# Common API Elements

Programming a V4L2 device consists of these steps:

- Opening the device

- Changing device properties, selecting a video and audio input, video standard, picture brightness a. o.

- Negotiating a data format

- Negotiating an input/output method

- The actual input/output loop

- Closing the device

In practice most steps are optional and can be executed out of order. It depends on the V4L2 device type, you can read about the details in . In this chapter we will discuss the basic concepts applicable to all devices.

# Opening and Closing Devices

## Device Naming

V4L2 drivers are implemented as kernel modules, loaded manually by the system administrator or automatically when a device is first opened. The driver modules plug into the "videodev" kernel module. It provides helper functions and a common application interface specified in this document.

Each driver thus loaded registers one or more device nodes with major number 81 and a minor number between 0 and 255. Assigning minor numbers to V4L2 devices is entirely up to the system administrator, this is primarily intended to solve conflicts between devices. Access permissions are associated with character device special files, hence we must ensure device numbers cannot change with the module load order. To this end minor numbers are no longer automatically assigned by the "videodev" module as in V4L but requested by the driver. The defaults will suffice for most people unless two drivers compete for the same minor numbers. The module options to select minor numbers are named after the device special file with a "_nr" suffix. For example "video_nr" for `/dev/video` video capture devices. The number is an offset to the base minor number associated with the device type. In earlier versions of the V4L2 API the module options where named after the device special file with a "unit_" prefix, expressing the minor number itself, not an offset. Rationale for this change is unknown. Lastly the naming and semantics are just a convention among driver writers, the point to note is that minor numbers are not supposed to be hardcoded into drivers. When the driver supports multiple devices of the same type more than one minor number can be assigned, separated by commas:

    > insmod mydriver.o video_nr=0,1 radio_nr=0,1

In `/etc/modules.conf` this may be written as:

    alias char-major-81-0 mydriver
    alias char-major-81-1 mydriver
    alias char-major-81-64 mydriver
    options mydriver video_nr=0,1 radio_nr=0,1

When an application attempts to open a device special file with major number 81 and minor number 0, 1, or 64, load "mydriver" (and the "videodev" module it depends upon).

Register the first two video capture devices with minor number 0 and 1 (base number is 0), the first two radio device with minor number 64 and 65 (base 64).

When no minor number is given as module option the driver supplies a default. recommends the base minor numbers to be used for the various device types. Obviously minor numbers must be unique. When the number is already in use the offending device will not be registered.

By convention system administrators create various character device special files with these major and minor numbers in the `/dev` directory. The names recommended for the different V4L2 device types are listed in .

The creation of character special files (with mknod) is a privileged operation and devices cannot be opened by major and minor number. That means applications cannot reliable scan for loaded or installed drivers. The user must enter a device name, or the application can try the conventional device names.

Under the device filesystem (devfs) the minor number options are ignored. V4L2 drivers (or by proxy the "videodev" module) automatically create the required device files in the `/dev/v4l` directory using the conventional device names above.

## Related Devices

Devices can support several related functions. For example video capturing, video overlay and VBI capturing are related because these functions share, amongst other, the same video input and tuner frequency. V4L and earlier versions of V4L2 used the same device name and minor number for video capturing and overlay, but different ones for VBI. Experience showed this approach has several problems Given a device file name one cannot reliable find related devices. For once names are arbitrary and in a system with multiple devices, where only some support VBI capturing, a `/dev/video2` is not necessarily related to `/dev/vbi2`. The V4L `VIDIOCGUNIT` ioctl would require a search for a device file with a particular major and minor number. , and to make things worse the V4L videodev module used to prohibit multiple opens of a device.

As a remedy the present version of the V4L2 API relaxed the concept of device types with specific names and minor numbers. For compatibility with old applications drivers must still register different minor numbers to assign a default function to the device. But if related functions are supported by the driver they must be available under all registered minor numbers. The desired function can be selected after opening the device as described in .

Imagine a driver supporting video capturing, video overlay, raw VBI capturing, and FM radio reception. It registers three devices with minor number 0, 64 and 224 (this numbering scheme is inherited from the V4L API). Regardless if `/dev/video` (81, 0) or `/dev/vbi` (81, 224) is opened the application can select any one of the video capturing, overlay or VBI capturing functions. Without programming (e. g. reading from the device with dd or cat) `/dev/video` captures video images, while `/dev/vbi` captures raw VBI data. `/dev/radio` (81, 64) is invariable a radio device, unrelated to the video functions. Being unrelated does not imply the devices can be used at the same time, however. The &func-open; function may very well return an &EBUSY;.

