Chapter 17. Autoplugging
In Chapter 10, 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 Chapter 19. 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 MIME 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 mimetype to another, for example for media decoding.
17.1. MIME-types as a way to identity 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 Section 8.2). We have explained that a capability is a combination of a mimetype 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 MIME-type is needed. A full list of MIME-types and accompanying properties can be found in the Plugin Writer's Guide.
An element must associate a MIME-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 Chapter 10, we've learned to build a music player for Ogg/Vorbis files. Let's look at the MIME-types associated with each pad in this pipeline. Figure 17-1 shows what MIME-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.