stream ID". A GstStream does not automatically imply the stream is present within a pipeline or element.

Any element that can introduce new streams in a pipeline should create the appropriate GstStream object, and can convey that object via the EventType.STREAM_START event and/or the GstStreamCollection.

Elements that do not modify the nature of the stream can add extra information on it (such as enrich the GstCaps, or GstTagList). This is typically done by parsing elements.

Gstvalue

GValue implementations specific to GStreamer.

Note that operations on the same GValue from multiple threads may lead to undefined behaviour.

Gstversion

Use the GST_VERSION_* macros e.g. when defining own plugins. The GStreamer runtime checks if these plugin and core version match and refuses to use a plugin compiled against a different version of GStreamer. You can also use the macros to keep the GStreamer version information in your application.

Use the gst_version() function if you want to know which version of GStreamer you are currently linked against.

The version macros get defined by including "gst/gst.h".