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diff --git a/Documentation/DocBook/v4l/common.xml b/Documentation/DocBook/v4l/common.xml new file mode 100644 index 0000000..b1a81d2 --- /dev/null +++ b/Documentation/DocBook/v4l/common.xml @@ -0,0 +1,1160 @@ + <title>Common API Elements</title> + + <para>Programming a V4L2 device consists of these +steps:</para> + + <itemizedlist> + <listitem> + <para>Opening the device</para> + </listitem> + <listitem> + <para>Changing device properties, selecting a video and audio +input, video standard, picture brightness a. o.</para> + </listitem> + <listitem> + <para>Negotiating a data format</para> + </listitem> + <listitem> + <para>Negotiating an input/output method</para> + </listitem> + <listitem> + <para>The actual input/output loop</para> + </listitem> + <listitem> + <para>Closing the device</para> + </listitem> + </itemizedlist> + + <para>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 <xref linkend="devices" />. In this chapter we will discuss +the basic concepts applicable to all devices.</para> + + <section id="open"> + <title>Opening and Closing Devices</title> + + <section> + <title>Device Naming</title> + + <para>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.</para> + + <para>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.<footnote> + <para>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.</para> + </footnote> The module options to select minor numbers are named +after the device special file with a "_nr" suffix. For example "video_nr" +for <filename>/dev/video</filename> video capture devices. The number is +an offset to the base minor number associated with the device type. +<footnote> + <para>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.</para> + </footnote> When the driver supports multiple devices of the same +type more than one minor number can be assigned, separated by commas: +<informalexample> + <screen> +> insmod mydriver.o video_nr=0,1 radio_nr=0,1</screen> + </informalexample></para> + + <para>In <filename>/etc/modules.conf</filename> this may be +written as: <informalexample> + <screen> +alias char-major-81-0 mydriver +alias char-major-81-1 mydriver +alias char-major-81-64 mydriver <co id="alias" /> +options mydriver video_nr=0,1 radio_nr=0,1 <co id="options" /> + </screen> + <calloutlist> + <callout arearefs="alias"> + <para>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).</para> + </callout> + <callout arearefs="options"> + <para>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).</para> + </callout> + </calloutlist> + </informalexample> When no minor number is given as module +option the driver supplies a default. <xref linkend="devices" /> +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 <emphasis>offending device</emphasis> will not be +registered. <!-- Blessed by Linus Torvalds on +linux-kernel@vger.kernel.org, 2002-11-20. --></para> + + <para>By convention system administrators create various +character device special files with these major and minor numbers in +the <filename>/dev</filename> directory. The names recomended for the +different V4L2 device types are listed in <xref linkend="devices" />. +</para> + + <para>The creation of character special files (with +<application>mknod</application>) is a privileged operation and +devices cannot be opened by major and minor number. That means +applications cannot <emphasis>reliable</emphasis> scan for loaded or +installed drivers. The user must enter a device name, or the +application can try the conventional device names.</para> + + <para>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 +<filename>/dev/v4l</filename> directory using the conventional device +names above.</para> + </section> + + <section id="related"> + <title>Related Devices</title> + + <para>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<footnote> + <para>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 +<filename>/dev/video2</filename> is not necessarily related to +<filename>/dev/vbi2</filename>. The V4L +<constant>VIDIOCGUNIT</constant> ioctl would require a search for a +device file with a particular major and minor number.</para> + </footnote>, and to make things worse the V4L videodev module +used to prohibit multiple opens of a device.</para> + + <para>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 <xref linkend="devices" />.</para> + + <para>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 +<filename>/dev/video</filename> (81, 0) or +<filename>/dev/vbi</filename> (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 <application>dd</application> or <application>cat</application>) +<filename>/dev/video</filename> captures video images, while +<filename>/dev/vbi</filename> captures raw VBI data. +<filename>/dev/radio</filename> (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;.</para> + + <para>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;.</para> + </section> + + <section> + <title>Multiple Opens</title> + + <para>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 +<emphasis>simultaneously</emphasis>, multiple opens allow concurrent +use of the device by forked processes or specialized applications.</para> + + <para>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.</para> + + <para>Mere opening a V4L2 device does not grant exclusive +access.<footnote> + <para>Drivers could recognize the +<constant>O_EXCL</constant> open flag. Presently this is not required, +so applications cannot know if it really works.</para> + </footnote> 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 +<xref linkend="app-pri" />.