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