mirror of
https://gitlab.freedesktop.org/gstreamer/gstreamer.git
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817 lines
29 KiB
XML
817 lines
29 KiB
XML
<chapter id="chapter-allocation" xreflabel="Memory allocation">
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<title>Memory allocation</title>
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<para>
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Memory allocation and management is a very important topic in
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multimedia. High definition video uses many megabytes to store
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one single frame of video. It is important to reuse the memory
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when possible instead of constantly allocating and freeing
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the memory.
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</para>
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<para>
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Multimedia systems usually use special purpose chips, such as
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DSPs or GPUs to perform the heavy lifting (especially for video).
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These special purpose chips have usually strict requirements
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for the memory that they can operate on and how the memory
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is accessed.
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</para>
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<para>
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This chapter talks about the memory management features that
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&GStreamer; plugins can use. We will first talk about the
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lowlevel <classname>GstMemory</classname> object that manages
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access to a piece of memory. We then continue with
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<classname>GstBuffer</classname> that is used to exchange data
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between plugins (and the application) and that uses
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<classname>GstMemory</classname>. We talk about
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<classname>GstMeta</classname> that can be placed on buffers to
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give extra info about the buffer and its memory.
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For efficiently managing buffers of the same size, we take a
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look at <classname>GstBufferPool</classname>. To conclude this
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chapter we take a look at the GST_QUERY_ALLOCATION query that
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is used to negotiate memory management options between elements.
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</para>
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<sect1 id="section-allocation-memory" xreflabel="GstMemory">
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<title>GstMemory</title>
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<para>
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<classname>GstMemory</classname> is an object that manages a region
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of memory. The memory object points to a region of memory of
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<quote>maxsize</quote>. The area in this memory starting at
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<quote>offset</quote> and for <quote>size</quote> bytes is the
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accessible region in the memory. the maxsize of the memory can
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never be changed after the object is created, however, the offset
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and size can be changed.
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</para>
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<sect2 id="section-allocation-allocator" xreflabel="GstAllocator">
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<title>GstAllocator</title>
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<para>
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<classname>GstMemory</classname> objects are created by a
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<classname>GstAllocator</classname> object. Most allocators implement the
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default <function>gst_allocator_alloc()</function> method but some allocator
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might implement a different method, for example when additional parameters
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are needed to allocate the specific memory.
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</para>
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<para>
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Different allocators exist for, for example, system memory, shared memory
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and memory backed by a DMAbuf file descriptor. To implement support for a
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new kind of memory type, you must implement a new allocator object as shown
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below.
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</para>
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</sect2>
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<sect2 id="section-allocation-memory-ex" xreflabel="GstMemory-ex">
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<title>GstMemory API example</title>
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<para>
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Data access to the memory wrapped by the <classname>GstMemory</classname>
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object is always protected with a <function>gst_memory_map()</function>
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and <function>gst_memory_unmap()</function> pair. An access mode
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(read/write) must be given when mapping memory. The map
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function returns a pointer to the valid memory region that can
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then be accessed according to the requested access mode.
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</para>
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<para>
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Below is an example of making a <classname>GstMemory</classname>
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object and using the <function>gst_memory_map()</function> to
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access the memory region.
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</para>
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<programlisting>
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<![CDATA[
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[...]
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GstMemory *mem;
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GstMapInfo info;
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gint i;
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/* allocate 100 bytes */
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mem = gst_allocator_alloc (NULL, 100, NULL);
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/* get access to the memory in write mode */
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gst_memory_map (mem, &info, GST_MAP_WRITE);
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/* fill with pattern */
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for (i = 0; i < info.size; i++)
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info.data[i] = i;
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/* release memory */
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gst_memory_unmap (mem, &info);
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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-allocation-allocator-ex" xreflabel="GstAllocator-ex">
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<title>Implementing a GstAllocator</title>
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<para>
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WRITEME
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</para>
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</sect2>
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</sect1>
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<sect1 id="section-allocation-buffer" xreflabel="GstBuffer">
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<title>GstBuffer</title>
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<para>
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A <classname>GstBuffer</classname> is an lightweight object that
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is passed from an upstream to a downstream element and contains
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memory and metadata. It represents the multimedia content that
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is pushed or pull downstream by elements.
