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