</para> + </section> + + <section> + <title>Shared Data Streams</title> + + <para>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. <!-- For example O_EXCL when the +application does not want to be preempted, PROT_READ mmapped buffers +which can be mapped twice, what happens when image formats do not +match etc.--></para> + </section> + + <section> + <title>Functions</title> + + <para>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.</para> + </section> + </section> + + <section id="querycap"> + <title>Querying Capabilities</title> + + <para>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.</para> + + <para>The &VIDIOC-QUERYCAP; ioctl is available to check if the kernel +device is compatible with this specification, and to query the <link +linkend="devices">functions</link> and <link linkend="io">I/O +methods</link> supported by the device. 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.</para> + + <para>All V4L2 drivers must support +<constant>VIDIOC_QUERYCAP</constant>. Applications should always call +this ioctl after opening the device.</para> + </section> + + <section id="app-pri"> + <title>Application Priority</title> + + <para>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.</para> + + <para>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 +<constant>VIDIOC_S_PRIORITY</constant> after verifying the device with +the &VIDIOC-QUERYCAP; ioctl.</para> + + <para>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.</para> + </section> + + <section id="video"> + <title>Video Inputs and Outputs</title> + + <para>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.</para> + + <para>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 <constant>VIDIOC_ENUMINPUT</constant> +ioctl also contains signal status information applicable when the +current video input is queried.</para> + + <para>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.</para> + + <!-- + <figure id=io-tree> + <title>Input and output enumeration is the root of most device properties.</title> + <mediaobject> + <imageobject> + <imagedata fileref="links.pdf" format="ps" /> + </imageobject> + <imageobject> + <imagedata fileref="links.gif" format="gif" /> + </imageobject> + <textobject> + <phrase>Links between various device property structures.</phrase> + </textobject> + </mediaobject> + </figure> + --> + + <example> + <title>Information about the current video input</title> + + <programlisting> +&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); + </programlisting> + </example> + + <example> + <title>Switching to the first video input</title> + + <programlisting> +int index; + +index = 0; + +if (-1 == ioctl (fd, &VIDIOC-S-INPUT;, &index)) { + perror ("VIDIOC_S_INPUT"); + exit (EXIT_FAILURE); +} + </programlisting> + </example> + </section> + + <section id="audio"> + <title>Audio Inputs and Outputs</title> + + <para>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 +<emphasis>is</emphasis> an audio source, but this API associates +tuners with video inputs or outputs only, and radio devices have +none of these.<footnote> + <para>Actually &v4l2-audio; ought to have a +<structfield>tuner</structfield> field like &v4l2-input;, not only +making the API more consistent but also permitting radio devices with +multiple tuners.</para> + </footnote> A connector on a TV card to loop back the received +audio signal to a sound card is not considered an audio output.</para> + + <para>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 <structfield>audioset</structfield> 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.</para> + + <para>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 <constant>VIDIOC_ENUMAUDIO</constant> ioctl +also contains signal status information applicable when the current +audio input is queried.</para> + + <para>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 <constant>VIDIOC_ENUMAUDIO</constant> and +<constant>VIDIOC_ENUMAUDOUT</constant> do, not just an index.</para> + + <para>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.</para> + + <para>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 <constant>V4L2_CAP_AUDIO</constant> flag in the +&v4l2-capability; returned by the &VIDIOC-QUERYCAP; ioctl.</para> + + <example> + <title>Information about the current audio input</title> + + <programlisting> +&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); + </programlisting> + </example> + + <example> + <title>Switching to the first audio input</title> + + <programlisting> +&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); +} + </programlisting> + </example> + </section> + + <section id="tuner"> + <title>Tuners and Modulators</title> + + <section> + <title>Tuners</title> + + <para>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 <structfield>type</structfield> field of the respective +&v4l2-input; returned by the &VIDIOC-ENUMINPUT; ioctl is set to +<constant>V4L2_INPUT_TYPE_TUNER</constant> and its +<structfield>tuner</structfield> field contains the index number of +the tuner.</para> + + <para>Radio devices have exactly one tuner with index zero, no +video inputs.</para> + + <para>To query and change tuner properties applications use the +&VIDIOC-G-TUNER; and &VIDIOC-S-TUNER; ioctl, respectively. The +&v4l2-tuner; returned by <constant>VIDIOC_G_TUNER</constant> also +contains signal status information applicable when the tuner of the +current video input, or a radio tuner is queried. Note that +<constant>VIDIOC_S_TUNER</constant> 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 +<constant>V4L2_CAP_TUNER</constant> flag in the &v4l2-capability; +returned by the &VIDIOC-QUERYCAP; ioctl when the device has one or +more tuners.