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</para>
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<para>
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The buffer contains one or more <classname>GstMemory</classname>
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objects that represent the data in the buffer.
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</para>
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<para>
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Metadata in the buffer consists of:
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</para>
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<itemizedlist mark="opencircle">
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<listitem>
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<para>
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DTS and PTS timestamps. These represent the decoding and
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presentation timestamps of the buffer content and is used by
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synchronizing elements to schedule buffers. Both these timestamps
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can be GST_CLOCK_TIME_NONE when unknown/undefined.
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</para>
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</listitem>
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<listitem>
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<para>
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The duration of the buffer contents. This duration can be
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GST_CLOCK_TIME_NONE when unknown/undefined.
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</para>
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</listitem>
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<listitem>
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<para>
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Media specific offsets and offset_end. For video this is the
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frame number in the stream and for audio the sample number. Other
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definitions for other media exist.
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</para>
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</listitem>
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<listitem>
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<para>
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Arbitrary structures via <classname>GstMeta</classname>, see below.
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</para>
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</listitem>
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</itemizedlist>
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<sect2 id="section-allocation-writability" xreflabel="GstBuffer-write">
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<title>GstBuffer writability</title>
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<para>
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A buffer is writable when the refcount of the object is exactly 1, meaning
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that only one object is holding a ref to the buffer. You can only
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modify anything in the buffer when the buffer is writable. This means
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that you need to call <function>gst_buffer_make_writable()</function>
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before changing the timestamps, offsets, metadata or adding and
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removing memory blocks.
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</para>
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</sect2>
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<sect2 id="section-allocation-buffer-ex" xreflabel="GstBuffer-ex">
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<title>GstBuffer API examples</title>
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<para>
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You can create a buffer with <function>gst_buffer_new ()</function>
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and then add memory objects to it or you can use a convenience function
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<function>gst_buffer_new_allocate ()</function> which combines the
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two. It's also possible to wrap existing memory with
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<function>gst_buffer_new_wrapped_full () </function> where you can
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give the function to call when the memory should be freed.
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</para>
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<para>
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You can access the memory of the buffer by getting and mapping the
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<classname>GstMemory</classname> objects individually or by using
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<function>gst_buffer_map ()</function>. The latter merges all the
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memory into one big block and then gives you a pointer to this block.
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</para>
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<para>
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Below is an example of how to create a buffer and access its memory.
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</para>
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<programlisting>
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<![CDATA[
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[...]
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GstBuffer *buffer;
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GstMemory *mem;
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GstMapInfo info;
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/* make empty buffer */
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buffer = gst_buffer_new ();
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/* make memory holding 100 bytes */
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mem = gst_allocator_alloc (NULL, 100, NULL);
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/* add the buffer */
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gst_buffer_append_memory (buffer, mem);
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[...]
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/* get WRITE access to the memory and fill with 0xff */
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gst_buffer_map (buffer, &info, GST_MAP_WRITE);
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memset (info.data, 0xff, info.size);
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gst_buffer_unmap (buffer, &info);
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[...]
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/* free the buffer */
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gst_buffer_unref (buffer);
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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-allocation-meta" xreflabel="GstMeta">
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<title>GstMeta</title>
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<para>
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With the <classname>GstMeta</classname> system you can add arbitrary
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structures on buffers. These structures describe extra properties
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of the buffer such as cropping, stride, region of interest etc.
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</para>
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<para>
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The metadata system separates API specification (what the metadata
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and its API look like) and the implementation (how it works). This makes
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it possible to make different implementations of the same API,
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for example, depending on the hardware you are running on.
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</para>
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<sect2 id="section-allocation-meta-ex" xreflabel="GstMeta-ex">
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<title>GstMeta API example</title>
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<para>
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After allocating a new buffer, you can add metadata to the buffer
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with the metadata specific API. This means that you will need to
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link to the header file where the metadata is defined to use
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its API.