Besides video input or output the hardware may also support audio sampling or playback. If so, these functions are implemented as OSS or ALSA PCM devices and eventually OSS or ALSA audio mixer. The V4L2 API makes no provisions yet to find these related devices. If you have an idea please write to the linux-media mailing list: &v4l-ml;.

## Multiple Opens

In general, V4L2 devices can be opened more than once. When this is supported by the driver, users can for example start a "panel" application to change controls like brightness or audio volume, while another application captures video and audio. In other words, panel applications are comparable to an OSS or ALSA audio mixer application. When a device supports multiple functions like capturing and overlay simultaneously, multiple opens allow concurrent use of the device by forked processes or specialized applications.

Multiple opens are optional, although drivers should permit at least concurrent accesses without data exchange, &ie; panel applications. This implies &func-open; can return an &EBUSY; when the device is already in use, as well as &func-ioctl; functions initiating data exchange (namely the &VIDIOC-S-FMT; ioctl), and the &func-read; and &func-write; functions.

Mere opening a V4L2 device does not grant exclusive access. Drivers could recognize the `O_EXCL` open flag. Presently this is not required, so applications cannot know if it really works. Initiating data exchange however assigns the right to read or write the requested type of data, and to change related properties, to this file descriptor. Applications can request additional access privileges using the priority mechanism described in .

## Shared Data Streams

V4L2 drivers should not support multiple applications reading or writing the same data stream on a device by copying buffers, time multiplexing or similar means. This is better handled by a proxy application in user space. When the driver supports stream sharing anyway it must be implemented transparently. The V4L2 API does not specify how conflicts are solved.

## Functions

To open and close V4L2 devices applications use the &func-open; and &func-close; function, respectively. Devices are programmed using the &func-ioctl; function as explained in the following sections.

# Querying Capabilities

Because V4L2 covers a wide variety of devices not all aspects of the API are equally applicable to all types of devices. Furthermore devices of the same type have different capabilities and this specification permits the omission of a few complicated and less important parts of the API.

The &VIDIOC-QUERYCAP; ioctl is available to check if the kernel device is compatible with this specification, and to query the functions and I/O methods supported by the device.

Starting with kernel version 3.1, VIDIOC-QUERYCAP will return the V4L2 API version used by the driver, with generally matches the Kernel version. There's no need of using &VIDIOC-QUERYCAP; to check if an specific ioctl is supported, the V4L2 core now returns ENOIOCTLCMD if a driver doesn't provide support for an ioctl.

Other features can be queried by calling the respective ioctl, for example &VIDIOC-ENUMINPUT; to learn about the number, types and names of video connectors on the device. Although abstraction is a major objective of this API, the ioctl also allows driver specific applications to reliable identify the driver.

All V4L2 drivers must support `VIDIOC_QUERYCAP`. Applications should always call this ioctl after opening the device.

# Application Priority

When multiple applications share a device it may be desirable to assign them different priorities. Contrary to the traditional "rm -rf /" school of thought a video recording application could for example block other applications from changing video controls or switching the current TV channel. Another objective is to permit low priority applications working in background, which can be preempted by user controlled applications and automatically regain control of the device at a later time.

Since these features cannot be implemented entirely in user space V4L2 defines the &VIDIOC-G-PRIORITY; and &VIDIOC-S-PRIORITY; ioctls to request and query the access priority associate with a file descriptor. Opening a device assigns a medium priority, compatible with earlier versions of V4L2 and drivers not supporting these ioctls. Applications requiring a different priority will usually call `VIDIOC_S_PRIORITY` after verifying the device with the &VIDIOC-QUERYCAP; ioctl.

Ioctls changing driver properties, such as &VIDIOC-S-INPUT;, return an &EBUSY; after another application obtained higher priority. An event mechanism to notify applications about asynchronous property changes has been proposed but not added yet.

# Video Inputs and Outputs

Video inputs and outputs are physical connectors of a device. These can be for example RF connectors (antenna/cable), CVBS a.k.a. Composite Video, S-Video or RGB connectors. Only video and VBI capture devices have inputs, output devices have outputs, at least one each. Radio devices have no video inputs or outputs.

To learn about the number and attributes of the available inputs and outputs applications can enumerate them with the &VIDIOC-ENUMINPUT; and &VIDIOC-ENUMOUTPUT; ioctl, respectively. The &v4l2-input; returned by the `VIDIOC_ENUMINPUT` ioctl also contains signal status information applicable when the current video input is queried.

The &VIDIOC-G-INPUT; and &VIDIOC-G-OUTPUT; ioctl return the index of the current video input or output. To select a different input or output applications call the &VIDIOC-S-INPUT; and &VIDIOC-S-OUTPUT; ioctl. Drivers must implement all the input ioctls when the device has one or more inputs, all the output ioctls when the device has one or more outputs.

