1 .. SPDX-License-Identifier: GFDL-1.1-no-invari !! 1 .. Permission is granted to copy, distribute and/or modify this
2 .. c:namespace:: V4L !! 2 .. document under the terms of the GNU Free Documentation License,
>> 3 .. Version 1.1 or any later version published by the Free Software
>> 4 .. Foundation, with no Invariant Sections, no Front-Cover Texts
>> 5 .. and no Back-Cover Texts. A copy of the license is included at
>> 6 .. Documentation/userspace-api/media/fdl-appendix.rst.
>> 7 ..
>> 8 .. TODO: replace it to GFDL-1.1-or-later WITH no-invariant-sections
3 9
4 .. _output: 10 .. _output:
5 11
6 ********************** 12 **********************
7 Video Output Interface 13 Video Output Interface
8 ********************** 14 **********************
9 15
10 Video output devices encode stills or image se 16 Video output devices encode stills or image sequences as analog video
11 signal. With this interface applications can c 17 signal. With this interface applications can control the encoding
12 process and move images from user space to the 18 process and move images from user space to the driver.
13 19
14 Conventionally V4L2 video output devices are a 20 Conventionally V4L2 video output devices are accessed through character
15 device special files named ``/dev/video`` and 21 device special files named ``/dev/video`` and ``/dev/video0`` to
16 ``/dev/video63`` with major number 81 and mino 22 ``/dev/video63`` with major number 81 and minor numbers 0 to 63.
17 ``/dev/video`` is typically a symbolic link to 23 ``/dev/video`` is typically a symbolic link to the preferred video
18 device. 24 device.
19 25
20 .. note:: The same device file names are used 26 .. note:: The same device file names are used also for video capture devices.
21 27
>> 28
22 Querying Capabilities 29 Querying Capabilities
23 ===================== 30 =====================
24 31
25 Devices supporting the video output interface 32 Devices supporting the video output interface set the
26 ``V4L2_CAP_VIDEO_OUTPUT`` or ``V4L2_CAP_VIDEO_ 33 ``V4L2_CAP_VIDEO_OUTPUT`` or ``V4L2_CAP_VIDEO_OUTPUT_MPLANE`` flag in
27 the ``capabilities`` field of struct 34 the ``capabilities`` field of struct
28 :c:type:`v4l2_capability` returned by the 35 :c:type:`v4l2_capability` returned by the
29 :ref:`VIDIOC_QUERYCAP` ioctl. As secondary dev 36 :ref:`VIDIOC_QUERYCAP` ioctl. As secondary device
30 functions they may also support the :ref:`raw 37 functions they may also support the :ref:`raw VBI output <raw-vbi>`
31 (``V4L2_CAP_VBI_OUTPUT``) interface. At least 38 (``V4L2_CAP_VBI_OUTPUT``) interface. At least one of the read/write or
32 streaming I/O methods must be supported. Modul 39 streaming I/O methods must be supported. Modulators and audio outputs
33 are optional. 40 are optional.
34 41
>> 42
35 Supplemental Functions 43 Supplemental Functions
36 ====================== 44 ======================
37 45
38 Video output devices shall support :ref:`audio 46 Video output devices shall support :ref:`audio output <audio>`,
39 :ref:`modulator <tuner>`, :ref:`controls <cont 47 :ref:`modulator <tuner>`, :ref:`controls <control>`,
40 :ref:`cropping and scaling <crop>` and 48 :ref:`cropping and scaling <crop>` and
41 :ref:`streaming parameter <streaming-par>` ioc 49 :ref:`streaming parameter <streaming-par>` ioctls as needed. The
42 :ref:`video output <video>` ioctls must be sup 50 :ref:`video output <video>` ioctls must be supported by all video
43 output devices. 51 output devices.
44 52
>> 53
45 Image Format Negotiation 54 Image Format Negotiation
46 ======================== 55 ========================
47 56
48 The output is determined by cropping and image 57 The output is determined by cropping and image format parameters. The
49 former select an area of the video picture whe 58 former select an area of the video picture where the image will appear,
50 the latter how images are stored in memory, i. 59 the latter how images are stored in memory, i. e. in RGB or YUV format,
51 the number of bits per pixel or width and heig 60 the number of bits per pixel or width and height. Together they also
52 define how images are scaled in the process. 61 define how images are scaled in the process.
53 62
54 As usual these parameters are *not* reset at : !! 63 As usual these parameters are *not* reset at :ref:`open() <func-open>`
55 time to permit Unix tool chains, programming a 64 time to permit Unix tool chains, programming a device and then writing
56 to it as if it was a plain file. Well written 65 to it as if it was a plain file. Well written V4L2 applications ensure
57 they really get what they want, including crop 66 they really get what they want, including cropping and scaling.
