Mauro Carvalho Chehab | 8e080c2 | 2009-09-13 22:16:04 -0300 | [diff] [blame] | 1 | <title>Common API Elements</title> |
| 2 | |
| 3 | <para>Programming a V4L2 device consists of these |
| 4 | steps:</para> |
| 5 | |
| 6 | <itemizedlist> |
| 7 | <listitem> |
| 8 | <para>Opening the device</para> |
| 9 | </listitem> |
| 10 | <listitem> |
| 11 | <para>Changing device properties, selecting a video and audio |
| 12 | input, video standard, picture brightness a. o.</para> |
| 13 | </listitem> |
| 14 | <listitem> |
| 15 | <para>Negotiating a data format</para> |
| 16 | </listitem> |
| 17 | <listitem> |
| 18 | <para>Negotiating an input/output method</para> |
| 19 | </listitem> |
| 20 | <listitem> |
| 21 | <para>The actual input/output loop</para> |
| 22 | </listitem> |
| 23 | <listitem> |
| 24 | <para>Closing the device</para> |
| 25 | </listitem> |
| 26 | </itemizedlist> |
| 27 | |
| 28 | <para>In practice most steps are optional and can be executed out of |
| 29 | order. It depends on the V4L2 device type, you can read about the |
| 30 | details in <xref linkend="devices" />. In this chapter we will discuss |
| 31 | the basic concepts applicable to all devices.</para> |
| 32 | |
| 33 | <section id="open"> |
| 34 | <title>Opening and Closing Devices</title> |
| 35 | |
| 36 | <section> |
| 37 | <title>Device Naming</title> |
| 38 | |
| 39 | <para>V4L2 drivers are implemented as kernel modules, loaded |
| 40 | manually by the system administrator or automatically when a device is |
| 41 | first opened. The driver modules plug into the "videodev" kernel |
| 42 | module. It provides helper functions and a common application |
| 43 | interface specified in this document.</para> |
| 44 | |
| 45 | <para>Each driver thus loaded registers one or more device nodes |
| 46 | with major number 81 and a minor number between 0 and 255. Assigning |
| 47 | minor numbers to V4L2 devices is entirely up to the system administrator, |
| 48 | this is primarily intended to solve conflicts between devices.<footnote> |
| 49 | <para>Access permissions are associated with character |
| 50 | device special files, hence we must ensure device numbers cannot |
| 51 | change with the module load order. To this end minor numbers are no |
| 52 | longer automatically assigned by the "videodev" module as in V4L but |
| 53 | requested by the driver. The defaults will suffice for most people |
| 54 | unless two drivers compete for the same minor numbers.</para> |
| 55 | </footnote> The module options to select minor numbers are named |
| 56 | after the device special file with a "_nr" suffix. For example "video_nr" |
| 57 | for <filename>/dev/video</filename> video capture devices. The number is |
| 58 | an offset to the base minor number associated with the device type. |
| 59 | <footnote> |
| 60 | <para>In earlier versions of the V4L2 API the module options |
| 61 | where named after the device special file with a "unit_" prefix, expressing |
| 62 | the minor number itself, not an offset. Rationale for this change is unknown. |
| 63 | Lastly the naming and semantics are just a convention among driver writers, |
| 64 | the point to note is that minor numbers are not supposed to be hardcoded |
| 65 | into drivers.</para> |
| 66 | </footnote> When the driver supports multiple devices of the same |
| 67 | type more than one minor number can be assigned, separated by commas: |
| 68 | <informalexample> |
| 69 | <screen> |
| 70 | > insmod mydriver.o video_nr=0,1 radio_nr=0,1</screen> |
| 71 | </informalexample></para> |
| 72 | |
| 73 | <para>In <filename>/etc/modules.conf</filename> this may be |
| 74 | written as: <informalexample> |
| 75 | <screen> |
| 76 | alias char-major-81-0 mydriver |
| 77 | alias char-major-81-1 mydriver |
| 78 | alias char-major-81-64 mydriver <co id="alias" /> |
| 79 | options mydriver video_nr=0,1 radio_nr=0,1 <co id="options" /> |
| 80 | </screen> |
| 81 | <calloutlist> |
| 82 | <callout arearefs="alias"> |
| 83 | <para>When an application attempts to open a device |
| 84 | special file with major number 81 and minor number 0, 1, or 64, load |
| 85 | "mydriver" (and the "videodev" module it depends upon).</para> |
| 86 | </callout> |
| 87 | <callout arearefs="options"> |
| 88 | <para>Register the first two video capture devices with |
| 89 | minor number 0 and 1 (base number is 0), the first two radio device |
| 90 | with minor number 64 and 65 (base 64).</para> |
| 91 | </callout> |
| 92 | </calloutlist> |
| 93 | </informalexample> When no minor number is given as module |