## Information about the current video input

    &v4l2-input; input;
    int index;

    if (-1 == ioctl (fd, &VIDIOC-G-INPUT;, &index)) {
    	perror ("VIDIOC_G_INPUT");
    	exit (EXIT_FAILURE);
    }

    memset (&input, 0, sizeof (input));
    input.index = index;

    if (-1 == ioctl (fd, &VIDIOC-ENUMINPUT;, &input)) {
    	perror ("VIDIOC_ENUMINPUT");
    	exit (EXIT_FAILURE);
    }

    printf ("Current input: %s\n", input.name);

## Switching to the first video input

    int index;

    index = 0;

    if (-1 == ioctl (fd, &VIDIOC-S-INPUT;, &index)) {
    	perror ("VIDIOC_S_INPUT");
    	exit (EXIT_FAILURE);
    }

# Audio Inputs and Outputs

Audio inputs and outputs are physical connectors of a device. Video capture devices have inputs, output devices have outputs, zero or more each. Radio devices have no audio inputs or outputs. They have exactly one tuner which in fact is an audio source, but this API associates tuners with video inputs or outputs only, and radio devices have none of these. Actually &v4l2-audio; ought to have a tuner field like &v4l2-input;, not only making the API more consistent but also permitting radio devices with multiple tuners. A connector on a TV card to loop back the received audio signal to a sound card is not considered an audio output.

Audio and video inputs and outputs are associated. Selecting a video source also selects an audio source. This is most evident when the video and audio source is a tuner. Further audio connectors can combine with more than one video input or output. Assumed two composite video inputs and two audio inputs exist, there may be up to four valid combinations. The relation of video and audio connectors is defined in the audioset field of the respective &v4l2-input; or &v4l2-output;, where each bit represents the index number, starting at zero, of one audio input or output.

To learn about the number and attributes of the available inputs and outputs applications can enumerate them with the &VIDIOC-ENUMAUDIO; and &VIDIOC-ENUMAUDOUT; ioctl, respectively. The &v4l2-audio; returned by the `VIDIOC_ENUMAUDIO` ioctl also contains signal status information applicable when the current audio input is queried.

The &VIDIOC-G-AUDIO; and &VIDIOC-G-AUDOUT; ioctl report the current audio input and output, respectively. Note that, unlike &VIDIOC-G-INPUT; and &VIDIOC-G-OUTPUT; these ioctls return a structure as `VIDIOC_ENUMAUDIO` and `VIDIOC_ENUMAUDOUT` do, not just an index.

To select an audio input and change its properties applications call the &VIDIOC-S-AUDIO; ioctl. To select an audio output (which presently has no changeable properties) applications call the &VIDIOC-S-AUDOUT; ioctl.

Drivers must implement all input ioctls when the device has one or more inputs, all output ioctls when the device has one or more outputs. When the device has any audio inputs or outputs the driver must set the `V4L2_CAP_AUDIO` flag in the &v4l2-capability; returned by the &VIDIOC-QUERYCAP; ioctl.

## Information about the current audio input

    &v4l2-audio; audio;

    memset (&audio, 0, sizeof (audio));

    if (-1 == ioctl (fd, &VIDIOC-G-AUDIO;, &audio)) {
    	perror ("VIDIOC_G_AUDIO");
    	exit (EXIT_FAILURE);
    }

    printf ("Current input: %s\n", audio.name);

## Switching to the first audio input

    &v4l2-audio; audio;

    memset (&audio, 0, sizeof (audio)); /* clear audio.mode, audio.reserved */

    audio.index = 0;

    if (-1 == ioctl (fd, &VIDIOC-S-AUDIO;, &audio)) {
    	perror ("VIDIOC_S_AUDIO");
    	exit (EXIT_FAILURE);
    }

# Tuners and Modulators

## Tuners

Video input devices can have one or more tuners demodulating a RF signal. Each tuner is associated with one or more video inputs, depending on the number of RF connectors on the tuner. The type field of the respective &v4l2-input; returned by the &VIDIOC-ENUMINPUT; ioctl is set to `V4L2_INPUT_TYPE_TUNER` and its tuner field contains the index number of the tuner.

Radio devices have exactly one tuner with index zero, no video inputs.

To query and change tuner properties applications use the &VIDIOC-G-TUNER; and &VIDIOC-S-TUNER; ioctl, respectively. The &v4l2-tuner; returned by `VIDIOC_G_TUNER` also contains signal status information applicable when the tuner of the current video input, or a radio tuner is queried. Note that `VIDIOC_S_TUNER` does not switch the current tuner, when there is more than one at all. The tuner is solely determined by the current video input. Drivers must support both ioctls and set the `V4L2_CAP_TUNER` flag in the &v4l2-capability; returned by the &VIDIOC-QUERYCAP; ioctl when the device has one or more tuners.

