[media] doc-rst: use the right markup for footnotes

each. Radio devices have no audio inputs or outputs. They have exactly

one tuner which in fact *is* an audio source, but this API associates

tuners with video inputs or outputs only, and radio devices have none of

these. [1]_ A connector on a TV card to loop back the received audio

these. [#f1]_ A connector on a TV card to loop back the received audio

signal to a sound card is not considered an audio output.

Audio and video inputs and outputs are associated. Selecting a video

	exit(EXIT_FAILURE);

    }

.. [1]

.. [#f1]

   Actually struct :ref:`v4l2_audio <v4l2-audio>` ought to have a

   ``tuner`` field like struct :ref:`v4l2_input <v4l2-input>`, not

   only making the API more consistent but also permitting radio devices

All controls are accessed using an ID value. V4L2 defines several IDs

for specific purposes. Drivers can also implement their own custom

controls using ``V4L2_CID_PRIVATE_BASE``  [1]_ and higher values. The

controls using ``V4L2_CID_PRIVATE_BASE``  [#f1]_ and higher values. The

pre-defined control IDs have the prefix ``V4L2_CID_``, and are listed in

:ref:`control-id`. The ID is used when querying the attributes of a

control, and when getting or setting the current value.

    /* Errors ignored */

    ioctl(fd, VIDIOC_S_CTRL, &control);

.. [1]

.. [#f1]

   The use of ``V4L2_CID_PRIVATE_BASE`` is problematic because different

   drivers may use the same ``V4L2_CID_PRIVATE_BASE`` ID for different

   controls. This makes it hard to programatically set such controls

overlay. This must be supported by all drivers capable of simultaneous

capturing and overlay. Optionally these drivers may also permit

capturing and overlay with a single file descriptor for compatibility

with V4L and earlier versions of V4L2. [1]_

with V4L and earlier versions of V4L2. [#f1]_

Querying Capabilities

    ((__u8 *) bitmap)[w.width * y + x / 8] & (1 << (x & 7))

where ``0`` ≤ x < ``w.width`` and ``0`` ≤ y <``w.height``. [2]_

where ``0`` ≤ x < ``w.width`` and ``0`` ≤ y <``w.height``. [#f2]_

When a clipping bit mask is not supported the driver ignores this field,

its contents after calling :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>` are

both, or despite negotiating chroma-keying, the results are undefined.

Regardless of the chosen method, the clipping abilities of the hardware

may be limited in quantity or quality. The results when these limits are

exceeded are undefined. [3]_

exceeded are undefined. [#f3]_

``__u8 global_alpha``

    The global alpha value used to blend the framebuffer with video

.. _v4l2-clip:

struct v4l2_clip [4]_

struct v4l2_clip [#f4]_

---------------------

``struct v4l2_rect c``

To start or stop the frame buffer overlay applications call the

:ref:`VIDIOC_OVERLAY` ioctl.

.. [1]

.. [#f1]

   A common application of two file descriptors is the XFree86

   :ref:`Xv/V4L <xvideo>` interface driver and a V4L2 application.

   While the X server controls video overlay, the application can take

   Hence as a complexity trade-off drivers *must* support two file

   descriptors and *may* support single fd operation.

.. [2]

.. [#f2]

   Should we require ``w.width`` to be a multiple of eight?

.. [3]

.. [#f3]

   When the image is written into frame buffer memory it will be

   undesirable if the driver clips out less pixels than expected,

   because the application and graphics system are not aware these

   regions need to be refreshed. The driver should clip out more pixels

   or not write the image at all.

.. [4]

.. [#f4]

   The X Window system defines "regions" which are vectors of ``struct

   BoxRec { short x1, y1, x2, y2; }`` with ``width = x2 - x1`` and

   ``height = y2 - y1``, so one cannot pass X11 clip lists directly.

information is transmitted, allowing some time while the electron beam

of a cathode ray tube TV returns to the top of the screen. Using an

oscilloscope you will find here the vertical synchronization pulses and

short data packages ASK modulated [1]_ onto the video signal. These are

short data packages ASK modulated [#f1]_ onto the video signal. These are

transmissions of services such as Teletext or Closed Caption.

Subject of this interface type is raw VBI data, as sampled off a video

       -  ``sample_format``

       -  Defines the sample format as in :ref:`pixfmt`, a

	  four-character-code. [2]_ Usually this is ``V4L2_PIX_FMT_GREY``,

	  four-character-code. [#f2]_ Usually this is ``V4L2_PIX_FMT_GREY``,

	  i. e. each sample consists of 8 bits with lower values oriented

	  towards the black level. Do not assume any other correlation of

	  values with the signal level. For example, the MSB does not

       -  __u32

       -  ``start``\ [2]_

       -  ``start``\ [#f2]_

       -  This is the scanning system line number associated with the first

	  line of the VBI image, of the first and the second field

       -  __u32

       -  ``count``\ [2]_

       -  ``count``\ [#f2]_

       -  The number of lines in the first and second field image,

	  respectively.

       -  __u32

       -  ``reserved``\ [2]_

       -  ``reserved``\ [#f2]_

       -  This array is reserved for future extensions. Drivers and

	  applications must set it to zero.

	  or bottom field depending on the video standard. When this flag is

	  set the first or second field may be stored first, however the

	  fields are still in correct temporal order with the older field

	  first in memory. [3]_

	  first in memory. [#f3]_

    -  .. row 2

are temporarily unavailable, for example the device is already in use by

another process.

.. [1]

.. [#f1]

   ASK: Amplitude-Shift Keying. A high signal level represents a '1'

   bit, a low level a '0' bit.

.. [2]

.. [#f2]

   A few devices may be unable to sample VBI data at all but can extend

   the video capture window to the VBI region.

.. [3]

.. [#f3]

   Most VBI services transmit on both fields, but some have different

   semantics depending on the field number. These cannot be reliable

   decoded or encoded when ``V4L2_VBI_UNSYNC`` is set.

	  specified in this field. For example, if ``service_set`` is

	  initialized with ``V4L2_SLICED_TELETEXT_B | V4L2_SLICED_WSS_625``,

	  a driver for the cx25840 video decoder sets lines 7-22 of both

	  fields [1]_ to ``V4L2_SLICED_TELETEXT_B`` and line 23 of the first

	  fields [#f1]_ to ``V4L2_SLICED_TELETEXT_B`` and line 23 of the first

	  field to ``V4L2_SLICED_WSS_625``. If ``service_set`` is set to

	  zero, then the values of ``service_lines`` will be used instead.

.. [1]

.. [#f1]

   According to :ref:`ETS 300 706 <ets300706>` lines 6-22 of the first

   field and lines 5-22 of the second field may carry Teletext data.