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</para>
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<para>
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By convention, a metadata API with name <classname>FooBar</classname>
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should provide two methods, a
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<function>gst_buffer_add_foo_bar_meta ()</function> and a
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<function>gst_buffer_get_foo_bar_meta ()</function>. Both functions
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should return a pointer to a <classname>FooBarMeta</classname>
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structure that contains the metadata fields. Some of the
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<function>_add_*_meta ()</function> can have extra parameters that
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will usually be used to configure the metadata structure for you.
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</para>
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<para>
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Let's have a look at the metadata that is used to specify a cropping
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region for video frames.
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</para>
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<programlisting>
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<![CDATA[
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#include <gst/video/gstvideometa.h>
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[...]
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GstVideoCropMeta *meta;
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/* buffer points to a video frame, add some cropping metadata */
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meta = gst_buffer_add_video_crop_meta (buffer);
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/* configure the cropping metadata */
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meta->x = 8;
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meta->y = 8;
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meta->width = 120;
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meta->height = 80;
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[...]
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]]>
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</programlisting>
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<para>
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An element can then use the metadata on the buffer when rendering
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the frame like this:
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</para>
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<programlisting>
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<![CDATA[
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#include <gst/video/gstvideometa.h>
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[...]
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GstVideoCropMeta *meta;
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/* buffer points to a video frame, get the cropping metadata */
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meta = gst_buffer_get_video_crop_meta (buffer);
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if (meta) {
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/* render frame with cropping */
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_render_frame_cropped (buffer, meta->x, meta->y, meta->width, meta->height);
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} else {
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/* render frame */
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_render_frame (buffer);
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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-allocation-meta-new" xreflabel="GstMeta-new">
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<title>Implementing new GstMeta</title>
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<para>
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In the next sections we show how you can add new metadata to the
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system and use it on buffers.
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</para>
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<sect3 id="section-allocation-meta-api" xreflabel="GstMeta-api">
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<title>Define the metadata API</title>
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<para>
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First we need to define what our API will look like and we
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will have to register this API to the system. This is important
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because this API definition will be used when elements negotiate
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what kind of metadata they will exchange. The API definition
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also contains arbitrary tags that give hints about what the
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metadata contains. This is important when we see how metadata
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is preserved when buffers pass through the pipeline.
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</para>
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<para>
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If you are making a new implementation of an existing API,
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you can skip this step and move on to the implementation step.
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</para>
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<para>
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First we start with making the
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<filename>my-example-meta.h</filename> header file that will contain
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the definition of the API and structure for our metadata.
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</para>
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<programlisting>
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<![CDATA[
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#include <gst/gst.h>
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typedef struct _MyExampleMeta MyExampleMeta;
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struct _MyExampleMeta {
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GstMeta meta;
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gint age;
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gchar *name;
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};
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GType my_example_meta_api_get_type (void);
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#define MY_EXAMPLE_META_API_TYPE (my_example_meta_api_get_type())
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#define gst_buffer_get_my_example_meta(b) \
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((MyExampleMeta*)gst_buffer_get_meta((b),MY_EXAMPLE_META_API_TYPE))
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]]>
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</programlisting>
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<para>
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The metadata API definition consists of the definition of the
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structure that holds a gint and a string. The first field in
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the structure must be <classname>GstMeta</classname>.
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</para>
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<para>
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We also define a <function>my_example_meta_api_get_type ()</function>
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function that will register out metadata API definition. We
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also define a convenience macro
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<function>gst_buffer_get_my_example_meta ()</function> that simply
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finds and returns the metadata with our new API.
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</para>
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<para>
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Next let's have a look at how the
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<function>my_example_meta_api_get_type ()</function> function is
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implemented in the <filename>my-example-meta.c</filename> file.
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</para>
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<programlisting>
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<![CDATA[
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#include "my-example-meta.h"
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GType
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my_example_meta_api_get_type (void)
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{
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static volatile GType type;
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static const gchar *tags[] = { "foo", "bar", NULL };
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if (g_once_init_enter (&type)) {
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GType _type = gst_meta_api_type_register ("MyExampleMetaAPI", tags);
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g_once_init_leave (&type, _type);
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}
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return type;
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}
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]]>
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</programlisting>
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<para>
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As you can see, it simply uses the
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<function>gst_meta_api_type_register ()</function> function to
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register a name for the api and some tags. The result is a
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new pointer GType that defines the newly registered API.