## Modulators

Video output devices can have one or more modulators, uh, modulating a video signal for radiation or connection to the antenna input of a TV set or video recorder. Each modulator is associated with one or more video outputs, depending on the number of RF connectors on the modulator. The type field of the respective &v4l2-output; returned by the &VIDIOC-ENUMOUTPUT; ioctl is set to `V4L2_OUTPUT_TYPE_MODULATOR` and its modulator field contains the index number of the modulator. This specification does not define radio output devices.

To query and change modulator properties applications use the &VIDIOC-G-MODULATOR; and &VIDIOC-S-MODULATOR; ioctl. Note that `VIDIOC_S_MODULATOR` does not switch the current modulator, when there is more than one at all. The modulator is solely determined by the current video output. Drivers must support both ioctls and set the `V4L2_CAP_MODULATOR` flag in the &v4l2-capability; returned by the &VIDIOC-QUERYCAP; ioctl when the device has one or more modulators.

## Radio Frequency

To get and set the tuner or modulator radio frequency applications use the &VIDIOC-G-FREQUENCY; and &VIDIOC-S-FREQUENCY; ioctl which both take a pointer to a &v4l2-frequency;. These ioctls are used for TV and radio devices alike. Drivers must support both ioctls when the tuner or modulator ioctls are supported, or when the device is a radio device.

# Video Standards

Video devices typically support one or more different video standards or variations of standards. Each video input and output may support another set of standards. This set is reported by the std field of &v4l2-input; and &v4l2-output; returned by the &VIDIOC-ENUMINPUT; and &VIDIOC-ENUMOUTPUT; ioctl, respectively.

V4L2 defines one bit for each analog video standard currently in use worldwide, and sets aside bits for driver defined standards, &eg; hybrid standards to watch NTSC video tapes on PAL TVs and vice versa. Applications can use the predefined bits to select a particular standard, although presenting the user a menu of supported standards is preferred. To enumerate and query the attributes of the supported standards applications use the &VIDIOC-ENUMSTD; ioctl.

Many of the defined standards are actually just variations of a few major standards. The hardware may in fact not distinguish between them, or do so internal and switch automatically. Therefore enumerated standards also contain sets of one or more standard bits.

Assume a hypothetic tuner capable of demodulating B/PAL, G/PAL and I/PAL signals. The first enumerated standard is a set of B and G/PAL, switched automatically depending on the selected radio frequency in UHF or VHF band. Enumeration gives a "PAL-B/G" or "PAL-I" choice. Similar a Composite input may collapse standards, enumerating "PAL-B/G/H/I", "NTSC-M" and "SECAM-D/K". Some users are already confused by technical terms PAL, NTSC and SECAM. There is no point asking them to distinguish between B, G, D, or K when the software or hardware can do that automatically.

To query and select the standard used by the current video input or output applications call the &VIDIOC-G-STD; and &VIDIOC-S-STD; ioctl, respectively. The received standard can be sensed with the &VIDIOC-QUERYSTD; ioctl. Note parameter of all these ioctls is a pointer to a &v4l2-std-id; type (a standard set), not an index into the standard enumeration. An alternative to the current scheme is to use pointers to indices as arguments of `VIDIOC_G_STD` and `VIDIOC_S_STD`, the &v4l2-input; and &v4l2-output; std field would be a set of indices like audioset. Indices are consistent with the rest of the API and identify the standard unambiguously. In the present scheme of things an enumerated standard is looked up by &v4l2-std-id;. Now the standards supported by the inputs of a device can overlap. Just assume the tuner and composite input in the example above both exist on a device. An enumeration of "PAL-B/G", "PAL-H/I" suggests a choice which does not exist. We cannot merge or omit sets, because applications would be unable to find the standards reported by `VIDIOC_G_STD`. That leaves separate enumerations for each input. Also selecting a standard by &v4l2-std-id; can be ambiguous. Advantage of this method is that applications need not identify the standard indirectly, after enumerating.So in summary, the lookup itself is unavoidable. The difference is only whether the lookup is necessary to find an enumerated standard or to switch to a standard by &v4l2-std-id;. Drivers must implement all video standard ioctls when the device has one or more video inputs or outputs.

Special rules apply to USB cameras where the notion of video standards makes little sense. More generally any capture device, output devices accordingly, which is

- incapable of capturing fields or frames at the nominal rate of the video standard, or

- where timestamps refer to the instant the field or frame was received by the driver, not the capture time, or

- where sequence numbers refer to the frames received by the driver, not the captured frames.

