As we have seen in the previous chapter (GstElement), the pads are the elements connections with the outside world. The specific type of media that the element can handle will be exposed by the pads. The description of this media type is done with capabilities (GstCaps) Since the pads play a very important role in how the element is viewed by the outside world, a mechanism is implemented to describe the pad by using capabilities. We will briefly describe what capabilities are, enough for you to get a basic understanding of the concepts. You will find more information on how to create capabilities in the filter-writer-guide. A capability is attached to a pad in order to describe what type of media the pad can handle. A capability is named and consists of a MIME type and a set of properties. Its data structure is: struct _GstCaps { gchar *name; /* the name of this caps */ guint16 id; /* type id (major type) */ GstProps *properties; /* properties for this capability */ }; Below is a dump of the capabilities of the element mpg123, as shown by gstreamer-inspect. You can see two pads: sink and src. Both pads have capability information attached to them. The sink pad (input pad) is called 'sink' and takes data of MIME type 'audio/mp3'. It also has three properties: layer, bitrate and framed. The src pad (output pad) is called 'src' and outputs data of MIME type 'audio/raw'. It also has four properties: format, depth, rate and channels. Pads: SINK: 'sink' .... Capabilities: 'mpg123_sink': MIME type: 'audio/mp3': layer: Integer range: 1 - 3 bitrate: Integer range: 8 - 320 framed: Boolean: TRUE SRC: 'src' .... Capabilities: 'mpg123_src': MIME type: 'audio/raw': format: Integer: 16 depth: Integer: 16 rate: Integer range: 11025 - 48000 channels: List: Integer: 1 Integer: 2 Properties are used to describe extra information for the capabilities. The properties basically exist of a key (a string) and a value. There are different possibile value types that can be used: An integer value: the property has this exact value. An integer range value. The property denotes a range of possible values. In the case of the mpg123 element: the src pad has a property rate that can go from 11025 to 48000. A boolean value. a fourcc value: this is a value that is commonly used to describe an encoding for video, as used be the AVI specification. A list value: the property can take any value from a list. Capabilities describe in great detail the type of media that is handled by the pads. They are mostly used for: Autoplugging: automatically finding plugins for a set of capabilities Compatibility detection: when two pads are connected, GStreamer can verify if the two pads are talking about the same media types.