Pads
As we have seen in , the pads are the element's
interface to 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(see
)
Pads are either source or sink pads. The terminology is defined from the
view of the element itself: elements accept data on their sink pads, and
send data out on their source pads. Sink pads are drawn on the left,
while source pads are drawn on the right of an element. In general,
data flows from left to right in the graph.
In reality, there is no objection to data flowing from a
source pad to the sink pad of an element upstream. Data will, however,
always flow from a source pad of one element to the sink pad of
another.
Getting pads from an element
Once you have created an element, you can get one of its pads with:
GstPad *srcpad;
...
srcpad = gst_element_get_pad (element, "src");
...
This function will get the pad named "src" from the given element.
Alternatively, you can request a GList of pads from the element. The
following code example will print the names of all the pads of an
element.
GList *pads;
...
pads = gst_element_get_pad_list (element);
while (pads) {
GstPad *pad = GST_PAD (pads->data);
g_print ("pad name %s\n", gst_pad_get_name (pad));
pads = g_list_next (pads);
}
...
Useful pad functions
You can get the name of a pad with gst_pad_get_name () and set its name with
get_pad_set_name().
gst_pad_get_direction (GstPad *pad) can be used to query if the pad
is a sink or a source pad. Remember that a source pad is a pad that
can output data and a sink pad is one that accepts data.
You can get the parent of the pad, this is the element that this pad belongs to,
with get_pad_get_parent(GstPad *pad). This function will return a pointer to a
GstElement.
Types of pads
Dynamic pads
Some elements might not have all of their pads when the element is
created. This
can happen, for example, with an MPEG system demultiplexer. The
demultiplexer will create its pads at runtime when it detects the
different elementary streams in the MPEG system stream.
Running gst-inspect mpegdemux will show that
the element has only one pad: a sink pad called 'sink'. The other pads are
"dormant". You can see this in the pad template because there is
an 'Exists: Sometimes'
property. Depending on the type of MPEG file you play, the pads will
be created. We
will see that this is very important when you are going to create dynamic
pipelines later on in this manual.
You can attach a signal to an element to inform you when the element has created
a new pad from one of its padtemplates. The following piece of code is an example
of how to do this:
static void
pad_link_func (GstElement *parser, GstPad *pad, GstElement *pipeline)
{
g_print("***** a new pad %s was created\n", gst_pad_get_name(pad));
gst_element_set_state (pipeline, GST_STATE_PAUSED);
if (strncmp (gst_pad_get_name (pad), "private_stream_1.0", 18) == 0) {
// set up an AC3 decoder pipeline
...
// link pad to the AC3 decoder pipeline
...
}
gst_element_set_state (GST_ELEMENT (audio_thread), GST_STATE_READY);
}
int
main(int argc, char *argv[])
{
GstElement *pipeline;
GstElement *mpeg2parser;
// create pipeline and do something useful
...
mpeg2parser = gst_element_factory_make ("mpegdemux", "mpegdemux");
g_signal_connect (G_OBJECT (mpeg2parser), "new_pad", pad_link_func, pipeline);
...
// start the pipeline
gst_element_set_state (GST_ELEMENT (pipeline), GST_STATE_PLAYING);
...
}
A pipeline cannot be changed in the PLAYING state.
Request pads
An element can also have request pads. These pads are not created
automatically but are only created on demand. This is very useful
for multiplexers, aggregators and tee elements.
The tee element, for example, has one input pad and a request padtemplate for the
output pads. Whenever an element wants to get an output pad from the tee element, it
has to request the pad.
The following piece of code can be used to get a pad from the tee element. After
the pad has been requested, it can be used to link another element to it.
...
GstPad *pad;
...
element = gst_element_factory_make ("tee", "element");
pad = gst_element_get_request_pad (element, "src%d");
g_print ("new pad %s\n", gst_pad_get_name (pad));
...
The gst_element_get_request_pad method can be used to get a pad
from the element based on the name_template of the padtemplate.
It is also possible to request a pad that is compatible with another
pad template. This is very useful if you want to link an element
to a multiplexer element and you need to request a pad that is
compatible. The gst_element_get_compatible_pad is used to request
a compatible pad, as is shown in the next example.
...
GstPadTemplate *templ;
GstPad *pad;
...
element = gst_element_factory_make ("tee", "element");
mad = gst_element_factory_make ("mad", "mad");
templ = gst_element_get_pad_template_by_name (mad, "sink");
pad = gst_element_get_compatible_pad (element, templ);
g_print ("new pad %s\n", gst_pad_get_name (pad));
...
Capabilities of a pad
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 data that can
flow through 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
Plugin Writer's Guide.
What are capabilities ?
Capabilities are attached to a pad in order to describe
what type of media the pad can handle.
