89 lines
3.5 KiB
ReStructuredText
89 lines
3.5 KiB
ReStructuredText
.. SPDX-License-Identifier: GFDL-1.1-no-invariants-or-later
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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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Devices that capture metadata by line have the struct v4l2_fmtdesc
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``V4L2_FMT_FLAG_META_LINE_BASED`` flag set for :c:func:`VIDIOC_ENUM_FMT`. Such
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devices can typically also :ref:`capture image data <capture>`. This primarily
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involves devices that receive the data from a different devices such as a camera
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sensor.
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.. c:type:: v4l2_meta_format
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.. tabularcolumns:: |p{1.4cm}|p{2.4cm}|p{13.5cm}|
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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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* - __u32
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- ``width``
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- Width of a line of metadata in Data Units. Valid when
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:c:type`v4l2_fmtdesc` flag ``V4L2_FMT_FLAG_META_LINE_BASED`` is set,
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otherwise zero. See :c:func:`VIDIOC_ENUM_FMT`.
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* - __u32
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- ``height``
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- Number of rows of metadata. Valid when :c:type`v4l2_fmtdesc` flag
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``V4L2_FMT_FLAG_META_LINE_BASED`` is set, otherwise zero. See
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:c:func:`VIDIOC_ENUM_FMT`.
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* - __u32
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- ``bytesperline``
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- Offset in bytes between the beginning of two consecutive lines. Valid
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when :c:type`v4l2_fmtdesc` flag ``V4L2_FMT_FLAG_META_LINE_BASED`` is
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set, otherwise zero. See :c:func:`VIDIOC_ENUM_FMT`.
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