| 94 | option the driver supplies a default. <xref linkend="devices" /> |
| 95 | recommends the base minor numbers to be used for the various device |
| 96 | types. Obviously minor numbers must be unique. When the number is |
| 97 | already in use the <emphasis>offending device</emphasis> will not be |
| 98 | registered. <!-- Blessed by Linus Torvalds on |
| 99 | linux-kernel@vger.kernel.org, 2002-11-20. --></para> |
| 100 | |
| 101 | <para>By convention system administrators create various |
| 102 | character device special files with these major and minor numbers in |
Lucas De Marchi | 25985ed | 2011-03-30 22:57:33 -0300 | [diff] [blame] | 103 | the <filename>/dev</filename> directory. The names recommended for the |
Mauro Carvalho Chehab | 8e080c2 | 2009-09-13 22:16:04 -0300 | [diff] [blame] | 104 | different V4L2 device types are listed in <xref linkend="devices" />. |
| 105 | </para> |
| 106 | |
| 107 | <para>The creation of character special files (with |
| 108 | <application>mknod</application>) is a privileged operation and |
| 109 | devices cannot be opened by major and minor number. That means |
| 110 | applications cannot <emphasis>reliable</emphasis> scan for loaded or |
| 111 | installed drivers. The user must enter a device name, or the |
| 112 | application can try the conventional device names.</para> |
| 113 | |
| 114 | <para>Under the device filesystem (devfs) the minor number |
| 115 | options are ignored. V4L2 drivers (or by proxy the "videodev" module) |
| 116 | automatically create the required device files in the |
| 117 | <filename>/dev/v4l</filename> directory using the conventional device |
| 118 | names above.</para> |
| 119 | </section> |
| 120 | |
| 121 | <section id="related"> |
| 122 | <title>Related Devices</title> |
| 123 | |
| 124 | <para>Devices can support several related functions. For example |
| 125 | video capturing, video overlay and VBI capturing are related because |
| 126 | these functions share, amongst other, the same video input and tuner |
| 127 | frequency. V4L and earlier versions of V4L2 used the same device name |
| 128 | and minor number for video capturing and overlay, but different ones |
| 129 | for VBI. Experience showed this approach has several problems<footnote> |
| 130 | <para>Given a device file name one cannot reliable find |
| 131 | related devices. For once names are arbitrary and in a system with |
| 132 | multiple devices, where only some support VBI capturing, a |
| 133 | <filename>/dev/video2</filename> is not necessarily related to |
| 134 | <filename>/dev/vbi2</filename>. The V4L |
| 135 | <constant>VIDIOCGUNIT</constant> ioctl would require a search for a |
| 136 | device file with a particular major and minor number.</para> |
| 137 | </footnote>, and to make things worse the V4L videodev module |
| 138 | used to prohibit multiple opens of a device.</para> |
| 139 | |
| 140 | <para>As a remedy the present version of the V4L2 API relaxed the |
| 141 | concept of device types with specific names and minor numbers. For |
| 142 | compatibility with old applications drivers must still register different |
| 143 | minor numbers to assign a default function to the device. But if related |
| 144 | functions are supported by the driver they must be available under all |
| 145 | registered minor numbers. The desired function can be selected after |
| 146 | opening the device as described in <xref linkend="devices" />.</para> |
| 147 | |
| 148 | <para>Imagine a driver supporting video capturing, video |
| 149 | overlay, raw VBI capturing, and FM radio reception. It registers three |
| 150 | devices with minor number 0, 64 and 224 (this numbering scheme is |
| 151 | inherited from the V4L API). Regardless if |
| 152 | <filename>/dev/video</filename> (81, 0) or |
| 153 | <filename>/dev/vbi</filename> (81, 224) is opened the application can |
| 154 | select any one of the video capturing, overlay or VBI capturing |
| 155 | functions. Without programming (e. g. reading from the device |
| 156 | with <application>dd</application> or <application>cat</application>) |
| 157 | <filename>/dev/video</filename> captures video images, while |
| 158 | <filename>/dev/vbi</filename> captures raw VBI data. |
| 159 | <filename>/dev/radio</filename> (81, 64) is invariable a radio device, |
| 160 | unrelated to the video functions. Being unrelated does not imply the |
| 161 | devices can be used at the same time, however. The &func-open; |
| 162 | function may very well return an &EBUSY;.</para> |
| 163 | |
