mirror of
https://gitlab.freedesktop.org/gstreamer/gstreamer.git
synced 2024-12-21 07:46:38 +00:00
601 lines
20 KiB
XML
601 lines
20 KiB
XML
<chapter id="chapter-negotiation" xreflabel="Caps negotiation">
|
|
<title>Caps negotiation</title>
|
|
<para>
|
|
Caps negotiation is the act of finding a media format (GstCaps) between
|
|
elements that they can handle. This process in &GStreamer; can in most
|
|
cases find an optimal solution for the complete pipeline. In this section
|
|
we explain how this works.
|
|
</para>
|
|
|
|
<sect1 id="section-nego-basics">
|
|
<title>Caps negotiation basics</title>
|
|
<para>
|
|
In &GStreamer;, negotiation of the media format always follows the
|
|
following simple rules:
|
|
</para>
|
|
<itemizedlist>
|
|
<listitem>
|
|
<para>
|
|
A downstream element suggest a format on its sinkpad and places the
|
|
suggestion in the result of the CAPS query performed on the sinkpad.
|
|
See also <xref linkend="section-nego-getcaps"/>.
|
|
</para>
|
|
</listitem>
|
|
<listitem>
|
|
<para>
|
|
An upstream element decides on a format. It sends the selected media
|
|
format downstream on its source pad with a CAPS event. Downstream
|
|
elements reconfigure themselves to handle the media type in the CAPS
|
|
event on the sinkpad.
|
|
</para>
|
|
</listitem>
|
|
<listitem>
|
|
<para>
|
|
A downstream element can inform upstream that it would like to
|
|
suggest a new format by sending a RECONFIGURE event upstream. The
|
|
RECONFIGURE event simply instructs an upstream element to restart
|
|
the negotiation phase. Because the element that sent out the
|
|
RECONFIGURE event is now suggesting another format, the format
|
|
in the pipeline might change.
|
|
</para>
|
|
</listitem>
|
|
</itemizedlist>
|
|
<para>
|
|
In addition to the CAPS and RECONFIGURE event and the CAPS query, there
|
|
is an ACCEPT_CAPS query to quickly check if a certain caps can
|
|
be accepted by an element.
|
|
</para>
|
|
<para>
|
|
All negotiation follows these simple rules. Let's take a look at some
|
|
typical uses cases and how negotiation happens.
|
|
</para>
|
|
</sect1>
|
|
|
|
<sect1 id="section-nego-usecases">
|
|
<title>Caps negotiation use cases</title>
|
|
<para>
|
|
In what follows we will look at some use cases for push-mode scheduling.
|
|
The pull-mode scheduling negotiation phase is discussed in
|
|
<xref linkend="section-nego-pullmode"/> and is actually similar as we
|
|
will see.
|
|
</para>
|
|
<para>
|
|
Since the sink pads only suggest formats and the source pads need to
|
|
decide, the most complicated work is done in the source pads.
|
|
We can identify 3 caps negotiation use cases for the source pads:
|
|
</para>
|
|
<itemizedlist>
|
|
<listitem>
|
|
<para>
|
|
Fixed negotiation. An element can output one format only.
|
|
See <xref linkend="section-nego-fixed"/>.
|
|
</para>
|
|
</listitem>
|
|
<listitem>
|
|
<para>
|
|
Transform negotiation. There is a (fixed) transform between the
|
|
input and output format of the element, usually based on some
|
|
element property. The caps that the element will produce depend
|
|
on the upstream caps and the caps that the element can accept
|
|
depend on the downstream caps.
|
|
See <xref linkend="section-nego-transform"/>.
|
|
</para>
|
|
</listitem>
|
|
<listitem>
|
|
<para>
|
|
Dynamic negotiation. An element can output many formats.
|
|
See <xref linkend="section-nego-dynamic"/>.