58 67
59 Cropping initialization at minimum requires to 68 Cropping initialization at minimum requires to reset the parameters to
60 defaults. An example is given in :ref:`crop`. 69 defaults. An example is given in :ref:`crop`.
61 70
62 To query the current image format applications 71 To query the current image format applications set the ``type`` field of
63 a struct :c:type:`v4l2_format` to 72 a struct :c:type:`v4l2_format` to
64 ``V4L2_BUF_TYPE_VIDEO_OUTPUT`` or ``V4L2_BUF_T 73 ``V4L2_BUF_TYPE_VIDEO_OUTPUT`` or ``V4L2_BUF_TYPE_VIDEO_OUTPUT_MPLANE``
65 and call the :ref:`VIDIOC_G_FMT <VIDIOC_G_FMT> 74 and call the :ref:`VIDIOC_G_FMT <VIDIOC_G_FMT>` ioctl with a pointer
66 to this structure. Drivers fill the struct 75 to this structure. Drivers fill the struct
67 :c:type:`v4l2_pix_format` ``pix`` or the struc 76 :c:type:`v4l2_pix_format` ``pix`` or the struct
68 :c:type:`v4l2_pix_format_mplane` ``pix_mp`` 77 :c:type:`v4l2_pix_format_mplane` ``pix_mp``
69 member of the ``fmt`` union. 78 member of the ``fmt`` union.
70 79
71 To request different parameters applications s 80 To request different parameters applications set the ``type`` field of a
72 struct :c:type:`v4l2_format` as above and init 81 struct :c:type:`v4l2_format` as above and initialize all
73 fields of the struct :c:type:`v4l2_pix_format` 82 fields of the struct :c:type:`v4l2_pix_format`
74 ``vbi`` member of the ``fmt`` union, or better 83 ``vbi`` member of the ``fmt`` union, or better just modify the results
75 of :ref:`VIDIOC_G_FMT <VIDIOC_G_FMT>`, and cal 84 of :ref:`VIDIOC_G_FMT <VIDIOC_G_FMT>`, and call the :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>`
76 ioctl with a pointer to this structure. Driver 85 ioctl with a pointer to this structure. Drivers may adjust the
77 parameters and finally return the actual param 86 parameters and finally return the actual parameters as :ref:`VIDIOC_G_FMT <VIDIOC_G_FMT>`
78 does. 87 does.
79 88
80 Like :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>` the :r 89 Like :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>` the :ref:`VIDIOC_TRY_FMT <VIDIOC_G_FMT>` ioctl
81 can be used to learn about hardware limitation 90 can be used to learn about hardware limitations without disabling I/O or
82 possibly time consuming hardware preparations. 91 possibly time consuming hardware preparations.
83 92
84 The contents of struct :c:type:`v4l2_pix_forma 93 The contents of struct :c:type:`v4l2_pix_format` and
85 struct :c:type:`v4l2_pix_format_mplane` are 94 struct :c:type:`v4l2_pix_format_mplane` are
86 discussed in :ref:`pixfmt`. See also the speci 95 discussed in :ref:`pixfmt`. See also the specification of the
87 :ref:`VIDIOC_G_FMT <VIDIOC_G_FMT>`, :ref:`VIDI 96 :ref:`VIDIOC_G_FMT <VIDIOC_G_FMT>`, :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>` and :ref:`VIDIOC_TRY_FMT <VIDIOC_G_FMT>` ioctls for
88 details. Video output devices must implement b 97 details. Video output devices must implement both the :ref:`VIDIOC_G_FMT <VIDIOC_G_FMT>`
89 and :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>` ioctl, 98 and :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>` ioctl, even if :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>` ignores all
90 requests and always returns default parameters 99 requests and always returns default parameters as :ref:`VIDIOC_G_FMT <VIDIOC_G_FMT>` does.
91 :ref:`VIDIOC_TRY_FMT <VIDIOC_G_FMT>` is option 100 :ref:`VIDIOC_TRY_FMT <VIDIOC_G_FMT>` is optional.
>> 101
92 102
93 Writing Images 103 Writing Images
94 ============== 104 ==============
95 105
96 A video output device may support the :ref:`wr 106 A video output device may support the :ref:`write() function <rw>`
97 and/or streaming (:ref:`memory mapping <mmap>` 107 and/or streaming (:ref:`memory mapping <mmap>` or
98 :ref:`user pointer <userp>`) I/O. See :ref:`io 108 :ref:`user pointer <userp>`) I/O. See :ref:`io` for details.
Linux® is a registered trademark of Linus Torvalds in the United States and other countries.
TOMOYO® is a registered trademark of NTT DATA CORPORATION.