Gstreamer introduction
Preface
What is GStreamer?
GStreamer is a framework for creating streaming media applications. The fundamental design comes from the video pipeline at Oregon Graduate Institute, as well as some ideas from DirectShow.
GStreamer's development framework makes it possible to write any type of streaming multimedia application. The GStreamer framework is designed to make it easy to write applications that handle audio or video or both. It isn't restricted to audio and video, and can process any kind of data flow. The pipeline design is made to have little overhead above what the applied filters induce. This makes GStreamer a good framework for designing even high-end audio applications which put high demands on latency or performance.
One of the most obvious uses of GStreamer is using it to build a media player. GStreamer already includes components for building a media player that can support a very wide variety of formats, including MP3, Ogg/Vorbis, MPEG-1/2, AVI, Quicktime, mod, and more. GStreamer, however, is much more than just another media player. Its main advantages are that the pluggable components can be mixed and matched into arbitrary pipelines so that it's possible to write a full-fledged video or audio editing application.
The framework is based on plugins that will provide the various codec and other functionality. The plugins can be linked and arranged in a pipeline. This pipeline defines the flow of the data.
The GStreamer core function is to provide a framework for plugins, data flow, synchronization and media type handling/negotiation. It also provides an API to write applications using the various plugins.
Who Should Read This Guide?
This guide explains how to write new modules for GStreamer. The guide is relevant to several groups of people:
Anyone who wants to add support for new ways of processing data in GStreamer. For example, a person in this group might want to create a new data format converter, a new visualization tool, or a new decoder or encoder.
Anyone who wants to add support for new input and output devices. For example, people in this group might want to add the ability to write to a new video output system or read data from a digital camera or special microphone.
Anyone who wants to extend GStreamer in any way. You need to have an understanding of how the plugin system works before you can understand the constraints that the plugin system places on the rest of the code. Also, you might be surprised after reading this at how much can be done with plugins.
This guide is not relevant to you if you only want to use the existing functionality of GStreamer, or if you just want to use an application that uses GStreamer. If you are only interested in using existing plugins to write a new application - and there are quite a lot of plugins already - you might want to check the GStreamer Application Development Manual. If you are just trying to get help with a GStreamer application, then you should check with the user manual for that particular application.
Preliminary Reading
This guide assumes that you are somewhat familiar with the basic workings of GStreamer. For a gentle introduction to programming concepts in GStreamer, you may wish to read the GStreamer Application Development Manual first. Also check out the other documentation available on the GStreamer web site.
In order to understand this manual, you will need to have a basic understanding of the C language. Since GStreamer adheres to the GObject programming model, this guide also assumes that you understand the basics of GObject programming. You may also want to have a look at Eric Harlow's book Developing Linux Applications with GTK+ and GDK.
Structure of This Guide
To help you navigate through this guide, it is divided into several large parts. Each part addresses a particular broad topic concerning GStreamer plugin development. The parts of this guide are laid out in the following order:
Building a Plugin - Introduction to the structure of a plugin, using an example audio filter for illustration.
This part covers all the basic steps you generally need to perform to build a plugin, such as registering the element with GStreamer and setting up the basics so it can receive data from and send data to neighbour elements. The discussion begins by giving examples of generating the basic structures and registering an element in Constructing the Boilerplate. Then, you will learn how to write the code to get a basic filter plugin working in Specifying the pads, The chain function and What are states?.
After that, we will show some of the GObject concepts on how to make an element configurable for applications and how to do application-element interaction in Adding Properties and Signals. Next, you will learn to build a quick test application to test all that you've just learned in Building a Test Application. We will just touch upon basics here. For full-blown application development, you should look at the Application Development Manual.
Advanced Filter Concepts - Information on advanced features of GStreamer plugin development.
After learning about the basic steps, you should be able to create a functional audio or video filter plugin with some nice features. However, GStreamer offers more for plugin writers. This part of the guide includes chapters on more advanced topics, such as scheduling, media type definitions in GStreamer, clocks, interfaces and tagging. Since these features are purpose-specific, you can read them in any order, most of them don't require knowledge from other sections.
The first chapter, named Different scheduling modes, will explain some of the basics of element scheduling. It is not very in-depth, but is mostly some sort of an introduction on why other things work as they do. Read this chapter if you're interested in GStreamer internals. Next, we will apply this knowledge and discuss another type of data transmission than what you learned in The chain function: Different scheduling modes. Loop-based elements will give you more control over input rate. This is useful when writing, for example, muxers or demuxers.
Next, we will discuss media identification in GStreamer in Media Types and Properties. You will learn how to define new media types and get to know a list of standard media types defined in GStreamer.
In the next chapter, you will learn the concept of request- and sometimes-pads, which are pads that are created dynamically, either because the application asked for it (request) or because the media stream requires it (sometimes). This will be in Request and Sometimes pads.
The next chapter, Clocking, will explain the concept of clocks in GStreamer. You need this information when you want to know how elements should achieve audio/video synchronization.
The next few chapters will discuss advanced ways of doing application-element interaction. Previously, we learned on the GObject-ways of doing this in Adding Properties and Signals. We will discuss dynamic parameters, which are a way of defining element behaviour over time in advance, in Supporting Dynamic Parameters. Next, you will learn about interfaces in Interfaces. Interfaces are very target- specific ways of application-element interaction, based on GObject's GInterface. Lastly, you will learn about how metadata is handled in GStreamer in Tagging (Metadata and Streaminfo).
The last chapter, Events: Seeking, Navigation and More, will discuss the concept of events in GStreamer. Events are another way of doing application-element interaction. They take care of seeking, for example. They are also yet another a way in which elements interact with each other, such as letting each other know about media stream discontinuities, forwarding tags inside a pipeline and so on.
Creating special element types - Explanation of writing other plugin types.
Because the first two parts of the guide use an audio filter as an example, the concepts introduced apply to filter plugins. But many of the concepts apply equally to other plugin types, including sources, sinks, and autopluggers. This part of the guide presents the issues that arise when working on these more specialized plugin types. The chapter starts with a special focus on elements that can be written using a base-class (Pre-made base classes), and later also goes into writing special types of elements in Writing a Demuxer or Parser, Writing a N-to-1 Element or Muxer and Writing a Manager.
Appendices - Further information for plugin developers.
The appendices contain some information that stubbornly refuses to fit cleanly in other sections of the guide. Most of this section is not yet finished.
The remainder of this introductory part of the guide presents a short overview of the basic concepts involved in GStreamer plugin development. Topics covered include Elements and Plugins, Pads, GstMiniObject, Buffers and Events and Media types and Properties. If you are already familiar with this information, you can use this short overview to refresh your memory, or you can skip to Building a Plugin.
As you can see, there a lot to learn, so let's get started!
Creating compound and complex elements by extending from a GstBin. This will allow you to create plugins that have other plugins embedded in them.
Adding new media types to the registry along with typedetect functions. This will allow your plugin to operate on a completely new media type.