4.14. 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.
4.14.1. Querying Capabilities¶
Device nodes supporting the metadata capture interface set the
V4L2_CAP_META_CAPTURE
flag in the device_caps
field of the
v4l2_capability
structure returned by the 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
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.
4.14.2. Data Format Negotiation¶
The metadata device uses the Data Formats ioctls to select the capture format.
The metadata buffer content format is bound to that selected format. In addition
to the basic Data Formats ioctls, the VIDIOC_ENUM_FMT()
ioctl must be
supported as well.
To use the Data Formats ioctls applications set the type
field of the
v4l2_format
structure to V4L2_BUF_TYPE_META_CAPTURE
or to
V4L2_BUF_TYPE_META_OUTPUT
and use the 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 v4l2_format
structure
to 0.
-
v4l2_meta_format
¶
__u32 | dataformat |
The data format, set by the application. This is a little endian four character code. V4L2 defines metadata formats in Metadata Formats. |
__u32 | buffersize |
Maximum buffer size in bytes required for data. The value is set by the driver. |