Here the driver shall set the std field of &v4l2-input; and &v4l2-output; to zero, the `VIDIOC_G_STD`, `VIDIOC_S_STD`, `VIDIOC_QUERYSTD` and `VIDIOC_ENUMSTD` ioctls shall return the &EINVAL;. See for a rationale. Probably even USB cameras follow some well known video standard. It might have been better to explicitly indicate elsewhere if a device cannot live up to normal expectations, instead of this exception.

## Information about the current video standard

    &v4l2-std-id; std_id;
    &v4l2-standard; standard;

    if (-1 == ioctl (fd, &VIDIOC-G-STD;, &std_id)) {
    	/* Note when VIDIOC_ENUMSTD always returns EINVAL this
    	   is no video device or it falls under the USB exception,
    	   and VIDIOC_G_STD returning EINVAL is no error. */

    	perror ("VIDIOC_G_STD");
    	exit (EXIT_FAILURE);
    }

    memset (&standard, 0, sizeof (standard));
    standard.index = 0;

    while (0 == ioctl (fd, &VIDIOC-ENUMSTD;, &standard)) {
    	if (standard.id & std_id) {
    	       printf ("Current video standard: %s\n", standard.name);
    	       exit (EXIT_SUCCESS);
    	}

    	standard.index++;
    }

    /* EINVAL indicates the end of the enumeration, which cannot be
       empty unless this device falls under the USB exception. */

    if (errno == EINVAL || standard.index == 0) {
    	perror ("VIDIOC_ENUMSTD");
    	exit (EXIT_FAILURE);
    }

## Listing the video standards supported by the current input

    &v4l2-input; input;
    &v4l2-standard; standard;

    memset (&input, 0, sizeof (input));

    if (-1 == ioctl (fd, &VIDIOC-G-INPUT;, &input.index)) {
    	perror ("VIDIOC_G_INPUT");
    	exit (EXIT_FAILURE);
    }

    if (-1 == ioctl (fd, &VIDIOC-ENUMINPUT;, &input)) {
    	perror ("VIDIOC_ENUM_INPUT");
    	exit (EXIT_FAILURE);
    }

    printf ("Current input %s supports:\n", input.name);

    memset (&standard, 0, sizeof (standard));
    standard.index = 0;

    while (0 == ioctl (fd, &VIDIOC-ENUMSTD;, &standard)) {
    	if (standard.id & input.std)
    		printf ("%s\n", standard.name);

    	standard.index++;
    }

    /* EINVAL indicates the end of the enumeration, which cannot be
       empty unless this device falls under the USB exception. */

    if (errno != EINVAL || standard.index == 0) {
    	perror ("VIDIOC_ENUMSTD");
    	exit (EXIT_FAILURE);
    }

## Selecting a new video standard

    &v4l2-input; input;
    &v4l2-std-id; std_id;

    memset (&input, 0, sizeof (input));

    if (-1 == ioctl (fd, &VIDIOC-G-INPUT;, &input.index)) {
    	perror ("VIDIOC_G_INPUT");
    	exit (EXIT_FAILURE);
    }

    if (-1 == ioctl (fd, &VIDIOC-ENUMINPUT;, &input)) {
    	perror ("VIDIOC_ENUM_INPUT");
    	exit (EXIT_FAILURE);
    }

    if (0 == (input.std & V4L2_STD_PAL_BG)) {
    	fprintf (stderr, "Oops. B/G PAL is not supported.\n");
    	exit (EXIT_FAILURE);
    }

    /* Note this is also supposed to work when only B
       or G/PAL is supported. */

    std_id = V4L2_STD_PAL_BG;

    if (-1 == ioctl (fd, &VIDIOC-S-STD;, &std_id)) {
    	perror ("VIDIOC_S_STD");
    	exit (EXIT_FAILURE);
    }

## Digital Video (DV) Timings

The video standards discussed so far has been dealing with Analog TV and the corresponding video timings. Today there are many more different hardware interfaces such as High Definition TV interfaces (HDMI), VGA, DVI connectors etc., that carry video signals and there is a need to extend the API to select the video timings for these interfaces. Since it is not possible to extend the &v4l2-std-id; due to the limited bits available, a new set of IOCTLs is added to set/get video timings at the input and output:

- DV Presets: Digital Video (DV) presets. These are IDs representing a video timing at the input/output. Presets are pre-defined timings implemented by the hardware according to video standards. A __u32 data type is used to represent a preset unlike the bit mask that is used in &v4l2-std-id; allowing future extensions to support as many different presets as needed.

- Custom DV Timings: This will allow applications to define more detailed custom video timings for the interface. This includes parameters such as width, height, polarities, frontporch, backporch etc.