</para> + </section> + + <section> + <title>Modulators</title> + + <para>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 <structfield>type</structfield> field of the +respective &v4l2-output; returned by the &VIDIOC-ENUMOUTPUT; ioctl is +set to <constant>V4L2_OUTPUT_TYPE_MODULATOR</constant> and its +<structfield>modulator</structfield> field contains the index number +of the modulator. This specification does not define radio output +devices.</para> + + <para>To query and change modulator properties applications use +the &VIDIOC-G-MODULATOR; and &VIDIOC-S-MODULATOR; ioctl. Note that +<constant>VIDIOC_S_MODULATOR</constant> 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 <constant>V4L2_CAP_MODULATOR</constant> flag in +the &v4l2-capability; returned by the &VIDIOC-QUERYCAP; ioctl when the +device has one or more modulators.</para> + </section> + + <section> + <title>Radio Frequency</title> + + <para>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.</para> + </section> + </section> + + <section id="standard"> + <title>Video Standards</title> + + <para>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 +<structfield>std</structfield> field of &v4l2-input; and +&v4l2-output; returned by the &VIDIOC-ENUMINPUT; and +&VIDIOC-ENUMOUTPUT; ioctl, respectively.</para> + + <para>V4L2 defines one bit for each analog video standard +currently in use worldwide, and sets aside bits for driver defined +standards, ⪚ 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.</para> + + <para>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.</para> + + <para>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".<footnote> + <para>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.</para> + </footnote></para> + + <para>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 <emphasis>received</emphasis> +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), <emphasis>not</emphasis> an index into the standard enumeration.<footnote> + <para>An alternative to the current scheme is to use pointers +to indices as arguments of <constant>VIDIOC_G_STD</constant> and +<constant>VIDIOC_S_STD</constant>, the &v4l2-input; and +&v4l2-output; <structfield>std</structfield> field would be a set of +indices like <structfield>audioset</structfield>.</para> + <para>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 +<constant>VIDIOC_G_STD</constant>. 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.</para><para>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;.</para> + </footnote> Drivers must implement all video standard ioctls +when the device has one or more video inputs or outputs.</para> + + <para>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 <itemizedlist> + <listitem> + <para>incapable of capturing fields or frames at the nominal +rate of the video standard, or</para> + </listitem> + <listitem> + <para>where <link linkend="buffer">timestamps</link> refer +to the instant the field or frame was received by the driver, not the +capture time, or</para> + </listitem> + <listitem> + <para>where <link linkend="buffer">sequence numbers</link> +refer to the frames received by the driver, not the captured +frames.</para> + </listitem> + </itemizedlist> Here the driver shall set the +<structfield>std</structfield> field of &v4l2-input; and &v4l2-output; +to zero, the <constant>VIDIOC_G_STD</constant>, +<constant>VIDIOC_S_STD</constant>, +<constant>VIDIOC_QUERYSTD</constant> and +<constant>VIDIOC_ENUMSTD</constant> ioctls shall return the +&EINVAL;.<footnote> + <para>See <xref linkend="buffer" /> 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.</para> + </footnote></para> + + <example> + <title>Information about the current video standard</title> + + <programlisting> +&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); +} + </programlisting> + </example> + + <example> + <title>Listing the video standards supported by the current +input</title> + + <programlisting> +&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); +} + </programlisting> + </example> + + <example> + <title>Selecting a new video standard</title> + + <programlisting> +&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 + <emphasis>or</emphasis> 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); +} + </programlisting> + </example> + </section> + + &sub-controls; + + <section id="format"> + <title>Data Formats</title> + + <section> + <title>Data Format Negotiation</title> + + <para>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.</para> + + <para>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 <emphasis>could</emphasis> do, +without actually selecting a new data format. The data formats +supported by the V4L2 API are covered in the respective device section +in <xref linkend="devices" />. For a closer look at image formats see +<xref linkend="pixfmt" />.</para> + + <para>The <constant>VIDIOC_S_FMT</constant> 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 <constant>VIDIOC_S_FMT</constant> assigns a logical stream +(video data, VBI data etc.) exclusively to one file descriptor.</para> + + <para>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.</para> + + <para>When applications omit the +<constant>VIDIOC_S_FMT</constant> 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.</para> + + <para>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 <emphasis>may</emphasis> support a +switch using <constant>VIDIOC_S_FMT</constant>.</para> + + <para>All drivers exchanging data with +applications must support the <constant>VIDIOC_G_FMT</constant> and +<constant>VIDIOC_S_FMT</constant> ioctl. Implementation of the +<constant>VIDIOC_TRY_FMT</constant> is highly recommended but +optional.