| 164 | <para>Besides video input or output the hardware may also |
| 165 | support audio sampling or playback. If so, these functions are |
| 166 | implemented as OSS or ALSA PCM devices and eventually OSS or ALSA |
| 167 | audio mixer. The V4L2 API makes no provisions yet to find these |
| 168 | related devices. If you have an idea please write to the linux-media |
| 169 | mailing list: &v4l-ml;.</para> |
| 170 | </section> |
| 171 | |
| 172 | <section> |
| 173 | <title>Multiple Opens</title> |
| 174 | |
| 175 | <para>In general, V4L2 devices can be opened more than once. |
| 176 | When this is supported by the driver, users can for example start a |
| 177 | "panel" application to change controls like brightness or audio |
| 178 | volume, while another application captures video and audio. In other words, panel |
| 179 | applications are comparable to an OSS or ALSA audio mixer application. |
| 180 | When a device supports multiple functions like capturing and overlay |
| 181 | <emphasis>simultaneously</emphasis>, multiple opens allow concurrent |
| 182 | use of the device by forked processes or specialized applications.</para> |
| 183 | |
| 184 | <para>Multiple opens are optional, although drivers should |
| 185 | permit at least concurrent accesses without data exchange, &ie; panel |
| 186 | applications. This implies &func-open; can return an &EBUSY; when the |
| 187 | device is already in use, as well as &func-ioctl; functions initiating |
| 188 | data exchange (namely the &VIDIOC-S-FMT; ioctl), and the &func-read; |
| 189 | and &func-write; functions.</para> |
| 190 | |
| 191 | <para>Mere opening a V4L2 device does not grant exclusive |
| 192 | access.<footnote> |
| 193 | <para>Drivers could recognize the |
| 194 | <constant>O_EXCL</constant> open flag. Presently this is not required, |
| 195 | so applications cannot know if it really works.</para> |
| 196 | </footnote> Initiating data exchange however assigns the right |
| 197 | to read or write the requested type of data, and to change related |
| 198 | properties, to this file descriptor. Applications can request |
| 199 | additional access privileges using the priority mechanism described in |
| 200 | <xref linkend="app-pri" />.</para> |
| 201 | </section> |
| 202 | |
| 203 | <section> |
| 204 | <title>Shared Data Streams</title> |
| 205 | |
| 206 | <para>V4L2 drivers should not support multiple applications |
| 207 | reading or writing the same data stream on a device by copying |
| 208 | buffers, time multiplexing or similar means. This is better handled by |
| 209 | a proxy application in user space. When the driver supports stream |
| 210 | sharing anyway it must be implemented transparently. The V4L2 API does |
| 211 | not specify how conflicts are solved. <!-- For example O_EXCL when the |
| 212 | application does not want to be preempted, PROT_READ mmapped buffers |
| 213 | which can be mapped twice, what happens when image formats do not |
| 214 | match etc.--></para> |
| 215 | </section> |
| 216 | |
| 217 | <section> |
| 218 | <title>Functions</title> |
| 219 | |
| 220 | <para>To open and close V4L2 devices applications use the |
| 221 | &func-open; and &func-close; function, respectively. Devices are |
| 222 | programmed using the &func-ioctl; function as explained in the |
| 223 | following sections.</para> |
| 224 | </section> |
| 225 | </section> |
| 226 | |
| 227 | <section id="querycap"> |
| 228 | <title>Querying Capabilities</title> |
| 229 | |
| 230 | <para>Because V4L2 covers a wide variety of devices not all |
| 231 | aspects of the API are equally applicable to all types of devices. |
| 232 | Furthermore devices of the same type have different capabilities and |
| 233 | this specification permits the omission of a few complicated and less |
| 234 | important parts of the API.</para> |
| 235 | |
| 236 | <para>The &VIDIOC-QUERYCAP; ioctl is available to check if the kernel |
| 237 | device is compatible with this specification, and to query the <link |
| 238 | linkend="devices">functions</link> and <link linkend="io">I/O |
| 239 | methods</link> supported by the device. Other features can be queried |
| 240 | by calling the respective ioctl, for example &VIDIOC-ENUMINPUT; |
| 241 | to learn about the number, types and names of video connectors on the |
| 242 | device. Although abstraction is a major objective of this API, the |
| 243 | ioctl also allows driver specific applications to reliable identify |
| 244 | the driver.</para> |
| 245 | |
| 246 | <para>All V4L2 drivers must support |
