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/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.

注釈

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 or V4L2_CAP_VIDEO_CAPTURE_MPLANE flag in the 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 optional.

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 open() 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 a struct v4l2_format to V4L2_BUF_TYPE_VIDEO_CAPTURE or V4L2_BUF_TYPE_VIDEO_CAPTURE_MPLANE and call the VIDIOC_G_FMT ioctl with a pointer to this structure. Drivers fill the struct v4l2_pix_format pix or the struct v4l2_pix_format_mplane pix_mp member of the fmt union.

To request different parameters applications set the type field of a struct v4l2_format as above and initialize all fields of the struct v4l2_pix_format 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 does.

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 v4l2_pix_format and struct v4l2_pix_format_mplane are 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.