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</para>
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</sect3>
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<sect3 id="section-allocation-meta-impl" xreflabel="GstMeta-impl">
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<title>Implementing a metadata API</title>
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<para>
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Next we can make an implementation for a registered metadata
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API GType. The implementation detail of a metadata API
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are kept in a <classname>GstMetaInfo</classname> structure
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that you will make available to the users of your metadata
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API implementation with a <function>my_example_meta_get_info ()</function>
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function and a convenience <function>MY_EXAMPLE_META_INFO</function>
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macro. You will also make a method to add your metadata
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implementation to a <classname>GstBuffer</classname>.
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Your <filename>my-example-meta.h</filename> header file will
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need these additions:
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</para>
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<programlisting>
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<![CDATA[
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[...]
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/* implementation */
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const GstMetaInfo *my_example_meta_get_info (void);
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#define MY_EXAMPLE_META_INFO (my_example_meta_get_info())
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MyExampleMeta * gst_buffer_add_my_example_meta (GstBuffer *buffer,
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gint age,
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const gchar *name);
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]]>
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</programlisting>
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<para>
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Let's have a look at how these functions are
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implemented in the <filename>my-example-meta.c</filename> file.
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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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my_example_meta_init (GstMeta * meta, gpointer params, GstBuffer * buffer)
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{
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MyExampleMeta *emeta = (MyExampleMeta *) meta;
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emeta->age = 0;
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emeta->name = NULL;
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return TRUE;
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}
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static gboolean
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my_example_meta_transform (GstBuffer * transbuf, GstMeta * meta,
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GstBuffer * buffer, GQuark type, gpointer data)
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{
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MyExampleMeta *emeta = (MyExampleMeta *) meta;
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/* we always copy no matter what transform */
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gst_buffer_add_my_example_meta (transbuf, emeta->age, emeta->name);
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return TRUE;
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}
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static void
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my_example_meta_free (GstMeta * meta, GstBuffer * buffer)
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{
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MyExampleMeta *emeta = (MyExampleMeta *) meta;
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g_free (emeta->name)
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emeta->name = NULL;
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}
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const GstMetaInfo *
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my_example_meta_get_info (void)
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{
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static const GstMetaInfo *meta_info = NULL;
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if (g_once_init_enter (&meta_info)) {
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const GstMetaInfo *mi = gst_meta_register (MY_EXAMPLE_META_API_TYPE,
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"MyExampleMeta",
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sizeof (MyExampleMeta),
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my_example_meta_init,
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my_example_meta_free,
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my_example_meta_transform);
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g_once_init_leave (&meta_info, mi);
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}
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return meta_info;
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}
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MyExampleMeta *
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gst_buffer_add_my_example_meta (GstBuffer *buffer,
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gint age,
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const gchar *name)
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{
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MyExampleMeta *meta;
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g_return_val_if_fail (GST_IS_BUFFER (buffer), NULL);
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meta = (MyExampleMeta *) gst_buffer_add_meta (buffer,
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MY_EXAMPLE_META_INFO, NULL);
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meta->age = age;
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meta->name = g_strdup (name);
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return meta;
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}
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]]>
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</programlisting>
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<para>
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<function>gst_meta_register ()</function> registers the implementation
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details, like the API that you implement and the size of the
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metadata structure along with methods to initialize and free the
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memory area. You can also implement a transform function that will
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be called when a certain transformation (identified by the quark and
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quark specific data) is performed on a buffer.
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</para>
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<para>
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Lastly, you implement a <function>gst_buffer_add_*_meta()</function>
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that adds the metadata implementation to a buffer and sets the
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values of the metadata.
