.. SPDX-License-Identifier: GFDL-1.1-no-invariants-or-later

.. _metadata:

Metadata Interface

Metadata refers to any non-image data that supplements video frames with additional information. This may include statistics computed over the image, frame capture parameters supplied by the image source or device specific parameters for specifying how the device processes images. This interface is intended for transfer of metadata between the userspace and the hardware and control of that operation.

The metadata interface is implemented on video device nodes. The device can be dedicated to metadata or can support both video and metadata as specified in its reported capabilities.

Querying Capabilities

Device nodes supporting the metadata capture interface set the V4L2_CAP_META_CAPTURE flag in the device_caps field of the :c:type:v4l2_capability structure returned by the :c:func:VIDIOC_QUERYCAP ioctl. That flag means the device can capture metadata to memory. Similarly, device nodes supporting metadata output interface set the V4L2_CAP_META_OUTPUT flag in the device_caps field of :c:type:v4l2_capability structure. That flag means the device can read metadata from memory.

At least one of the read/write or streaming I/O methods must be supported.

Data Format Negotiation

The metadata device uses the :ref:format ioctls to select the capture format. The metadata buffer content format is bound to that selected format. In addition to the basic :ref:format ioctls, the :c:func:VIDIOC_ENUM_FMT ioctl must be supported as well.

To use the :ref:format ioctls applications set the type field of the :c:type:v4l2_format structure to V4L2_BUF_TYPE_META_CAPTURE or to V4L2_BUF_TYPE_META_OUTPUT and use the :c:type:v4l2_meta_format meta member of the fmt union as needed per the desired operation. Both drivers and applications must set the remainder of the :c:type:v4l2_format structure to 0.

.. c:type:: v4l2_meta_format

.. tabularcolumns:: |p{1.4cm}|p{2.4cm}|p{13.5cm}|

.. flat-table:: struct v4l2_meta_format :header-rows: 0 :stub-columns: 0 :widths: 1 1 2

* - __u32
  - ``dataformat``
  - The data format, set by the application. This is a little endian
    :ref:`four character code <v4l2-fourcc>`. V4L2 defines metadata formats
    in :ref:`meta-formats`.
* - __u32
  - ``buffersize``
  - Maximum buffer size in bytes required for data. The value is set by the
    driver.