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132 lines
4.1 KiB
XML
132 lines
4.1 KiB
XML
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<!-- ############ chapter ############# -->
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<chapter id="chapter-building-chainfn">
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<title>The chain function</title>
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<para>
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The chain function is the function in which all data processing takes
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place. In the case of a simple filter, <function>_chain ()</function>
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functions are mostly linear functions - so for each incoming buffer,
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one buffer will go out, too. Below is a very simple implementation of
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a chain function:
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</para>
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<programlisting><!-- example-begin chain.c a --><!--
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#include "init.func"
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#include "caps.func"
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static gboolean
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gst_my_filter_event (GstPad * pad, GstEvent * event)
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{
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return gst_pad_event_default (pad, event);
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}
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--><!-- example-end chain.c a -->
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<!-- example-begin chain.c b -->
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static GstFlowReturn
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gst_my_filter_chain (GstPad *pad,
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GstBuffer *buf)
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{
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GstMyFilter *filter = GST_MY_FILTER (GST_OBJECT_PARENT (pad));
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if (!filter->silent)
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g_print ("Have data of size %u bytes!\n", GST_BUFFER_SIZE (buf));
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return gst_pad_push (filter->srcpad, buf);
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}
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<!-- example-end chain.c b -->
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<!-- example-begin chain.c c --><!--
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static GstStateChangeReturn
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gst_my_filter_change_state (GstElement * element, GstStateChange transition)
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{
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return GST_CALL_PARENT_WITH_DEFAULT (GST_ELEMENT_CLASS,
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change_state, (element, transition), GST_STATE_CHANGE_SUCCESS);
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}
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#include "register.func"
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--><!-- example-end chain.c c --></programlisting>
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<para>
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Obviously, the above doesn't do much useful. Instead of printing that the
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data is in, you would normally process the data there. Remember, however,
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that buffers are not always writeable. In more advanced elements (the ones
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that do event processing), you may want to additionally specify an event
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handling function, which will be called when stream-events are sent (such
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as end-of-stream, newsegment, tags, etc.).
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</para>
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<programlisting>
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static void
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gst_my_filter_init (GstMyFilter * filter)
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{
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[..]
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gst_pad_set_event_function (filter->sinkpad,
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gst_my_filter_event);
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[..]
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}
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<!-- example-begin chain2.c a --><!--
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#include "init.func"
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#include "caps.func"
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#include "chain.func"
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--><!-- example-end chain2.c a -->
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<!-- example-begin chain.func a --><!--
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static void
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gst_my_filter_stop_processing (GstMyFilter * filter)
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{
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}
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static GstBuffer *
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gst_my_filter_process_data (GstMyFilter * filter, const GstBuffer * buf)
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{
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return NULL;
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}
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--><!-- example-end chain.func a -->
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<!-- example-begin chain.func b -->
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static gboolean
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gst_my_filter_event (GstPad *pad,
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GstEvent *event)
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{
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GstMyFilter *filter = GST_MY_FILTER (GST_OBJECT_PARENT (pad));
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switch (GST_EVENT_TYPE (event)) {
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case GST_EVENT_EOS:
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/* end-of-stream, we should close down all stream leftovers here */
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gst_my_filter_stop_processing (filter);
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break;
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default:
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break;
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}
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return gst_pad_event_default (pad, event);
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}
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static GstFlowReturn
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gst_my_filter_chain (GstPad *pad,
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GstBuffer *buf)
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{
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GstMyFilter *filter = GST_MY_FILTER (gst_pad_get_parent (pad));
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GstBuffer *outbuf;
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outbuf = gst_my_filter_process_data (filter, buf);
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gst_buffer_unref (buf);
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if (!outbuf) {
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/* something went wrong - signal an error */
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GST_ELEMENT_ERROR (GST_ELEMENT (filter), STREAM, FAILED, (NULL), (NULL));
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return GST_FLOW_ERROR;
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}
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return gst_pad_push (filter->srcpad, outbuf);
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}
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<!-- example-end chain.func b -->
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<!-- example-begin chain2.c b --><!--
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static GstStateChangeReturn
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gst_my_filter_change_state (GstElement * element, GstStateChange transition)
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{
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return GST_CALL_PARENT_WITH_DEFAULT (GST_ELEMENT_CLASS,
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change_state, (element, transition), GST_STATE_CHANGE_SUCCESS);
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}
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#include "register.func"
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--><!-- example-end chain2.c b --></programlisting>
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<para>
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In some cases, it might be useful for an element to have control over the
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input data rate, too. In that case, you probably want to write a so-called
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<emphasis>loop-based</emphasis> element. Source elements (with only source
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pads) can also be <emphasis>get-based</emphasis> elements. These concepts
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will be explained in the advanced section of this guide, and in the section
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that specifically discusses source pads.
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</para>
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</chapter>
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