|
|
</para>
|
|
</listitem>
|
|
</itemizedlist>
|
|
|
|
<sect2 id="section-nego-fixed">
|
|
<title>Fixed negotiation</title>
|
|
<para>
|
|
In this case, the source pad can only produce a fixed format. Usually
|
|
this format is encoded inside the media. No downstream element can
|
|
ask for a different format, the only way that the source pad will
|
|
renegotiate is when the element decides to change the caps itself.
|
|
</para>
|
|
<para>
|
|
Elements that could implement fixed caps (on their source pads) are,
|
|
in general, all elements that are not renegotiable. Examples include:
|
|
</para>
|
|
<itemizedlist>
|
|
<listitem>
|
|
<para>
|
|
A typefinder, since the type found is part of the actual data stream
|
|
and can thus not be re-negotiated. The typefinder will look at the
|
|
stream of bytes, figure out the type, send a CAPS event with the
|
|
caps and then push buffers of the type.
|
|
</para>
|
|
</listitem>
|
|
<listitem>
|
|
<para>
|
|
Pretty much all demuxers, since the contained elementary data
|
|
streams are defined in the file headers, and thus not
|
|
renegotiable.
|
|
</para>
|
|
</listitem>
|
|
<listitem>
|
|
<para>
|
|
Some decoders, where the format is embedded in the data stream
|
|
and not part of the peercaps <emphasis>and</emphasis> where the
|
|
decoder itself is not reconfigurable, too.
|
|
</para>
|
|
</listitem>
|
|
<listitem>
|
|
<para>
|
|
Some sources that produce a fixed format.
|
|
</para>
|
|
</listitem>
|
|
</itemizedlist>
|
|
<para>
|
|
<function>gst_pad_use_fixed_caps()</function> is used on the source
|
|
pad with fixed caps. As long as the pad is not negotiated, the default
|
|
CAPS query will return the caps presented in the padtemplate. As soon
|
|
as the pad is negotiated, the CAPS query will return the negotiated
|
|
caps (and nothing else). These are the relevant code snippets for fixed
|
|
caps source pads.
|
|
</para>
|
|
<programlisting>
|
|
<![CDATA[
|
|
[..]
|
|
pad = gst_pad_new_from_static_template (..);
|
|
gst_pad_use_fixed_caps (pad);
|
|
[..]
|
|
]]>
|
|
</programlisting>
|
|
<para>
|
|
The fixed caps can then be set on the pad by calling
|
|
<function>gst_pad_set_caps ()</function>.
|
|
</para>
|
|
<programlisting>
|
|
<![CDATA[
|
|
[..]
|
|
caps = gst_caps_new_simple ("audio/x-raw",
|
|
"format", G_TYPE_STRING, GST_AUDIO_NE(F32),
|
|
"rate", G_TYPE_INT, <samplerate>,
|
|
"channels", G_TYPE_INT, <num-channels>, NULL);
|
|
if (!gst_pad_set_caps (pad, caps)) {
|
|
GST_ELEMENT_ERROR (element, CORE, NEGOTIATION, (NULL),
|
|
("Some debug information here"));
|
|
return GST_FLOW_ERROR;
|
|
}
|
|
[..]
|
|
]]>
|
|
</programlisting>
|
|
<para>
|
|
These types of elements also don't have a relation between the input
|
|
format and the output format, the input caps simply don't contain the
|
|
information needed to produce the output caps.
|
|
</para>
|
|
<para>
|
|
All other elements that need to be configured for the format should
|
|
implement full caps negotiation, which will be explained in the next
|
|
few sections.
|
|
</para>
|
|
</sect2>
|
|
|
|
<sect2 id="section-nego-transform">
|
|
<title>Transform negotiation</title>
|
|
<para>
|
|
In this negotiation technique, there is a fixed transform between
|
|
the element input caps and the output caps. This transformation
|
|
could be parameterized by element properties but not by the
|
|
content of the stream (see <xref linkend="section-nego-fixed"/>
|
|
for that use-case).
|
|
</para>
|
|
<para>
|
|
The caps that the element can accept depend on the (fixed
|
|
transformation) downstream caps. The caps that the element can
|
|
produce depend on the (fixed transformation of) the upstream
|
|
caps.