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</para>
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</sect3>
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</sect2>
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</sect1>
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<sect1 id="section-allocation-bufferpool" xreflabel="GstBufferPool">
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|
<title>GstBufferPool</title>
|
|
<para>
|
|
The <classname>GstBufferPool</classname> object provides a convenient
|
|
base class for managing lists of reusable buffers. Essential for this
|
|
object is that all the buffers have the same properties such as size,
|
|
padding, metadata and alignment.
|
|
</para>
|
|
<para>
|
|
A bufferpool object can be configured to manage a minimum and maximum
|
|
amount of buffers of a specific size. A bufferpool can also be
|
|
configured to use a specific <classname>GstAllocator</classname> for
|
|
the memory of the buffers. There is support in the bufferpool to enable
|
|
bufferpool specific options, such as adding <classname>GstMeta</classname>
|
|
to the buffers in the pool or such as enabling specific padding on
|
|
the memory in the buffers.
|
|
</para>
|
|
<para>
|
|
A Bufferpool can be inactivate and active. In the inactive state,
|
|
you can configure the pool. In the active state, you can't change
|
|
the configuration anymore but you can acquire and release buffers
|
|
from/to the pool.
|
|
</para>
|
|
<para>
|
|
In the following sections we take a look at how you can use
|
|
a bufferpool.
|
|
</para>
|
|
|
|
<sect2 id="section-allocation-pool-ex" xreflabel="GstBufferPool-ex">
|
|
<title>GstBufferPool API example</title>
|
|
<para>
|
|
Many different bufferpool implementations can exist; they are all
|
|
subclasses of the base class <classname>GstBufferPool</classname>.
|
|
For this example, we will assume we somehow have access to a
|
|
bufferpool, either because we created it ourselves or because
|
|
we were given one as a result of the ALLOCATION query as we will
|
|
see below.
|
|
</para>
|
|
<para>
|
|
The bufferpool is initially in the inactive state so that we can
|
|
configure it. Trying to configure a bufferpool that is not in the
|
|
inactive state will fail. Likewise, trying to activate a bufferpool
|
|
that is not configured will fail.
|
|
</para>
|
|
<programlisting>
|
|
<![CDATA[
|
|
GstStructure *config;
|
|
|
|
[...]
|
|
|
|
/* get config structure */
|
|
config = gst_buffer_pool_get_config (pool);
|
|
|
|
/* set caps, size, minimum and maximum buffers in the pool */
|
|
gst_buffer_pool_config_set_params (config, caps, size, min, max);
|
|
|
|
/* configure allocator and parameters */
|
|
gst_buffer_pool_config_set_allocator (config, allocator, ¶ms);
|
|
|
|
/* store the updated configuration again */
|
|
gst_buffer_pool_set_config (pool, config);
|
|
|
|
[...]
|
|
]]>
|
|
</programlisting>
|
|
<para>
|
|
The configuration of the bufferpool is maintained in a generic
|
|
<classname>GstStructure</classname> that can be obtained with
|
|
<function>gst_buffer_pool_get_config()</function>. Convenience
|
|
methods exist to get and set the configuration options in this
|
|
structure. After updating the structure, it is set as the current
|
|
configuration in the bufferpool again with
|
|
<function>gst_buffer_pool_set_config()</function>.
|
|
</para>
|
|
<para>
|
|
The following options can be configured on a bufferpool:
|
|
</para>
|
|
<itemizedlist mark="opencircle">
|
|
<listitem>
|
|
<para>
|
|
The caps of the buffers to allocate.
|
|
</para>
|
|
</listitem>
|
|
<listitem>
|
|
<para>
|
|
The size of the buffers. This is the suggested size of the
|
|
buffers in the pool. The pool might decide to allocate larger
|
|
buffers to add padding.
|
|
</para>
|
|
</listitem>
|
|
<listitem>
|
|
<para>
|
|
The minimum and maximum amount of buffers in the pool. When
|
|
minimum is set to > 0, the bufferpool will pre-allocate this
|
|
amount of buffers. When maximum is not 0, the bufferpool
|
|
will allocate up to maximum amount of buffers.
|
|
</para>
|
|
</listitem>
|
|
<listitem>
|
|
<para>
|
|
The allocator and parameters to use. Some bufferpools might
|
|
ignore the allocator and use its internal one.
|
|
</para>
|
|
</listitem>
|
|
<listitem>
|
|
<para>
|
|
Other arbitrary bufferpool options identified with a string.
