4.3. 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/video63 with major number 81 and minor numbers 0 to 63.
/dev/video is typically a symbolic link to the preferred video
The same device file names are used also for video capture devices.
4.3.1. Querying Capabilities¶
Devices supporting the video output interface set the
V4L2_CAP_VIDEO_OUTPUT_MPLANE flag in
capabilities field of struct
v4l2_capability returned by the
ioctl VIDIOC_QUERYCAP ioctl. As secondary device
functions they may also support the raw VBI output
V4L2_CAP_VBI_OUTPUT) interface. At least one of the read/write or
streaming I/O methods must be supported. Modulators and audio outputs
4.3.2. Supplemental Functions¶
Video output devices shall support audio output, modulator, controls, cropping and scaling and streaming parameter ioctls as needed. The video output ioctls must be supported by all video output devices.
4.3.3. Image Format Negotiation¶
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
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 Image Cropping, Insertion and Scaling – the CROP API.
To query the current image format applications set the
type field of
and call the VIDIOC_G_FMT ioctl with a pointer
to this structure. Drivers fill the struct
pix or the struct
member of the
To request different parameters applications set the
type field of a
v4l2_format as above and initialize all
fields of the struct
vbi member of the
fmt union, or better just modify the results
of VIDIOC_G_FMT, and call the VIDIOC_S_FMT
ioctl with a pointer to this structure. Drivers may adjust the
parameters and finally return the actual parameters as VIDIOC_G_FMT
Like VIDIOC_S_FMT the VIDIOC_TRY_FMT ioctl can be used to learn about hardware limitations without disabling I/O or possibly time consuming hardware preparations.
The contents of struct
discussed in Image Formats. See also the specification of the
VIDIOC_G_FMT, VIDIOC_S_FMT and VIDIOC_TRY_FMT ioctls for
details. Video output devices must implement both the VIDIOC_G_FMT
and VIDIOC_S_FMT ioctl, even if VIDIOC_S_FMT ignores all
requests and always returns default parameters as VIDIOC_G_FMT does.
VIDIOC_TRY_FMT is optional.
4.3.4. Writing Images¶
A video output device may support the write() function and/or streaming (memory mapping or user pointer) I/O. See Input/Output for details.