63 lines
2.9 KiB
XML
63 lines
2.9 KiB
XML
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<section id="planar-apis">
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<title>Single- and multi-planar APIs</title>
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<para>Some devices require data for each input or output video frame
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to be placed in discontiguous memory buffers. In such cases, one
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video frame has to be addressed using more than one memory address, i.e. one
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pointer per "plane". A plane is a sub-buffer of the current frame. For
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examples of such formats see <xref linkend="pixfmt" />.</para>
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<para>Initially, V4L2 API did not support multi-planar buffers and a set of
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extensions has been introduced to handle them. Those extensions constitute
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what is being referred to as the "multi-planar API".</para>
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<para>Some of the V4L2 API calls and structures are interpreted differently,
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depending on whether single- or multi-planar API is being used. An application
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can choose whether to use one or the other by passing a corresponding buffer
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type to its ioctl calls. Multi-planar versions of buffer types are suffixed
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with an `_MPLANE' string. For a list of available multi-planar buffer types
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see &v4l2-buf-type;.
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</para>
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<section>
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<title>Multi-planar formats</title>
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<para>Multi-planar API introduces new multi-planar formats. Those formats
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use a separate set of FourCC codes. It is important to distinguish between
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the multi-planar API and a multi-planar format. Multi-planar API calls can
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handle all single-planar formats as well (as long as they are passed in
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multi-planar API structures), while the single-planar API cannot
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handle multi-planar formats.</para>
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</section>
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<section>
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<title>Calls that distinguish between single and multi-planar APIs</title>
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<variablelist>
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<varlistentry>
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<term>&VIDIOC-QUERYCAP;</term>
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<listitem><para>Two additional multi-planar capabilities are added. They can
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be set together with non-multi-planar ones for devices that handle
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both single- and multi-planar formats.</para></listitem>
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</varlistentry>
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<varlistentry>
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<term>&VIDIOC-G-FMT;, &VIDIOC-S-FMT;, &VIDIOC-TRY-FMT;</term>
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<listitem><para>New structures for describing multi-planar formats are added:
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&v4l2-pix-format-mplane; and &v4l2-plane-pix-format;. Drivers may
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define new multi-planar formats, which have distinct FourCC codes from
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the existing single-planar ones.</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term>&VIDIOC-QBUF;, &VIDIOC-DQBUF;, &VIDIOC-QUERYBUF;</term>
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<listitem><para>A new &v4l2-plane; structure for describing planes is added.
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Arrays of this structure are passed in the new
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<structfield>m.planes</structfield> field of &v4l2-buffer;.</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term>&VIDIOC-REQBUFS;</term>
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<listitem><para>Will allocate multi-planar buffers as requested.</para></listitem>
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</varlistentry>
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</variablelist>
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</section>
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</section>
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