|
|
a bufferpool lists the supported options with
|
|
<function>gst_buffer_pool_get_options()</function> and you
|
|
can ask if an option is supported with
|
|
<function>gst_buffer_pool_has_option()</function>. The option
|
|
can be enabled by adding it to the configuration structure
|
|
with <function>gst_buffer_pool_config_add_option ()</function>.
|
|
These options are used to enable things like letting the
|
|
pool set metadata on the buffers or to add extra configuration
|
|
options for padding, for example.
|
|
</para>
|
|
</listitem>
|
|
</itemizedlist>
|
|
<para>
|
|
After the configuration is set on the bufferpool, the pool can
|
|
be activated with
|
|
<function>gst_buffer_pool_set_active (pool, TRUE)</function>. From
|
|
that point on you can use
|
|
<function>gst_buffer_pool_acquire_buffer ()</function> to retrieve
|
|
a buffer from the pool, like this:
|
|
</para>
|
|
<programlisting>
|
|
<![CDATA[
|
|
[...]
|
|
|
|
GstFlowReturn ret;
|
|
GstBuffer *buffer;
|
|
|
|
ret = gst_buffer_pool_acquire_buffer (pool, &buffer, NULL);
|
|
if (G_UNLIKELY (ret != GST_FLOW_OK))
|
|
goto pool_failed;
|
|
|
|
[...]
|
|
]]>
|
|
</programlisting>
|
|
<para>
|
|
It is important to check the return value of the acquire function
|
|
because it is possible that it fails: When your
|
|
element shuts down, it will deactivate the bufferpool and then
|
|
all calls to acquire will return GST_FLOW_FLUSHNG.
|
|
</para>
|
|
<para>
|
|
All buffers that are acquired from the pool will have their pool
|
|
member set to the original pool. When the last ref is decremented
|
|
on the buffer, &GStreamer; will automatically call
|
|
<function>gst_buffer_pool_release_buffer()</function> to release
|
|
the buffer back to the pool. You (or any other downstream element)
|
|
don't need to know if a buffer came from a pool, you can just
|
|
unref it.
|
|
</para>
|
|
</sect2>
|
|
|
|
<sect2 id="section-allocation-pool-impl" xreflabel="GstBufferPool-impl">
|
|
<title>Implementing a new GstBufferPool</title>
|
|
<para>
|
|
WRITEME
|
|
</para>
|
|
</sect2>
|
|
|
|
</sect1>
|
|
|
|
<sect1 id="section-allocation-query" xreflabel="GST_QUERY_ALLOCATION">
|
|
<title>GST_QUERY_ALLOCATION</title>
|
|
<para>
|
|
The ALLOCATION query is used to negotiate
|
|
<classname>GstMeta</classname>, <classname>GstBufferPool</classname>
|
|
and <classname>GstAllocator</classname> between elements. Negotiation
|
|
of the allocation strategy is always initiated and decided by a srcpad
|
|
after it has negotiated a format and before it decides to push buffers.
|
|
A sinkpad can suggest an allocation strategy but it is ultimately the
|
|
source pad that will decide based on the suggestions of the downstream
|
|
sink pad.
|
|
</para>
|
|
<para>
|
|
The source pad will do a GST_QUERY_ALLOCATION with the negotiated caps
|
|
as a parameter. This is needed so that the downstream element knows
|
|
what media type is being handled. A downstream sink pad can answer the
|
|
allocation query with the following results:
|
|
</para>
|
|
<itemizedlist mark="opencircle">
|
|
<listitem>
|
|
<para>
|
|
An array of possible <classname>GstBufferPool</classname> suggestions
|
|
with suggested size, minimum and maximum amount of buffers.
|
|
</para>
|
|
</listitem>
|
|
<listitem>
|
|
<para>
|
|
An array of GstAllocator objects along with suggested allocation
|
|
parameters such as flags, prefix, alignment and padding. These
|
|
allocators can also be configured in a bufferpool when this is
|
|
supported by the bufferpool.