|
|
</para>
|
|
<para>
|
|
This type of element can usually set caps on its source pad from
|
|
the <function>_event()</function> function on the sink pad when
|
|
it received the CAPS event. This means that the caps transform
|
|
function transforms a fixed caps into another fixed caps.
|
|
Examples of elements include:
|
|
</para>
|
|
<itemizedlist>
|
|
<listitem>
|
|
<para>
|
|
Videobox. It adds configurable border around a video frame
|
|
depending on object properties.
|
|
</para>
|
|
</listitem>
|
|
<listitem>
|
|
<para>
|
|
Identity elements. All elements that don't change the format
|
|
of the data, only the content. Video and audio effects are an
|
|
example. Other examples include elements that inspect the
|
|
stream.
|
|
</para>
|
|
</listitem>
|
|
<listitem>
|
|
<para>
|
|
Some decoders and encoders, where the output format is defined
|
|
by input format, like mulawdec and mulawenc. These decoders
|
|
usually have no headers that define the content of the stream.
|
|
They are usually more like conversion elements.
|
|
</para>
|
|
</listitem>
|
|
</itemizedlist>
|
|
<para>
|
|
Below is an example of a negotiation steps of a typical transform
|
|
element. In the sink pad CAPS event handler, we compute the caps
|
|
for the source pad and set those.
|
|
</para>
|
|
<programlisting>
|
|
<![CDATA[
|
|
[...]
|
|
|
|
static gboolean
|
|
gst_my_filter_setcaps (GstMyFilter *filter,
|
|
GstCaps *caps)
|
|
{
|
|
GstStructure *structure;
|
|
int rate, channels;
|
|
gboolean ret;
|
|
GstCaps *outcaps;
|
|
|
|
structure = gst_caps_get_structure (caps, 0);
|
|
ret = gst_structure_get_int (structure, "rate", &rate);
|
|
ret = ret && gst_structure_get_int (structure, "channels", &channels);
|
|
if (!ret)
|
|
return FALSE;
|
|
|
|
outcaps = gst_caps_new_simple ("audio/x-raw",
|
|
"format", G_TYPE_STRING, GST_AUDIO_NE(S16),
|
|
"rate", G_TYPE_INT, rate,
|
|
"channels", G_TYPE_INT, channels, NULL);
|
|
ret = gst_pad_set_caps (filter->srcpad, outcaps);
|
|
gst_caps_unref (outcaps);
|
|
|
|
return ret;
|
|
}
|
|
|
|
static gboolean
|
|
gst_my_filter_sink_event (GstPad *pad,
|
|
GstObject *parent,
|
|
GstEvent *event)
|
|
{
|
|
gboolean ret;
|
|
GstMyFilter *filter = GST_MY_FILTER (parent);
|
|
|
|
switch (GST_EVENT_TYPE (event)) {
|
|
case GST_EVENT_CAPS:
|
|
{
|
|
GstCaps *caps;
|
|
|
|
gst_event_parse_caps (event, &caps);
|
|
ret = gst_my_filter_setcaps (filter, caps);
|
|
break;
|
|
}
|
|
default:
|
|
ret = gst_pad_event_default (pad, parent, event);
|
|
break;
|
|
}
|
|
return ret;
|
|
}
|
|
|
|
[...]
|
|
]]>
|
|
</programlisting>
|
|
</sect2>
|
|
|
|
<sect2 id="section-nego-dynamic">
|
|
<title>Dynamic negotiation</title>
|
|
<para>
|
|
A last negotiation method is the most complex and powerful dynamic
|
|
negotiation.
|
|
</para>
|
|
<para>
|
|
Like with the transform negotiation in
|
|
<xref linkend="section-nego-transform"/>, dynamic negotiation will
|
|
perform a transformation on the downstream/upstream caps. Unlike the
|
|
transform negotiation, this transform will convert fixed caps to
|
|
unfixed caps. This means that the sink pad input caps can be converted
|
|
into unfixed (multiple) formats. The source pad will have to choose a
|
|
format from all the possibilities. It would usually like to choose a
|
|
format that requires the least amount of effort to produce but it does
|
|
not have to be. The selection of the format should also depend on the
|
|
caps that can be accepted downstream (see a QUERY_CAPS function in
|
|
<xref linkend="section-nego-getcaps"/>).
|
|
</para>
|
|
<para>
|
|
A typical flow goes like this:
|
|
</para>
|
|
<itemizedlist>
|
|
<listitem>
|
|
<para>
|
|
Caps are received on the sink pad of the element.
|
|
</para>
|
|
</listitem>
|
|
<listitem>
|
|
<para>
|
|
If the element prefers to operate in passthrough mode, check
|
|
if downstream accepts the caps with the ACCEPT_CAPS query. If it
|
|
does, we can complete negotiation and we can operate in
|
|
passthrough mode.
|
|
</para>
|
|
</listitem>
|
|
<listitem>
|
|
<para>
|
|
Calculate the possible caps for the source pad.
|
|
</para>
|
|
</listitem>
|
|
<listitem>
|
|
<para>
|
|
Query the downstream peer pad for the list of possible
|
|
caps.
|
|
</para>
|
|
</listitem>
|
|
<listitem>
|
|
<para>
|
|
Select from the downstream list the first caps that you can
|
|
transform to and set this as the output caps. You might have to
|
|
fixate the caps to some reasonable defaults to construct
|
|
fixed caps.
|
|
</para>
|
|
</listitem>
|
|
</itemizedlist>
|
|
<para>
|
|
Examples of this type of elements include:
|
|
</para>
|
|
<itemizedlist>
|
|
<listitem>
|
|
<para>
|
|
Converter elements such as videoconvert, audioconvert, audioresample,
|
|
videoscale, ...
|
|
</para>
|
|
</listitem>
|
|
<listitem>
|
|
<para>
|
|
Source elements such as audiotestsrc, videotestsrc, v4l2src,
|
|
pulsesrc, ...
|
|
</para>
|
|
</listitem>
|
|
</itemizedlist>
|
|
<para>
|
|
Let's look at the example of an element that can convert between
|
|
samplerates, so where input and output samplerate don't have to be
|
|
the same:
|
|
</para>
|
|
<programlisting>
|
|
<![CDATA[
|
|
static gboolean
|
|
gst_my_filter_setcaps (GstMyFilter *filter,
|
|
GstCaps *caps)
|
|
{
|
|
if (gst_pad_set_caps (filter->sinkpad, caps)) {
|
|
filter->passthrough = TRUE;
|
|
} else {
|
|
GstCaps *othercaps, *newcaps;
|
|
GstStructure *s = gst_caps_get_structure (caps, 0), *others;
|
|
|
|
/* no passthrough, setup internal conversion */
|
|
gst_structure_get_int (s, "channels", &filter->channels);
|
|
othercaps = gst_pad_get_allowed_caps (filter->srcpad);
|
|
others = gst_caps_get_structure (othercaps, 0);
|
|
gst_structure_set (others,
|
|
"channels", G_TYPE_INT, filter->channels, NULL);
|
|
|
|
/* now, the samplerate value can optionally have multiple values, so
|
|
* we "fixate" it, which means that one fixed value is chosen */
|
|
newcaps = gst_caps_copy_nth (othercaps, 0);
|
|
gst_caps_unref (othercaps);
|
|
gst_pad_fixate_caps (filter->srcpad, newcaps);
|
|
if (!gst_pad_set_caps (filter->srcpad, newcaps))
|
|
return FALSE;
|
|
|
|
/* we are now set up, configure internally */
|
|
filter->passthrough = FALSE;
|
|
gst_structure_get_int (s, "rate", &filter->from_samplerate);
|
|
others = gst_caps_get_structure (newcaps, 0);
|
|
gst_structure_get_int (others, "rate", &filter->to_samplerate);
|
|
}
|
|
|
|
return TRUE;
|
|
}
|
|
|
|
static gboolean
|
|
gst_my_filter_sink_event (GstPad *pad,
|
|
GstObject *parent,
|
|
GstEvent *event)
|
|
{
|
|
gboolean ret;
|
|
GstMyFilter *filter = GST_MY_FILTER (parent);
|
|
|
|
switch (GST_EVENT_TYPE (event)) {
|
|
case GST_EVENT_CAPS:
|
|
{
|
|
