62 lines
2.6 KiB
ReStructuredText
62 lines
2.6 KiB
ReStructuredText
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.. -*- coding: utf-8; mode: rst -*-
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.. _planar-apis:
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*****************************
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Single- and multi-planar APIs
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*****************************
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Some devices require data for each input or output video frame to be
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placed in discontiguous memory buffers. In such cases, one video frame
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has to be addressed using more than one memory address, i.e. one pointer
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per "plane". A plane is a sub-buffer of the current frame. For examples
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of such formats see :ref:`pixfmt`.
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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
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constitute what is being referred to as the "multi-planar API".
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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
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application can choose whether to use one or the other by passing a
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corresponding buffer type to its ioctl calls. Multi-planar versions of
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buffer types are suffixed with an ``_MPLANE`` string. For a list of
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available multi-planar buffer types see enum
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:c:type:`v4l2_buf_type`.
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Multi-planar formats
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====================
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Multi-planar API introduces new multi-planar formats. Those formats use
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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
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can handle all single-planar formats as well (as long as they are passed
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in multi-planar API structures), while the single-planar API cannot
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handle multi-planar formats.
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Calls that distinguish between single and multi-planar APIs
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===========================================================
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:ref:`VIDIOC_QUERYCAP <VIDIOC_QUERYCAP>`
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Two additional multi-planar capabilities are added. They can be set
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together with non-multi-planar ones for devices that handle both
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single- and multi-planar formats.
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:ref:`VIDIOC_G_FMT <VIDIOC_G_FMT>`, :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>`, :ref:`VIDIOC_TRY_FMT <VIDIOC_G_FMT>`
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New structures for describing multi-planar formats are added: struct
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:c:type:`v4l2_pix_format_mplane` and
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struct :c:type:`v4l2_plane_pix_format`.
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Drivers may define new multi-planar formats, which have distinct
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FourCC codes from the existing single-planar ones.
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:ref:`VIDIOC_QBUF <VIDIOC_QBUF>`, :ref:`VIDIOC_DQBUF <VIDIOC_QBUF>`, :ref:`VIDIOC_QUERYBUF <VIDIOC_QUERYBUF>`
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A new struct :c:type:`v4l2_plane` structure for
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describing planes is added. Arrays of this structure are passed in
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the new ``m.planes`` field of struct
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:c:type:`v4l2_buffer`.
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:ref:`VIDIOC_REQBUFS <VIDIOC_REQBUFS>`
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Will allocate multi-planar buffers as requested.
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