1.. SPDX-License-Identifier: GFDL-1.1-no-invariants-or-later 2.. c:namespace:: V4L 3 4.. _output: 5 6********************** 7Video Output Interface 8********************** 9 10Video output devices encode stills or image sequences as analog video 11signal. With this interface applications can control the encoding 12process and move images from user space to the driver. 13 14Conventionally V4L2 video output devices are accessed through character 15device 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 18device. 19 20.. note:: The same device file names are used also for video capture devices. 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 35Supplemental Functions 36====================== 37 38Video 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 43output devices. 44 45Image Format Negotiation 46======================== 47 48The output is determined by cropping and image format parameters. The 49former select an area of the video picture where the image will appear, 50the latter how images are stored in memory, i. e. in RGB or YUV format, 51the number of bits per pixel or width and height. Together they also 52define how images are scaled in the process. 53 54As usual these parameters are *not* reset at :c:func:`open()` 55time to permit Unix tool chains, programming a device and then writing 56to it as if it was a plain file. Well written V4L2 applications ensure 57they really get what they want, including cropping and scaling. 58 59Cropping initialization at minimum requires to reset the parameters to 60defaults. An example is given in :ref:`crop`. 61 62To query the current image format applications set the ``type`` field of 63a struct :c:type:`v4l2_format` to 64``V4L2_BUF_TYPE_VIDEO_OUTPUT`` or ``V4L2_BUF_TYPE_VIDEO_OUTPUT_MPLANE`` 65and call the :ref:`VIDIOC_G_FMT <VIDIOC_G_FMT>` ioctl with a pointer 66to 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`` 69member of the ``fmt`` union. 70 71To request different parameters applications set the ``type`` field of a 72struct :c:type:`v4l2_format` as above and initialize all 73fields of the struct :c:type:`v4l2_pix_format` 74``vbi`` member of the ``fmt`` union, or better just modify the results 75of :ref:`VIDIOC_G_FMT <VIDIOC_G_FMT>`, and call the :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>` 76ioctl with a pointer to this structure. Drivers may adjust the 77parameters and finally return the actual parameters as :ref:`VIDIOC_G_FMT <VIDIOC_G_FMT>` 78does. 79 80Like :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>` the :ref:`VIDIOC_TRY_FMT <VIDIOC_G_FMT>` ioctl 81can be used to learn about hardware limitations without disabling I/O or 82possibly time consuming hardware preparations. 83 84The contents of struct :c:type:`v4l2_pix_format` and 85struct :c:type:`v4l2_pix_format_mplane` are 86discussed 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 88details. Video output devices must implement both the :ref:`VIDIOC_G_FMT <VIDIOC_G_FMT>` 89and :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>` ioctl, even if :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>` ignores all 90requests 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 93Writing Images 94============== 95 96A video output device may support the :ref:`write() function <rw>` 97and/or streaming (:ref:`memory mapping <mmap>` or 98:ref:`user pointer <userp>`) I/O. See :ref:`io` for details. 99