Documentation/userspace-api/media/v4l/dev-output.rst
.. SPDX-License-Identifier: GFDL-1.1-no-invariants-or-later .. c:namespace:: V4L
.. _output:
Video Output Interface
Video output devices encode stills or image sequences as analog video signal. With this interface applications can control the encoding process and move images from user space to the driver.
Conventionally V4L2 video output devices are accessed through character
device special files named /dev/video and /dev/video0 to
/dev/video63 with major number 81 and minor numbers 0 to 63.
/dev/video is typically a symbolic link to the preferred video
device.
.. note:: The same device file names are used also for video capture devices.
Devices supporting the video output interface set the
V4L2_CAP_VIDEO_OUTPUT or V4L2_CAP_VIDEO_OUTPUT_MPLANE flag in
the capabilities field of struct
:c:type:v4l2_capability returned by the
:ref:VIDIOC_QUERYCAP ioctl. As secondary device
functions they may also support the :ref:raw VBI output <raw-vbi>
(V4L2_CAP_VBI_OUTPUT) interface. At least one of the read/write or
streaming I/O methods must be supported. Modulators and audio outputs
are optional.
Video output devices shall support :ref:audio output <audio>,
:ref:modulator <tuner>, :ref:controls <control>,
:ref:cropping and scaling <crop> and
:ref:streaming parameter <streaming-par> ioctls as needed. The
:ref:video output <video> ioctls must be supported by all video
output devices.
The output is determined by cropping and image format parameters. The former select an area of the video picture where the image will appear, the latter how images are stored in memory, i. e. in RGB or YUV format, the number of bits per pixel or width and height. Together they also define how images are scaled in the process.
As usual these parameters are not reset at :c:func:open()
time to permit Unix tool chains, programming a device and then writing
to it as if it was a plain file. Well written V4L2 applications ensure
they really get what they want, including cropping and scaling.
Cropping initialization at minimum requires to reset the parameters to
defaults. An example is given in :ref:crop.
To query the current image format applications set the type field of
a struct :c:type:v4l2_format to
V4L2_BUF_TYPE_VIDEO_OUTPUT or V4L2_BUF_TYPE_VIDEO_OUTPUT_MPLANE
and call the :ref:VIDIOC_G_FMT <VIDIOC_G_FMT> ioctl with a pointer
to this structure. Drivers fill the struct
:c:type:v4l2_pix_format pix or the struct
:c:type:v4l2_pix_format_mplane pix_mp
member of the fmt union.
To request different parameters applications set the type field of a
struct :c:type:v4l2_format as above and initialize all
fields of the struct :c:type:v4l2_pix_format
vbi member of the fmt union, or better just modify the results
of :ref:VIDIOC_G_FMT <VIDIOC_G_FMT>, and call the :ref:VIDIOC_S_FMT <VIDIOC_G_FMT>
ioctl with a pointer to this structure. Drivers may adjust the
parameters and finally return the actual parameters as :ref:VIDIOC_G_FMT <VIDIOC_G_FMT>
does.
Like :ref:VIDIOC_S_FMT <VIDIOC_G_FMT> the :ref:VIDIOC_TRY_FMT <VIDIOC_G_FMT> ioctl
can be used to learn about hardware limitations without disabling I/O or
possibly time consuming hardware preparations.
The contents of struct :c:type:v4l2_pix_format and
struct :c:type:v4l2_pix_format_mplane are
discussed in :ref:pixfmt. See also the specification of the
:ref:VIDIOC_G_FMT <VIDIOC_G_FMT>, :ref:VIDIOC_S_FMT <VIDIOC_G_FMT> and :ref:VIDIOC_TRY_FMT <VIDIOC_G_FMT> ioctls for
details. Video output devices must implement both the :ref:VIDIOC_G_FMT <VIDIOC_G_FMT>
and :ref:VIDIOC_S_FMT <VIDIOC_G_FMT> ioctl, even if :ref:VIDIOC_S_FMT <VIDIOC_G_FMT> ignores all
requests and always returns default parameters as :ref:VIDIOC_G_FMT <VIDIOC_G_FMT> does.
:ref:VIDIOC_TRY_FMT <VIDIOC_G_FMT> is optional.
A video output device may support the :ref:write() function <rw>
and/or streaming (:ref:memory mapping <mmap> or
:ref:user pointer <userp>) I/O. See :ref:io for details.