Capabilities is shorthand for "capability chain". A capability chain
is a chain of one capability or more.
The basic entity is a capability, and is defined by a name, a MIME
type and a set of properties. A capability can be chained to
another capability, which is why we commonly refer to a chain of
capability entities as "capabilities".
It is important to understand that the term "capabilities" refers
to a chain of one capability or more. This will be clearer when
you see the structure definition of a GstCaps
element.
Its structure is:
struct _GstCaps {
gchar *name; /* the name of this caps */
guint16 id; /* type id (major type) */
guint refcount; /* caps are refcounted */
GstProps *properties; /* properties for this capability */
GstCaps *next; /* caps can be chained together */
};
Below is a dump of the capabilities of the element mad, as shown by
gst-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 source 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 template: 'sink'
Availability: Always
Capabilities:
'mad_sink':
MIME type: 'audio/mp3':
SRC template: 'src'
Availability: Always
Capabilities:
'mad_src':
MIME type: 'audio/raw':
format: String: int
endianness: Integer: 1234
width: Integer: 16
depth: Integer: 16
channels: Integer range: 1 - 2
law: Integer: 0
signed: Boolean: TRUE
rate: Integer range: 11025 - 48000
What are properties ?
Properties are used to describe extra information for
capabilities. A property consists of a key (a string) and
a value. There are different possible value types that can be used:
basic types:
an integer value: the property has this exact value.
a boolean value: the property is either TRUE or FALSE.
a fourcc value: this is a value that is commonly used to
describe an encoding for video,
as used for example by the AVI specification.
fourcc values consist of four bytes.
The FOURCC
Definition List is the most complete resource
on the allowed fourcc values.
a float value: the property has this exact floating point value.
a string value.
range types:
an integer range value: the property denotes a range of
possible integer. For example, the wavparse element has
a source pad where the "rate" property can go from 8000 to
48000.
a float range value: the property denotes a range of possible
floating point values.
a list value: the property can take any value from a list of
basic value types or range types.
What capabilities are used for
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 linked, GStreamer
can verify if the two pads are talking about the same media types.
The process of linking two pads and checking if they are compatible
is called "caps negotiation".
Getting the capabilities of a pad
A pad can have a chain of capabilities attached to it. You can get the capabilities chain
with:
GstCaps *caps;
...
caps = gst_pad_get_caps (pad);
g_print ("pad name %s\n", gst_pad_get_name (pad));
while (caps) {
g_print (" Capability name %s, MIME type %s\n",
gst_caps_get_name (cap),
gst_caps_get_mime (cap));
caps = caps->next;
}
...
Creating capability structures
While capabilities are mainly used inside a plugin to describe the
media type of the pads, the application programmer also has to have
basic understanding of capabilities in order to interface with the
plugins, specially when using the autopluggers.
As we said, a capability has a name, a mime-type and some
properties. The signature of the function to create a new
GstCaps structure is:
GstCaps* gst_caps_new (const gchar *name, const gchar *mime, GstProps *props);
You can therefore create a new capability with no properties like this:
GstCaps *newcaps;
newcaps = gst_caps_new ("my_caps", "audio/wav", NULL);
GstProps basically consist of a set of key-value pairs
and are created with a function with this signature:
GstProps* gst_props_new (const gchar *firstname, ...);
The keys are given as strings and the values are given with a set of macros:
GST_PROPS_INT(a): An integer value
GST_PROPS_FLOAT(a): A floating point value
GST_PROPS_FOURCC(a): A fourcc value
GST_PROPS_BOOLEAN(a): A boolean value
GST_PROPS_STRING(a): A string value
The values can also be specified as ranges with:
GST_PROPS_INT_RANGE(a,b): An integer range from a to b
GST_PROPS_FLOAT_RANGE(a,b): A float ragne from a to b
All of the above values can be given with a list too, using:
GST_PROPS_LIST(a,...): A list of property values.
A more complex capability with properties is created like this:
GstCaps *newcaps;
newcaps = gst_caps_new ("my_caps",
"audio/wav",
gst_props_new (
"bitrate", GST_PROPS_INT_RANGE (11025,22050),
"depth", GST_PROPS_INT (16),
"signed", GST_PROPS_LIST (
GST_PROPS_BOOLEAN (TRUE),
GST_PROPS_BOOLEAN (FALSE)
),
NULL
);
Optionally, the convenient shortcut macro can be used. The above complex
capability can be created with:
GstCaps *newcaps;
newcaps = GST_CAPS_NEW ("my_caps",
"audio/wav",
"bitrate", GST_PROPS_INT_RANGE (11025,22050),
"depth", GST_PROPS_INT (16),
"signed", GST_PROPS_LIST (
GST_PROPS_BOOLEAN (TRUE),
GST_PROPS_BOOLEAN (FALSE)
)
);