Mauro Carvalho Chehab | 54f38fc | 2020-03-04 10:21:39 +0100 | [diff] [blame] | 1 | .. Permission is granted to copy, distribute and/or modify this |
| 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 |
| 9 | |
| 10 | .. _output: |
| 11 | |
| 12 | ********************** |
| 13 | Video Output Interface |
| 14 | ********************** |
| 15 | |
| 16 | Video output devices encode stills or image sequences as analog video |
| 17 | signal. With this interface applications can control the encoding |
| 18 | process and move images from user space to the driver. |
| 19 | |
| 20 | Conventionally V4L2 video output devices are accessed through character |
| 21 | device special files named ``/dev/video`` and ``/dev/video0`` to |
| 22 | ``/dev/video63`` with major number 81 and minor numbers 0 to 63. |
| 23 | ``/dev/video`` is typically a symbolic link to the preferred video |
| 24 | device. |
| 25 | |
| 26 | .. note:: The same device file names are used also for video capture devices. |
| 27 | |
| 28 | |
| 29 | Querying Capabilities |
| 30 | ===================== |
| 31 | |
| 32 | Devices supporting the video output interface set the |
| 33 | ``V4L2_CAP_VIDEO_OUTPUT`` or ``V4L2_CAP_VIDEO_OUTPUT_MPLANE`` flag in |
| 34 | the ``capabilities`` field of struct |
| 35 | :c:type:`v4l2_capability` returned by the |
| 36 | :ref:`VIDIOC_QUERYCAP` ioctl. As secondary device |
| 37 | functions they may also support the :ref:`raw VBI output <raw-vbi>` |
| 38 | (``V4L2_CAP_VBI_OUTPUT``) interface. At least one of the read/write or |
| 39 | streaming I/O methods must be supported. Modulators and audio outputs |
| 40 | are optional. |
| 41 | |
| 42 | |
| 43 | Supplemental Functions |
| 44 | ====================== |
| 45 | |
| 46 | Video output devices shall support :ref:`audio output <audio>`, |
| 47 | :ref:`modulator <tuner>`, :ref:`controls <control>`, |
| 48 | :ref:`cropping and scaling <crop>` and |
| 49 | :ref:`streaming parameter <streaming-par>` ioctls as needed. The |
| 50 | :ref:`video output <video>` ioctls must be supported by all video |
| 51 | output devices. |
| 52 | |
| 53 | |
| 54 | Image Format Negotiation |
| 55 | ======================== |
| 56 | |
| 57 | The output is determined by cropping and image format parameters. The |
| 58 | former select an area of the video picture where the image will appear, |
| 59 | the latter how images are stored in memory, i. e. in RGB or YUV format, |
| 60 | the number of bits per pixel or width and height. Together they also |
| 61 | define how images are scaled in the process. |
| 62 | |
| 63 | As usual these parameters are *not* reset at :ref:`open() <func-open>` |
| 64 | time to permit Unix tool chains, programming a device and then writing |
| 65 | to it as if it was a plain file. Well written V4L2 applications ensure |
| 66 | they really get what they want, including cropping and scaling. |
| 67 | |
| 68 | Cropping initialization at minimum requires to reset the parameters to |
| 69 | defaults. An example is given in :ref:`crop`. |
| 70 | |
| 71 | To query the current image format applications set the ``type`` field of |
| 72 | a struct :c:type:`v4l2_format` to |
| 73 | ``V4L2_BUF_TYPE_VIDEO_OUTPUT`` or ``V4L2_BUF_TYPE_VIDEO_OUTPUT_MPLANE`` |
| 74 | and call the :ref:`VIDIOC_G_FMT <VIDIOC_G_FMT>` ioctl with a pointer |
| 75 | to this structure. Drivers fill the struct |
| 76 | :c:type:`v4l2_pix_format` ``pix`` or the struct |
| 77 | :c:type:`v4l2_pix_format_mplane` ``pix_mp`` |
| 78 | member of the ``fmt`` union. |
| 79 | |
| 80 | To request different parameters applications set the ``type`` field of a |
| 81 | struct :c:type:`v4l2_format` as above and initialize all |
| 82 | fields of the struct :c:type:`v4l2_pix_format` |
| 83 | ``vbi`` member of the ``fmt`` union, or better just modify the results |
| 84 | of :ref:`VIDIOC_G_FMT <VIDIOC_G_FMT>`, and call the :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>` |
| 85 | ioctl with a pointer to this structure. Drivers may adjust the |
| 86 | parameters and finally return the actual parameters as :ref:`VIDIOC_G_FMT <VIDIOC_G_FMT>` |
| 87 | does. |
| 88 | |
| 89 | Like :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>` the :ref:`VIDIOC_TRY_FMT <VIDIOC_G_FMT>` ioctl |
| 90 | can be used to learn about hardware limitations without disabling I/O or |
| 91 | possibly time consuming hardware preparations. |
| 92 | |
| 93 | The contents of struct :c:type:`v4l2_pix_format` and |
| 94 | struct :c:type:`v4l2_pix_format_mplane` are |
| 95 | discussed in :ref:`pixfmt`. See also the specification of the |
| 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 |
| 97 | details. Video output devices must implement both the :ref:`VIDIOC_G_FMT <VIDIOC_G_FMT>` |
| 98 | and :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>` ioctl, even if :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>` ignores all |
| 99 | requests and always returns default parameters as :ref:`VIDIOC_G_FMT <VIDIOC_G_FMT>` does. |
| 100 | :ref:`VIDIOC_TRY_FMT <VIDIOC_G_FMT>` is optional. |
| 101 | |
| 102 | |
| 103 | Writing Images |
| 104 | ============== |
| 105 | |
| 106 | A video output device may support the :ref:`write() function <rw>` |
| 107 | and/or streaming (:ref:`memory mapping <mmap>` or |
| 108 | :ref:`user pointer <userp>`) I/O. See :ref:`io` for details. |