4.1. Video Capture Interface¶
Video capture devices sample an analog video signal and store the digitized images in memory. Today nearly all devices can capture at full 25 or 30 frames/second. With this interface applications can control the capture process and move images from the driver into user space.
Conventionally V4L2 video capture 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 for video output devices.
4.1.1. Querying Capabilities¶
Devices supporting the video capture interface set the
V4L2_CAP_VIDEO_CAPTURE_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 video overlay
V4L2_CAP_VIDEO_OVERLAY) and the raw VBI capture
V4L2_CAP_VBI_CAPTURE) interface. At least one of the read/write or
streaming I/O methods must be supported. Tuners and audio inputs are
4.1.2. Supplemental Functions¶
Video capture devices shall support audio input, Tuners and Modulators, controls, cropping and scaling and streaming parameter ioctls as needed. The video input ioctls must be supported by all video capture devices.
4.1.3. Image Format Negotiation¶
The result of a capture operation is determined by cropping and image format parameters. The former select an area of the video picture to capture, 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 reading
from 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
V4L2_BUF_TYPE_VIDEO_CAPTURE_MPLANE 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 capture 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.
4.1.4. Reading Images¶
A video capture device may support the read() function and/or streaming (memory mapping or user pointer) I/O. See Input/Output for details.