1 .. SPDX-License-Identifier: GFDL-1.1-no-invari
2 .. c:namespace:: V4L
3
4 .. _output:
5
6 **********************
7 Video Output Interface
8 **********************
9
10 Video output devices encode stills or image se
11 signal. With this interface applications can c
12 process and move images from user space to the
13
14 Conventionally V4L2 video output devices are a
15 device special files named ``/dev/video`` and
16 ``/dev/video63`` with major number 81 and mino
17 ``/dev/video`` is typically a symbolic link to
18 device.
19
20 .. note:: The same device file names are used
21
22 Querying Capabilities
23 =====================
24
25 Devices supporting the video output interface
26 ``V4L2_CAP_VIDEO_OUTPUT`` or ``V4L2_CAP_VIDEO_
27 the ``capabilities`` field of struct
28 :c:type:`v4l2_capability` returned by the
29 :ref:`VIDIOC_QUERYCAP` ioctl. As secondary dev
30 functions they may also support the :ref:`raw
31 (``V4L2_CAP_VBI_OUTPUT``) interface. At least
32 streaming I/O methods must be supported. Modul
33 are optional.
34
35 Supplemental Functions
36 ======================
37
38 Video output devices shall support :ref:`audio
39 :ref:`modulator <tuner>`, :ref:`controls <cont
40 :ref:`cropping and scaling <crop>` and
41 :ref:`streaming parameter <streaming-par>` ioc
42 :ref:`video output <video>` ioctls must be sup
43 output devices.
44
45 Image Format Negotiation
46 ========================
47
48 The output is determined by cropping and image
49 former select an area of the video picture whe
50 the latter how images are stored in memory, i.
51 the number of bits per pixel or width and heig
52 define how images are scaled in the process.
53
54 As usual these parameters are *not* reset at :
55 time to permit Unix tool chains, programming a
56 to it as if it was a plain file. Well written
57 they really get what they want, including crop
58
59 Cropping initialization at minimum requires to
60 defaults. An example is given in :ref:`crop`.
61
62 To query the current image format applications
63 a struct :c:type:`v4l2_format` to
64 ``V4L2_BUF_TYPE_VIDEO_OUTPUT`` or ``V4L2_BUF_T
65 and call the :ref:`VIDIOC_G_FMT <VIDIOC_G_FMT>
66 to this structure. Drivers fill the struct
67 :c:type:`v4l2_pix_format` ``pix`` or the struc
68 :c:type:`v4l2_pix_format_mplane` ``pix_mp``
69 member of the ``fmt`` union.
70
71 To request different parameters applications s
72 struct :c:type:`v4l2_format` as above and init
73 fields of the struct :c:type:`v4l2_pix_format`
74 ``vbi`` member of the ``fmt`` union, or better
75 of :ref:`VIDIOC_G_FMT <VIDIOC_G_FMT>`, and cal
76 ioctl with a pointer to this structure. Driver
77 parameters and finally return the actual param
78 does.
79
80 Like :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>` the :r
81 can be used to learn about hardware limitation
82 possibly time consuming hardware preparations.
83
84 The contents of struct :c:type:`v4l2_pix_forma
85 struct :c:type:`v4l2_pix_format_mplane` are
86 discussed in :ref:`pixfmt`. See also the speci
87 :ref:`VIDIOC_G_FMT <VIDIOC_G_FMT>`, :ref:`VIDI
88 details. Video output devices must implement b
89 and :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>` ioctl,
90 requests and always returns default parameters
91 :ref:`VIDIOC_TRY_FMT <VIDIOC_G_FMT>` is option
92
93 Writing Images
94 ==============
95
96 A video output device may support the :ref:`wr
97 and/or streaming (:ref:`memory mapping <mmap>`
98 :ref:`user pointer <userp>`) I/O. See :ref:`io
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