GstCaps *caps;
|
|
|
|
gst_event_parse_caps (event, &caps);
|
|
ret = gst_my_filter_setcaps (filter, caps);
|
|
break;
|
|
}
|
|
default:
|
|
ret = gst_pad_event_default (pad, parent, event);
|
|
break;
|
|
}
|
|
return ret;
|
|
}
|
|
|
|
static GstFlowReturn
|
|
gst_my_filter_chain (GstPad *pad,
|
|
GstObject *parent,
|
|
GstBuffer *buf)
|
|
{
|
|
GstMyFilter *filter = GST_MY_FILTER (parent);
|
|
GstBuffer *out;
|
|
|
|
/* push on if in passthrough mode */
|
|
if (filter->passthrough)
|
|
return gst_pad_push (filter->srcpad, buf);
|
|
|
|
/* convert, push */
|
|
out = gst_my_filter_convert (filter, buf);
|
|
gst_buffer_unref (buf);
|
|
|
|
return gst_pad_push (filter->srcpad, out);
|
|
}
|
|
]]>
|
|
</programlisting>
|
|
</sect2>
|
|
</sect1>
|
|
|
|
<sect1 id="section-nego-upstream" xreflabel="Upstream caps (re)negotiation">
|
|
<title>Upstream caps (re)negotiation</title>
|
|
<para>
|
|
Upstream negotiation's primary use is to renegotiate (part of) an
|
|
already-negotiated pipeline to a new format. Some practical examples
|
|
include to select a different video size because the size of the video
|
|
window changed, and the video output itself is not capable of rescaling,
|
|
or because the audio channel configuration changed.
|
|
</para>
|
|
<para>
|
|
Upstream caps renegotiation is requested by sending a GST_EVENT_RECONFIGURE
|
|
event upstream. The idea is that it will instruct the upstream element
|
|
to reconfigure its caps by doing a new query for the allowed caps and then
|
|
choosing a new caps. The element that sends out the RECONFIGURE event
|
|
would influence the selection of the new caps by returning the new
|
|
preferred caps from its GST_QUERY_CAPS query function. The RECONFIGURE
|
|
event will set the GST_PAD_FLAG_NEED_RECONFIGURE on all pads that it
|
|
travels over.
|
|
</para>
|
|
<para>
|
|
It is important to note here that different elements actually have
|
|
different responsibilities here:
|
|
</para>
|
|
<itemizedlist>
|
|
<listitem>
|
|
<para>
|
|
Elements that want to propose a new format upstream need to first
|
|
check if the new caps are acceptable upstream with an ACCEPT_CAPS
|
|
query. Then they would send a RECONFIGURE event and be prepared to
|
|
answer the CAPS query with the new preferred format. It should be
|
|
noted that when there is no upstream element that can (or wants)
|
|
to renegotiate, the element needs to deal with the currently
|
|
configured format.
|
|
</para>
|
|
</listitem>
|
|
<listitem>
|
|
<para>
|
|
Elements that operate in transform negotiation according to
|
|
<xref linkend="section-nego-transform"/> pass the RECONFIGURE
|
|
event upstream. Because these elements simply do a fixed transform
|
|
based on the upstream caps, they need to send the event upstream
|
|
so that it can select a new format.
|
|
</para>
|
|
</listitem>
|
|
<listitem>
|
|
<para>
|
|
Elements that operate in fixed negotiation
|
|
(<xref linkend="section-nego-fixed"/>) drop the RECONFIGURE event.
|
|
These elements can't reconfigure and their output caps don't depend
|
|
on the upstream caps so the event can be dropped.