To enumerate and query the attributes of DV presets supported by a device, applications use the &VIDIOC-ENUM-DV-PRESETS; ioctl. To get the current DV preset, applications use the &VIDIOC-G-DV-PRESET; ioctl and to set a preset they use the &VIDIOC-S-DV-PRESET; ioctl.

To set custom DV timings for the device, applications use the &VIDIOC-S-DV-TIMINGS; ioctl and to get current custom DV timings they use the &VIDIOC-G-DV-TIMINGS; ioctl.

Applications can make use of the and flags to decide what ioctls are available to set the video timings for the device.

&sub-controls;

# Data Formats

## Data Format Negotiation

Different devices exchange different kinds of data with applications, for example video images, raw or sliced VBI data, RDS datagrams. Even within one kind many different formats are possible, in particular an abundance of image formats. Although drivers must provide a default and the selection persists across closing and reopening a device, applications should always negotiate a data format before engaging in data exchange. Negotiation means the application asks for a particular format and the driver selects and reports the best the hardware can do to satisfy the request. Of course applications can also just query the current selection.

A single mechanism exists to negotiate all data formats using the aggregate &v4l2-format; and the &VIDIOC-G-FMT; and &VIDIOC-S-FMT; ioctls. Additionally the &VIDIOC-TRY-FMT; ioctl can be used to examine what the hardware could do, without actually selecting a new data format. The data formats supported by the V4L2 API are covered in the respective device section in . For a closer look at image formats see .

The `VIDIOC_S_FMT` ioctl is a major turning-point in the initialization sequence. Prior to this point multiple panel applications can access the same device concurrently to select the current input, change controls or modify other properties. The first `VIDIOC_S_FMT` assigns a logical stream (video data, VBI data etc.) exclusively to one file descriptor.

Exclusive means no other application, more precisely no other file descriptor, can grab this stream or change device properties inconsistent with the negotiated parameters. A video standard change for example, when the new standard uses a different number of scan lines, can invalidate the selected image format. Therefore only the file descriptor owning the stream can make invalidating changes. Accordingly multiple file descriptors which grabbed different logical streams prevent each other from interfering with their settings. When for example video overlay is about to start or already in progress, simultaneous video capturing may be restricted to the same cropping and image size.

When applications omit the `VIDIOC_S_FMT` ioctl its locking side effects are implied by the next step, the selection of an I/O method with the &VIDIOC-REQBUFS; ioctl or implicit with the first &func-read; or &func-write; call.

Generally only one logical stream can be assigned to a file descriptor, the exception being drivers permitting simultaneous video capturing and overlay using the same file descriptor for compatibility with V4L and earlier versions of V4L2. Switching the logical stream or returning into "panel mode" is possible by closing and reopening the device. Drivers may support a switch using `VIDIOC_S_FMT`.

All drivers exchanging data with applications must support the `VIDIOC_G_FMT` and `VIDIOC_S_FMT` ioctl. Implementation of the `VIDIOC_TRY_FMT` is highly recommended but optional.

## Image Format Enumeration

Apart of the generic format negotiation functions a special ioctl to enumerate all image formats supported by video capture, overlay or output devices is available. Enumerating formats an application has no a-priori knowledge of (otherwise it could explicitly ask for them and need not enumerate) seems useless, but there are applications serving as proxy between drivers and the actual video applications for which this is useful.

The &VIDIOC-ENUM-FMT; ioctl must be supported by all drivers exchanging image data with applications.

Drivers are not supposed to convert image formats in kernel space. They must enumerate only formats directly supported by the hardware. If necessary driver writers should publish an example conversion routine or library for integration into applications.

&sub-planar-apis;

# Image Cropping, Insertion and Scaling

Some video capture devices can sample a subsection of the picture and shrink or enlarge it to an image of arbitrary size. We call these abilities cropping and scaling. Some video output devices can scale an image up or down and insert it at an arbitrary scan line and horizontal offset into a video signal.

Applications can use the following API to select an area in the video signal, query the default area and the hardware limits. Despite their name, the &VIDIOC-CROPCAP;, &VIDIOC-G-CROP; and &VIDIOC-S-CROP; ioctls apply to input as well as output devices.

Scaling requires a source and a target. On a video capture or overlay device the source is the video signal, and the cropping ioctls determine the area actually sampled. The target are images read by the application or overlaid onto the graphics screen. Their size (and position for an overlay) is negotiated with the &VIDIOC-G-FMT; and &VIDIOC-S-FMT; ioctls.

