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Capability Query Ioctl

The VIDIOCGCAP
ioctl call is used to obtain the capability information for a video device. The struct video_capability
object passed to the ioctl is completed and returned. It contains the following information

The type field lists the capability flags for the device. These are as follows

The
minimum and maximum sizes listed for a capture device do not imply all
that all height/width ratios or sizes within the range are possible. A
request to set a size will be honoured by the largest available capture
size whose capture is no large than the requested rectangle in either
direction. For example the quickcam has 3 fixed settings.

Frame Buffer

Capture
cards that drop data directly onto the frame buffer must be told the
base address of the frame buffer, its size and organisation. This is a
privileged ioctl and one that eventually X itself should set.

The VIDIOCSFBUF
ioctl sets the frame buffer parameters for a capture card. If the card
does not do direct writes to the frame buffer then this ioctl will be
unsupported. The VIDIOCGFBUF
ioctl returns the currently used parameters. The structure used in both cases is a struct video_buffer
.

Note
that these values reflect the physical layout of the frame buffer. The
visible area may be smaller. In fact under XFree86 this is commonly the
case. XFree86 DGA can provide the parameters required to set up this
ioctl. Setting the base address to NULL indicates there is no physical
frame buffer access.

Capture Windows

The capture area is described by a struct video_window
. This defines a capture area and the clipping information if relevant. The VIDIOCGWIN
ioctl recovers the current settings and the VIDIOCSWIN
sets new values. A successful call to VIDIOCSWIN
indicates that a suitable set of parameters have been chosen. They do
not indicate that exactly what was requested was granted. The program
should call VIDIOCGWIN
to check if the nearest match was suitable. The struct video_window
contains the following fields.

Clipping rectangles are passed as an array. Each clip consists of the following fields available to the user.

Merely setting the window does not enable capturing. Overlay capturing is activated by passing the VIDIOCCAPTURE
ioctl a value of 1, and disabled by passing it a value of 0.

Some
capture devices can capture a subfield of the image they actually see.
This is indicated when VIDEO_TYPE_SUBCAPTURE is defined. The
video_capture describes the time and special subfields to capture. The
video_capture structure contains the following fields.

The available flags are

Video Sources

Each video4linux video or audio device captures from one or more source channels
. Each channel can be queries with the VDIOCGCHAN
ioctl call. Before invoking this function the caller must set the
channel field to the channel that is being queried. On return the struct video_channel
is filled in with information about the nature of the channel itself.

The VIDIOCSCHAN
ioctl takes an integer argument and switches the capture to this input.
It is not defined whether parameters such as colour settings or tuning
are maintained across a channel switch. The caller should maintain
settings as desired for each channel. (This is reasonable as different
video inputs may have different properties).

The struct video_channel
consists of the following

The flags defined are

The types defined are

Image Properties

The image properties of the picture can be queried with the VIDIOCGPICT
ioctl which fills in a struct video_picture
. The VIDIOCSPICT
ioctl allows values to be changed. All values except for the palette type are scaled between 0-65535.

The struct video_picture
consists of the following fields

The following palettes are defined

Tuning

Each
video input channel can have one or more tuners associated with it.
Many devices will not have tuners. TV cards and radio cards will have
one or more tuners attached.

Tuners are described by a struct video_tuner
which can be obtained by the VIDIOCGTUNER
ioctl. Fill in the tuner number in the structure then pass the
structure to the ioctl to have the data filled in. The tuner can be
switched using VIDIOCSTUNER
which takes an integer argument giving the tuner to use. A struct tuner has the following fields

The following flags exist

The following modes are defined

Tuning frequencies are an unsigned 32bit value in 1/16th MHz or if the VIDEO_TUNER_LOW
flag is set they are in 1/16th KHz. The current frequency is obtained as an unsigned long via the VIDIOCGFREQ
ioctl and set by the VIDIOCSFREQ
ioctl.

Audio

TV and Radio devices have one or more audio inputs that may be selected. The audio properties are queried by passing a struct video_audio
to VIDIOCGAUDIO
ioctl. The VIDIOCSAUDIO
ioctl sets audio properties.

The structure contains the following fields

The following flags are defined

The following decoding modes are defined

Reading Images

Each call to the read
syscall returns the next available image from the device. It is up to
the caller to set the format and then to pass a suitable size buffer
and length to the function. Not all devices will support read
operations.

A second way to handle image
capture is via the mmap interface if supported. To use the mmap
interface a user first sets the desired image size and depth
properties. Next the VIDIOCGMBUF ioctl is issued. This reports the size
of buffer to mmap and the offset within the buffer for each frame. The
number of frames supported is device dependent and may only be one.

The video_mbuf structure contains the following fields

Once
the mmap has been made the VIDIOCMCAPTURE ioctl sets the image size you
wish to use (which should match or be below the initial query size).
Having done so it will begin capturing to the memory mapped buffer.
Whenever a buffer is "used" by the program it should called VIDIOCSYNC
to free this frame up and continue. to add:
VIDIOCSYNC takes
the frame number you are freeing as its argument. When the buffer is
unmapped or all the buffers are full capture ceases. While capturing to
memory the driver will make a "best effort" attempt to capture to
screen as well if requested. This normally means all frames that "miss"
memory mapped capture will go to the display.

A
final ioctl exists to allow a device to obtain related devices if a
driver has multiple components (for example video0 may not be
associated with vbi0 which would cause an intercast display program to
make a bad mistake). The VIDIOCGUNIT ioctl reports the unit numbers of
the associated devices if any exist. The video_unit structure has the
following fields.

RDS Datastreams

For
radio devices that support it, it is possible to receive Radio Data
System (RDS) data by means of a read() on the device. The data is
packed in groups of three, as follows: