Autoplugging
In the Chapter called Your first application, you've learned to build a simple media player for Ogg/Vorbis files. By using alternative elements, you are able to build media players for other media types, such as Ogg/Speex, MP3 or even video formats. However, you would rather want to build an application that can automatically detect the media type of a stream and automatically generate the best possible pipeline by looking at all available elements in a system. This process is called autoplugging, and GStreamer contains high-quality autopluggers. If you're looking for an autoplugger, don't read any further and go to the Chapter called Playback Components. This chapter will explain the concept of autoplugging and typefinding. It will explain what systems GStreamer includes to dynamically detect the type of a media stream, and how to generate a pipeline of decoder elements to playback this media. The same principles can also be used for transcoding. Because of the full dynamicity of this concept, GStreamer can be automatically extended to support new media types without needing any adaptations to its autopluggers.
We will first introduce the concept of Media types as a dynamic and extendible way of identifying media streams. After that, we will introduce the concept of typefinding to find the type of a media stream. Lastly, we will explain how autoplugging and the GStreamer registry can be used to setup a pipeline that will convert media from one mediatype to another, for example for media decoding.
Media types as a way to identify streams
We have previously introduced the concept of capabilities as a way for elements (or, rather, pads) to agree on a media type when streaming data from one element to the next (see the Section called Capabilities of a pad in the Chapter called Pads and capabilities). We have explained that a capability is a combination of a media type and a set of properties. For most container formats (those are the files that you will find on your hard disk; Ogg, for example, is a container format), no properties are needed to describe the stream. Only a media type is needed. A full list of media types and accompanying properties can be found in the Plugin Writer's Guide.
An element must associate a media type to its source and sink pads when it is loaded into the system. GStreamer knows about the different elements and what type of data they expect and emit through the GStreamer registry. This allows for very dynamic and extensible element creation as we will see.
In the Chapter called Your first application, we've learned to build a music player for Ogg/Vorbis files. Let's look at the media types associated with each pad in this pipeline. Figure 1 shows what media type belongs to each pad in this pipeline.
Now that we have an idea how GStreamer identifies known media streams, we can look at methods GStreamer uses to setup pipelines for media handling and for media type detection.