|
|
</para>
|
|
</listitem>
|
|
<listitem>
|
|
<para>
|
|
Elements that can be reconfigured on the source pad (source pads
|
|
implementing dynamic negotiation in
|
|
<xref linkend="section-nego-dynamic"/>) should check its
|
|
NEED_RECONFIGURE flag with
|
|
<function>gst_pad_check_reconfigure ()</function> and it should
|
|
start renegotiation when the function returns TRUE.
|
|
</para>
|
|
</listitem>
|
|
</itemizedlist>
|
|
</sect1>
|
|
|
|
<sect1 id="section-nego-getcaps" xreflabel="Implementing a CAPS query function">
|
|
<title>Implementing a CAPS query function</title>
|
|
<para>
|
|
A <function>_query ()</function>-function with the GST_QUERY_CAPS query
|
|
type is called when a peer element would like to know which formats
|
|
this pad supports, and in what order of preference. The return value
|
|
should be all formats that this elements supports, taking into account
|
|
limitations of peer elements further downstream or upstream, sorted by
|
|
order of preference, highest preference first.
|
|
</para>
|
|
<para>
|
|
</para>
|
|
<programlisting>
|
|
<![CDATA[
|
|
static gboolean
|
|
gst_my_filter_query (GstPad *pad, GstObject * parent, GstQuery * query)
|
|
{
|
|
gboolean ret;
|
|
GstMyFilter *filter = GST_MY_FILTER (parent);
|
|
|
|
switch (GST_QUERY_TYPE (query)) {
|
|
case GST_QUERY_CAPS
|
|
{
|
|
GstPad *otherpad;
|
|
GstCaps *temp, *caps, *filt, *tcaps;
|
|
gint i;
|
|
|
|
otherpad = (pad == filter->srcpad) ? filter->sinkpad :
|
|
filter->srcpad;
|
|
caps = gst_pad_get_allowed_caps (otherpad);
|
|
|
|
gst_query_parse_caps (query, &filt);
|
|
|
|
/* We support *any* samplerate, indifferent from the samplerate
|
|
* supported by the linked elements on both sides. */
|
|
for (i = 0; i < gst_caps_get_size (caps); i++) {
|
|
GstStructure *structure = gst_caps_get_structure (caps, i);
|
|
|
|
gst_structure_remove_field (structure, "rate");
|
|
}
|
|
|
|
/* make sure we only return results that intersect our
|
|
* padtemplate */
|
|
tcaps = gst_pad_get_pad_template_caps (pad);
|
|
if (tcaps) {
|
|
temp = gst_caps_intersect (caps, tcaps);
|
|
gst_caps_unref (caps);
|
|
gst_caps_unref (tcaps);
|
|
caps = temp;
|
|
}
|
|
/* filter against the query filter when needed */
|
|
if (filt) {
|
|
temp = gst_caps_intersect (caps, filt);
|
|
gst_caps_unref (caps);
|
|
caps = temp;
|
|
}
|
|
gst_query_set_caps_result (query, caps);
|
|
gst_caps_unref (caps);
|
|
ret = TRUE;
|
|
break;
|
|
}
|
|
default:
|
|
ret = gst_pad_query_default (pad, parent, query);
|
|
break;
|
|
}
|
|
return ret;
|
|
}
|
|
]]>
|
|
</programlisting>
|
|
</sect1>
|
|
|
|
<sect1 id="section-nego-pullmode">
|
|
<title>Pull-mode Caps negotiation</title>
|
|
<para>
|
|
WRITEME, the mechanism of pull-mode negotiation is not yet fully
|
|
understood.
|
|
</para>
|
|
|
|
<para>
|
|
Using all the knowledge you've acquired by reading this chapter, you
|
|
should be able to write an element that does correct caps negotiation.
|
|
If in doubt, look at other elements of the same type in our git
|
|
repository to get an idea of how they do what you want to do.
|
|
</para>
|
|
</sect1>
|
|
</chapter>
|