On a video output device the source are the images passed in by the application, and their size is again negotiated with the `VIDIOC_G/S_FMT` ioctls, or may be encoded in a compressed video stream. The target is the video signal, and the cropping ioctls determine the area where the images are inserted.

Source and target rectangles are defined even if the device does not support scaling or the `VIDIOC_G/S_CROP` ioctls. Their size (and position where applicable) will be fixed in this case. All capture and output device must support the `VIDIOC_CROPCAP` ioctl such that applications can determine if scaling takes place.

## Cropping Structures

### Image Cropping, Insertion and Scaling

The cropping, insertion and scaling process

For capture devices the coordinates of the top left corner, width and height of the area which can be sampled is given by the bounds substructure of the &v4l2-cropcap; returned by the `VIDIOC_CROPCAP` ioctl. To support a wide range of hardware this specification does not define an origin or units. However by convention drivers should horizontally count unscaled samples relative to 0H (the leading edge of the horizontal sync pulse, see ). Vertically ITU-R line numbers of the first field (, ), multiplied by two if the driver can capture both fields.

The top left corner, width and height of the source rectangle, that is the area actually sampled, is given by &v4l2-crop; using the same coordinate system as &v4l2-cropcap;. Applications can use the `VIDIOC_G_CROP` and `VIDIOC_S_CROP` ioctls to get and set this rectangle. It must lie completely within the capture boundaries and the driver may further adjust the requested size and/or position according to hardware limitations.

Each capture device has a default source rectangle, given by the defrect substructure of &v4l2-cropcap;. The center of this rectangle shall align with the center of the active picture area of the video signal, and cover what the driver writer considers the complete picture. Drivers shall reset the source rectangle to the default when the driver is first loaded, but not later.

For output devices these structures and ioctls are used accordingly, defining the target rectangle where the images will be inserted into the video signal.

## Scaling Adjustments

Video hardware can have various cropping, insertion and scaling limitations. It may only scale up or down, support only discrete scaling factors, or have different scaling abilities in horizontal and vertical direction. Also it may not support scaling at all. At the same time the &v4l2-crop; rectangle may have to be aligned, and both the source and target rectangles may have arbitrary upper and lower size limits. In particular the maximum width and height in &v4l2-crop; may be smaller than the &v4l2-cropcap;.bounds area. Therefore, as usual, drivers are expected to adjust the requested parameters and return the actual values selected.

Applications can change the source or the target rectangle first, as they may prefer a particular image size or a certain area in the video signal. If the driver has to adjust both to satisfy hardware limitations, the last requested rectangle shall take priority, and the driver should preferably adjust the opposite one. The &VIDIOC-TRY-FMT; ioctl however shall not change the driver state and therefore only adjust the requested rectangle.

Suppose scaling on a video capture device is restricted to a factor 1:1 or 2:1 in either direction and the target image size must be a multiple of 16 × 16 pixels. The source cropping rectangle is set to defaults, which are also the upper limit in this example, of 640 × 400 pixels at offset 0, 0. An application requests an image size of 300 × 225 pixels, assuming video will be scaled down from the "full picture" accordingly. The driver sets the image size to the closest possible values 304 × 224, then chooses the cropping rectangle closest to the requested size, that is 608 × 224 (224 × 2:1 would exceed the limit 400). The offset 0, 0 is still valid, thus unmodified. Given the default cropping rectangle reported by `VIDIOC_CROPCAP` the application can easily propose another offset to center the cropping rectangle.

Now the application may insist on covering an area using a picture aspect ratio closer to the original request, so it asks for a cropping rectangle of 608 × 456 pixels. The present scaling factors limit cropping to 640 × 384, so the driver returns the cropping size 608 × 384 and adjusts the image size to closest possible 304 × 192.

## Examples

Source and target rectangles shall remain unchanged across closing and reopening a device, such that piping data into or out of a device will work without special preparations. More advanced applications should ensure the parameters are suitable before starting I/O.

### Resetting the cropping parameters

(A video capture device is assumed; change `V4L2_BUF_TYPE_VIDEO_CAPTURE` for other devices.)

    &v4l2-cropcap; cropcap;
    &v4l2-crop; crop;

    memset (&cropcap, 0, sizeof (cropcap));
    cropcap.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;

    if (-1 == ioctl (fd, &VIDIOC-CROPCAP;, &cropcap)) {
    	perror ("VIDIOC_CROPCAP");
    	exit (EXIT_FAILURE);
    }

    memset (&crop, 0, sizeof (crop));
    crop.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
    crop.c = cropcap.defrect;

    /* Ignore if cropping is not supported (EINVAL). */

    if (-1 == ioctl (fd, &VIDIOC-S-CROP;, &crop)
        && errno != EINVAL) {
    	perror ("VIDIOC_S_CROP");
    	exit (EXIT_FAILURE);
    }

### Simple downscaling

(A video capture device is assumed.)

