1.. SPDX-License-Identifier: GFDL-1.1-no-invariants-or-later 2 3.. _metadata: 4 5****************** 6Metadata Interface 7****************** 8 9Metadata refers to any non-image data that supplements video frames with 10additional information. This may include statistics computed over the image, 11frame capture parameters supplied by the image source or device specific 12parameters for specifying how the device processes images. This interface is 13intended for transfer of metadata between the userspace and the hardware and 14control of that operation. 15 16The metadata interface is implemented on video device nodes. The device can be 17dedicated to metadata or can support both video and metadata as specified in its 18reported capabilities. 19 20Querying Capabilities 21===================== 22 23Device nodes supporting the metadata capture interface set the 24``V4L2_CAP_META_CAPTURE`` flag in the ``device_caps`` field of the 25:c:type:`v4l2_capability` structure returned by the :c:func:`VIDIOC_QUERYCAP` 26ioctl. That flag means the device can capture metadata to memory. Similarly, 27device nodes supporting metadata output interface set the 28``V4L2_CAP_META_OUTPUT`` flag in the ``device_caps`` field of 29:c:type:`v4l2_capability` structure. That flag means the device can read 30metadata from memory. 31 32At least one of the read/write or streaming I/O methods must be supported. 33 34 35Data Format Negotiation 36======================= 37 38The metadata device uses the :ref:`format` ioctls to select the capture format. 39The metadata buffer content format is bound to that selected format. In addition 40to the basic :ref:`format` ioctls, the :c:func:`VIDIOC_ENUM_FMT` ioctl must be 41supported as well. 42 43To use the :ref:`format` ioctls applications set the ``type`` field of the 44:c:type:`v4l2_format` structure to ``V4L2_BUF_TYPE_META_CAPTURE`` or to 45``V4L2_BUF_TYPE_META_OUTPUT`` and use the :c:type:`v4l2_meta_format` ``meta`` 46member of the ``fmt`` union as needed per the desired operation. Both drivers 47and applications must set the remainder of the :c:type:`v4l2_format` structure 48to 0. 49 50.. c:type:: v4l2_meta_format 51 52.. tabularcolumns:: |p{1.4cm}|p{2.4cm}|p{13.5cm}| 53 54.. flat-table:: struct v4l2_meta_format 55 :header-rows: 0 56 :stub-columns: 0 57 :widths: 1 1 2 58 59 * - __u32 60 - ``dataformat`` 61 - The data format, set by the application. This is a little endian 62 :ref:`four character code <v4l2-fourcc>`. V4L2 defines metadata formats 63 in :ref:`meta-formats`. 64 * - __u32 65 - ``buffersize`` 66 - Maximum buffer size in bytes required for data. The value is set by the 67 driver. 68