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