|
|
</para>
|
|
</listitem>
|
|
<listitem>
|
|
<para>
|
|
An array of supported <classname>GstMeta</classname> implementations
|
|
along with metadata specific parameters.
|
|
It is important that the upstream element knows what kind of
|
|
metadata is supported downstream before it places that metadata
|
|
on buffers.
|
|
</para>
|
|
</listitem>
|
|
</itemizedlist>
|
|
<para>
|
|
When the GST_QUERY_ALLOCATION returns, the source pad will select
|
|
from the available bufferpools, allocators and metadata how it will
|
|
allocate buffers.
|
|
</para>
|
|
|
|
<sect2 id="section-allocation-query-ex" xreflabel="Allocation-ex">
|
|
<title>ALLOCATION query example</title>
|
|
<para>
|
|
Below is an example of the ALLOCATION query.
|
|
</para>
|
|
<programlisting>
|
|
<![CDATA[
|
|
#include <gst/video/video.h>
|
|
#include <gst/video/gstvideometa.h>
|
|
#include <gst/video/gstvideopool.h>
|
|
|
|
GstCaps *caps;
|
|
GstQuery *query;
|
|
GstStructure *structure;
|
|
GstBufferPool *pool;
|
|
GstStructure *config;
|
|
guint size, min, max;
|
|
|
|
[...]
|
|
|
|
/* find a pool for the negotiated caps now */
|
|
query = gst_query_new_allocation (caps, TRUE);
|
|
|
|
if (!gst_pad_peer_query (scope->srcpad, query)) {
|
|
/* query failed, not a problem, we use the query defaults */
|
|
}
|
|
|
|
if (gst_query_get_n_allocation_pools (query) > 0) {
|
|
/* we got configuration from our peer, parse them */
|
|
gst_query_parse_nth_allocation_pool (query, 0, &pool, &size, &min, &max);
|
|
} else {
|
|
pool = NULL;
|
|
size = 0;
|
|
min = max = 0;
|
|
}
|
|
|
|
if (pool == NULL) {
|
|
/* we did not get a pool, make one ourselves then */
|
|
pool = gst_video_buffer_pool_new ();
|
|
}
|
|
|
|
config = gst_buffer_pool_get_config (pool);
|
|
gst_buffer_pool_config_add_option (config, GST_BUFFER_POOL_OPTION_VIDEO_META);
|
|
gst_buffer_pool_config_set_params (config, caps, size, min, max);
|
|
gst_buffer_pool_set_config (pool, config);
|
|
|
|
/* and activate */
|
|
gst_buffer_pool_set_active (pool, TRUE);
|
|
|
|
[...]
|
|
]]>
|
|
</programlisting>
|
|
<para>
|
|
This particular implementation will make a custom
|
|
<classname>GstVideoBufferPool</classname> object that is specialized
|
|
in allocating video buffers. You can also enable the pool to
|
|
put <classname>GstVideoMeta</classname> metadata on the buffers from
|
|
the pool doing
|
|
<function>gst_buffer_pool_config_add_option (config,
|
|
GST_BUFFER_POOL_OPTION_VIDEO_META)</function>.
|
|
</para>
|
|
</sect2>
|
|
|
|
<sect2 id="section-allocation-query-base" xreflabel="Allocation-base">
|
|
<title>The ALLOCATION query in base classes</title>
|
|
<para>
|
|
In many baseclasses you will see the following virtual methods for
|
|
influencing the allocation strategy:
|
|
</para>
|
|
<itemizedlist>
|
|
<listitem>
|
|
<para>
|
|
<function>propose_allocation ()</function> should suggest
|
|
allocation parameters for the upstream element.
|
|
</para>
|
|
</listitem>
|
|
<listitem>
|
|
<para>
|
|
<function>decide_allocation ()</function> should decide the
|
|
allocation parameters from the suggestions received from
|
|
downstream.
|
|
</para>
|
|
</listitem>
|
|
</itemizedlist>
|
|
<para>
|
|
Implementors of these methods should modify the given
|
|
<classname>GstQuery</classname> object by updating the pool options
|
|
and allocation options.
|
|
</para>
|
|
</sect2>
|
|
</sect1>
|
|
</chapter>
|