1.. SPDX-License-Identifier: GFDL-1.1-no-invariants-or-later
2
3.. _output:
4
5**********************
6Video Output Interface
7**********************
8
9Video output devices encode stills or image sequences as analog video
10signal. With this interface applications can control the encoding
11process and move images from user space to the driver.
12
13Conventionally V4L2 video output devices are accessed through character
14device special files named ``/dev/video`` and ``/dev/video0`` to
15``/dev/video63`` with major number 81 and minor numbers 0 to 63.
16``/dev/video`` is typically a symbolic link to the preferred video
17device.
18
19.. note:: The same device file names are used also for video capture devices.
20
21
22Querying Capabilities
23=====================
24
25Devices supporting the video output interface set the
26``V4L2_CAP_VIDEO_OUTPUT`` or ``V4L2_CAP_VIDEO_OUTPUT_MPLANE`` flag in
27the ``capabilities`` field of struct
28:c:type:`v4l2_capability` returned by the
29:ref:`VIDIOC_QUERYCAP` ioctl. As secondary device
30functions they may also support the :ref:`raw VBI output <raw-vbi>`
31(``V4L2_CAP_VBI_OUTPUT``) interface. At least one of the read/write or
32streaming I/O methods must be supported. Modulators and audio outputs
33are optional.
34
35
36Supplemental Functions
37======================
38
39Video output devices shall support :ref:`audio output <audio>`,
40:ref:`modulator <tuner>`, :ref:`controls <control>`,
41:ref:`cropping and scaling <crop>` and
42:ref:`streaming parameter <streaming-par>` ioctls as needed. The
43:ref:`video output <video>` ioctls must be supported by all video
44output devices.
45
46
47Image Format Negotiation
48========================
49
50The output is determined by cropping and image format parameters. The
51former select an area of the video picture where the image will appear,
52the latter how images are stored in memory, i. e. in RGB or YUV format,
53the number of bits per pixel or width and height. Together they also
54define how images are scaled in the process.
55
56As usual these parameters are *not* reset at :ref:`open() <func-open>`
57time to permit Unix tool chains, programming a device and then writing
58to it as if it was a plain file. Well written V4L2 applications ensure
59they really get what they want, including cropping and scaling.
60
61Cropping initialization at minimum requires to reset the parameters to
62defaults. An example is given in :ref:`crop`.
63
64To query the current image format applications set the ``type`` field of
65a struct :c:type:`v4l2_format` to
66``V4L2_BUF_TYPE_VIDEO_OUTPUT`` or ``V4L2_BUF_TYPE_VIDEO_OUTPUT_MPLANE``
67and call the :ref:`VIDIOC_G_FMT <VIDIOC_G_FMT>` ioctl with a pointer
68to this structure. Drivers fill the struct
69:c:type:`v4l2_pix_format` ``pix`` or the struct
70:c:type:`v4l2_pix_format_mplane` ``pix_mp``
71member of the ``fmt`` union.
72
73To request different parameters applications set the ``type`` field of a
74struct :c:type:`v4l2_format` as above and initialize all
75fields of the struct :c:type:`v4l2_pix_format`
76``vbi`` member of the ``fmt`` union, or better just modify the results
77of :ref:`VIDIOC_G_FMT <VIDIOC_G_FMT>`, and call the :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>`
78ioctl with a pointer to this structure. Drivers may adjust the
79parameters and finally return the actual parameters as :ref:`VIDIOC_G_FMT <VIDIOC_G_FMT>`
80does.
81
82Like :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>` the :ref:`VIDIOC_TRY_FMT <VIDIOC_G_FMT>` ioctl
83can be used to learn about hardware limitations without disabling I/O or
84possibly time consuming hardware preparations.
85
86The contents of struct :c:type:`v4l2_pix_format` and
87struct :c:type:`v4l2_pix_format_mplane` are
88discussed in :ref:`pixfmt`. See also the specification of the
89:ref:`VIDIOC_G_FMT <VIDIOC_G_FMT>`, :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>` and :ref:`VIDIOC_TRY_FMT <VIDIOC_G_FMT>` ioctls for
90details. Video output devices must implement both the :ref:`VIDIOC_G_FMT <VIDIOC_G_FMT>`
91and :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>` ioctl, even if :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>` ignores all
92requests and always returns default parameters as :ref:`VIDIOC_G_FMT <VIDIOC_G_FMT>` does.
93:ref:`VIDIOC_TRY_FMT <VIDIOC_G_FMT>` is optional.
94
95
96Writing Images
97==============
98
99A video output device may support the :ref:`write() function <rw>`
100and/or streaming (:ref:`memory mapping <mmap>` or
101:ref:`user pointer <userp>`) I/O. See :ref:`io` for details.
102