75 lines
2.9 KiB
ReStructuredText
75 lines
2.9 KiB
ReStructuredText
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.. Permission is granted to copy, distribute and/or modify this
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.. document under the terms of the GNU Free Documentation License,
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.. Version 1.1 or any later version published by the Free Software
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.. Foundation, with no Invariant Sections, no Front-Cover Texts
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.. and no Back-Cover Texts. A copy of the license is included at
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.. Documentation/media/uapi/fdl-appendix.rst.
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..
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.. TODO: replace it to GFDL-1.1-or-later WITH no-invariant-sections
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.. _metadata:
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******************
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Metadata Interface
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******************
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Metadata refers to any non-image data that supplements video frames with
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additional information. This may include statistics computed over the image,
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frame capture parameters supplied by the image source or device specific
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parameters for specifying how the device processes images. This interface is
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intended for transfer of metadata between the userspace and the hardware and
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control of that operation.
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The metadata interface is implemented on video device nodes. The device can be
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dedicated to metadata or can support both video and metadata as specified in its
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reported capabilities.
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Querying Capabilities
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=====================
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Device nodes supporting the metadata capture interface set the
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``V4L2_CAP_META_CAPTURE`` flag in the ``device_caps`` field of the
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:c:type:`v4l2_capability` structure returned by the :c:func:`VIDIOC_QUERYCAP`
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ioctl. That flag means the device can capture metadata to memory. Similarly,
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device nodes supporting metadata output interface set the
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``V4L2_CAP_META_OUTPUT`` flag in the ``device_caps`` field of
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:c:type:`v4l2_capability` structure. That flag means the device can read
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metadata from memory.
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At least one of the read/write or streaming I/O methods must be supported.
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Data Format Negotiation
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=======================
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The metadata device uses the :ref:`format` ioctls to select the capture format.
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The metadata buffer content format is bound to that selected format. In addition
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to the basic :ref:`format` ioctls, the :c:func:`VIDIOC_ENUM_FMT` ioctl must be
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supported as well.
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To use the :ref:`format` ioctls applications set the ``type`` field of the
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:c:type:`v4l2_format` structure to ``V4L2_BUF_TYPE_META_CAPTURE`` or to
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``V4L2_BUF_TYPE_META_OUTPUT`` and use the :c:type:`v4l2_meta_format` ``meta``
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member of the ``fmt`` union as needed per the desired operation. Both drivers
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and applications must set the remainder of the :c:type:`v4l2_format` structure
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to 0.
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.. c:type:: v4l2_meta_format
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.. tabularcolumns:: |p{1.4cm}|p{2.2cm}|p{13.9cm}|
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.. flat-table:: struct v4l2_meta_format
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:header-rows: 0
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:stub-columns: 0
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:widths: 1 1 2
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* - __u32
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- ``dataformat``
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- The data format, set by the application. This is a little endian
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:ref:`four character code <v4l2-fourcc>`. V4L2 defines metadata formats
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in :ref:`meta-formats`.
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* - __u32
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- ``buffersize``
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- Maximum buffer size in bytes required for data. The value is set by the
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driver.
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