    &v4l2-cropcap; cropcap;
    &v4l2-format; format;

    reset_cropping_parameters ();

    /* Scale down to 1/4 size of full picture. */

    memset (&format, 0, sizeof (format)); /* defaults */

    format.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;

    format.fmt.pix.width = cropcap.defrect.width >> 1;
    format.fmt.pix.height = cropcap.defrect.height >> 1;
    format.fmt.pix.pixelformat = V4L2_PIX_FMT_YUYV;

    if (-1 == ioctl (fd, &VIDIOC-S-FMT;, &format)) {
    	perror ("VIDIOC_S_FORMAT");
    	exit (EXIT_FAILURE);
    }

    /* We could check the actual image size now, the actual scaling factor
       or if the driver can scale at all. */

### Selecting an output area

    &v4l2-cropcap; cropcap;
    &v4l2-crop; crop;

    memset (&cropcap, 0, sizeof (cropcap));
    cropcap.type = V4L2_BUF_TYPE_VIDEO_OUTPUT;

    if (-1 == ioctl (fd, VIDIOC_CROPCAP;, &cropcap)) {
    	perror ("VIDIOC_CROPCAP");
    	exit (EXIT_FAILURE);
    }

    memset (&crop, 0, sizeof (crop));

    crop.type = V4L2_BUF_TYPE_VIDEO_OUTPUT;
    crop.c = cropcap.defrect;

    /* Scale the width and height to 50 % of their original size
       and center the output. */

    crop.c.width /= 2;
    crop.c.height /= 2;
    crop.c.left += crop.c.width / 2;
    crop.c.top += crop.c.height / 2;

    /* Ignore if cropping is not supported (EINVAL). */

    if (-1 == ioctl (fd, VIDIOC_S_CROP, &crop)
        && errno != EINVAL) {
    	perror ("VIDIOC_S_CROP");
    	exit (EXIT_FAILURE);
    }

### Current scaling factor and pixel aspect

(A video capture device is assumed.)

    &v4l2-cropcap; cropcap;
    &v4l2-crop; crop;
    &v4l2-format; format;
    double hscale, vscale;
    double aspect;
    int dwidth, dheight;

    memset (&cropcap, 0, sizeof (cropcap));
    cropcap.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;

    if (-1 == ioctl (fd, &VIDIOC-CROPCAP;, &cropcap)) {
    	perror ("VIDIOC_CROPCAP");
    	exit (EXIT_FAILURE);
    }

    memset (&crop, 0, sizeof (crop));
    crop.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;

    if (-1 == ioctl (fd, &VIDIOC-G-CROP;, &crop)) {
    	if (errno != EINVAL) {
    		perror ("VIDIOC_G_CROP");
    		exit (EXIT_FAILURE);
    	}

    	/* Cropping not supported. */
    	crop.c = cropcap.defrect;
    }

    memset (&format, 0, sizeof (format));
    format.fmt.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;

    if (-1 == ioctl (fd, &VIDIOC-G-FMT;, &format)) {
    	perror ("VIDIOC_G_FMT");
    	exit (EXIT_FAILURE);
    }

    /* The scaling applied by the driver. */

    hscale = format.fmt.pix.width / (double) crop.c.width;
    vscale = format.fmt.pix.height / (double) crop.c.height;

    aspect = cropcap.pixelaspect.numerator /
    	 (double) cropcap.pixelaspect.denominator;
    aspect = aspect * hscale / vscale;

    /* Devices following ITU-R BT.601 do not capture
       square pixels. For playback on a computer monitor
       we should scale the images to this size. */

    dwidth = format.fmt.pix.width / aspect;
    dheight = format.fmt.pix.height;

# Streaming Parameters

Streaming parameters are intended to optimize the video capture process as well as I/O. Presently applications can request a high quality capture mode with the &VIDIOC-S-PARM; ioctl.

The current video standard determines a nominal number of frames per second. If less than this number of frames is to be captured or output, applications can request frame skipping or duplicating on the driver side. This is especially useful when using the &func-read; or &func-write;, which are not augmented by timestamps or sequence counters, and to avoid unnecessary data copying.

Finally these ioctls can be used to determine the number of buffers used internally by a driver in read/write mode. For implications see the section discussing the &func-read; function.

To get and set the streaming parameters applications call the &VIDIOC-G-PARM; and &VIDIOC-S-PARM; ioctl, respectively. They take a pointer to a &v4l2-streamparm;, which contains a union holding separate parameters for input and output devices.

These ioctls are optional, drivers need not implement them. If so, they return the &EINVAL;.