</para> + </section> + + <section> + <title>Image Format Enumeration</title> + + <para>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.<footnote> + <para>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.</para> + </footnote></para> + + <para>The &VIDIOC-ENUM-FMT; ioctl must be supported +by all drivers exchanging image data with applications.</para> + + <important> + <para>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.</para> + </important> + </section> + </section> + + <section id="crop"> + <title>Image Cropping, Insertion and Scaling</title> + + <para>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.</para> + + <para>Applications can use the following API to select an area in +the video signal, query the default area and the hardware limits. +<emphasis>Despite their name, the &VIDIOC-CROPCAP;, &VIDIOC-G-CROP; +and &VIDIOC-S-CROP; ioctls apply to input as well as output +devices.</emphasis></para> + + <para>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.</para> + + <para>On a video output device the source are the images passed in +by the application, and their size is again negotiated with the +<constant>VIDIOC_G/S_FMT</constant> 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.</para> + + <para>Source and target rectangles are defined even if the device +does not support scaling or the <constant>VIDIOC_G/S_CROP</constant> +ioctls. Their size (and position where applicable) will be fixed in +this case. <emphasis>All capture and output device must support the +<constant>VIDIOC_CROPCAP</constant> ioctl such that applications can +determine if scaling takes place.</emphasis></para> + + <section> + <title>Cropping Structures</title> + + <figure id="crop-scale"> + <title>Image Cropping, Insertion and Scaling</title> + <mediaobject> + <imageobject> + <imagedata fileref="crop.pdf" format="PS" /> + </imageobject> + <imageobject> + <imagedata fileref="crop.gif" format="GIF" /> + </imageobject> + <textobject> + <phrase>The cropping, insertion and scaling process</phrase> + </textobject> + </mediaobject> + </figure> + + <para>For capture devices the coordinates of the top left +corner, width and height of the area which can be sampled is given by +the <structfield>bounds</structfield> substructure of the +&v4l2-cropcap; returned by the <constant>VIDIOC_CROPCAP</constant> +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 <xref linkend="vbi-hsync" />). +Vertically ITU-R line +numbers of the first field (<xref linkend="vbi-525" />, <xref +linkend="vbi-625" />), multiplied by two if the driver can capture both +fields.</para> + + <para>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 <constant>VIDIOC_G_CROP</constant> and +<constant>VIDIOC_S_CROP</constant> 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.</para> + + <para>Each capture device has a default source rectangle, given +by the <structfield>defrect</structfield> 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.</para> + + <para>For output devices these structures and ioctls are used +accordingly, defining the <emphasis>target</emphasis> rectangle where +the images will be inserted into the video signal.</para> + + </section> + + <section> + <title>Scaling Adjustments</title> + + <para>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 +<structfield>width</structfield> and <structfield>height</structfield> +in &v4l2-crop; may be smaller than the +&v4l2-cropcap;.<structfield>bounds</structfield> area. Therefore, as +usual, drivers are expected to adjust the requested parameters and +return the actual values selected.</para> + + <para>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.</para> + + <para>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 <constant>VIDIOC_CROPCAP</constant> the +application can easily propose another offset to center the cropping +rectangle.</para> + + <para>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.</para> + + </section> + + <section> + <title>Examples</title> + + <para>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.</para> + + <example> + <title>Resetting the cropping parameters</title> + + <para>(A video capture device is assumed; change +<constant>V4L2_BUF_TYPE_VIDEO_CAPTURE</constant> for other +devices.)</para> + + <programlisting> +&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); +} + </programlisting> + </example> + + <example> + <title>Simple downscaling</title> + + <para>(A video capture device is assumed.)</para> + + <programlisting> +&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. */ + </programlisting> + </example> + + <example> + <title>Selecting an output area</title> + + <programlisting> +&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); +} +</programlisting> + </example> + + <example> + <title>Current scaling factor and pixel aspect</title> + + <para>(A video capture device is assumed.)</para> + + <programlisting> +&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; + </programlisting> + </example> + </section> + </section> + + <section id="streaming-par"> + <title>Streaming Parameters</title> + + <para>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.</para> + + <para>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 unneccessary data copying.</para> + + <para>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.</para> + + <para>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.</para> + + <para>These ioctls are optional, drivers need not implement +them. If so, they return the &EINVAL;.</para> + </section> + + <!-- +Local Variables: +mode: sgml +sgml-parent-document: "v4l2.sgml" +indent-tabs-mode: nil +End: + --> |