| 247 | <constant>VIDIOC_QUERYCAP</constant>. Applications should always call |
| 248 | this ioctl after opening the device.</para> |
| 249 | </section> |
| 250 | |
| 251 | <section id="app-pri"> |
| 252 | <title>Application Priority</title> |
| 253 | |
| 254 | <para>When multiple applications share a device it may be |
| 255 | desirable to assign them different priorities. Contrary to the |
| 256 | traditional "rm -rf /" school of thought a video recording application |
| 257 | could for example block other applications from changing video |
| 258 | controls or switching the current TV channel. Another objective is to |
| 259 | permit low priority applications working in background, which can be |
| 260 | preempted by user controlled applications and automatically regain |
| 261 | control of the device at a later time.</para> |
| 262 | |
| 263 | <para>Since these features cannot be implemented entirely in user |
| 264 | space V4L2 defines the &VIDIOC-G-PRIORITY; and &VIDIOC-S-PRIORITY; |
| 265 | ioctls to request and query the access priority associate with a file |
| 266 | descriptor. Opening a device assigns a medium priority, compatible |
| 267 | with earlier versions of V4L2 and drivers not supporting these ioctls. |
| 268 | Applications requiring a different priority will usually call |
| 269 | <constant>VIDIOC_S_PRIORITY</constant> after verifying the device with |
| 270 | the &VIDIOC-QUERYCAP; ioctl.</para> |
| 271 | |
| 272 | <para>Ioctls changing driver properties, such as &VIDIOC-S-INPUT;, |
| 273 | return an &EBUSY; after another application obtained higher priority. |
| 274 | An event mechanism to notify applications about asynchronous property |
| 275 | changes has been proposed but not added yet.</para> |
| 276 | </section> |
| 277 | |
| 278 | <section id="video"> |
| 279 | <title>Video Inputs and Outputs</title> |
| 280 | |
| 281 | <para>Video inputs and outputs are physical connectors of a |
| 282 | device. These can be for example RF connectors (antenna/cable), CVBS |
| 283 | a.k.a. Composite Video, S-Video or RGB connectors. Only video and VBI |
| 284 | capture devices have inputs, output devices have outputs, at least one |
| 285 | each. Radio devices have no video inputs or outputs.</para> |
| 286 | |
| 287 | <para>To learn about the number and attributes of the |
| 288 | available inputs and outputs applications can enumerate them with the |
| 289 | &VIDIOC-ENUMINPUT; and &VIDIOC-ENUMOUTPUT; ioctl, respectively. The |
| 290 | &v4l2-input; returned by the <constant>VIDIOC_ENUMINPUT</constant> |
| 291 | ioctl also contains signal status information applicable when the |
| 292 | current video input is queried.</para> |
| 293 | |
| 294 | <para>The &VIDIOC-G-INPUT; and &VIDIOC-G-OUTPUT; ioctl return the |
| 295 | index of the current video input or output. To select a different |
| 296 | input or output applications call the &VIDIOC-S-INPUT; and |
| 297 | &VIDIOC-S-OUTPUT; ioctl. Drivers must implement all the input ioctls |
| 298 | when the device has one or more inputs, all the output ioctls when the |
| 299 | device has one or more outputs.</para> |
| 300 | |
| 301 | <!-- |
| 302 | <figure id=io-tree> |
| 303 | <title>Input and output enumeration is the root of most device properties.</title> |
| 304 | <mediaobject> |
| 305 | <imageobject> |
| 306 | <imagedata fileref="links.pdf" format="ps" /> |
| 307 | </imageobject> |
| 308 | <imageobject> |
| 309 | <imagedata fileref="links.gif" format="gif" /> |
| 310 | </imageobject> |
| 311 | <textobject> |
| 312 | <phrase>Links between various device property structures.</phrase> |
| 313 | </textobject> |
| 314 | </mediaobject> |
| 315 | </figure> |
| 316 | --> |
| 317 | |
| 318 | <example> |
| 319 | <title>Information about the current video input</title> |
| 320 | |
| 321 | <programlisting> |
| 322 | &v4l2-input; input; |
| 323 | int index; |
| 324 | |
| 325 | if (-1 == ioctl (fd, &VIDIOC-G-INPUT;, &index)) { |
| 326 | perror ("VIDIOC_G_INPUT"); |
| 327 | exit (EXIT_FAILURE); |
| 328 | } |
| 329 | |
| 330 | memset (&input, 0, sizeof (input)); |
| 331 | input.index = index; |
| 332 | |
| 333 | if (-1 == ioctl (fd, &VIDIOC-ENUMINPUT;, &input)) { |
| 334 | perror ("VIDIOC_ENUMINPUT"); |
| 335 | exit (EXIT_FAILURE); |
| 336 | } |
| 337 | |
| 338 | printf ("Current input: %s\n", input.name); |
| 339 | </programlisting> |
| 340 | </example> |
| 341 | |
| 342 | <example> |
| 343 | <title>Switching to the first video input</title> |
| 344 | |
| 345 | <programlisting> |
| 346 | int index; |
| 347 | |
| 348 | index = 0; |
| 349 | |
| 350 | if (-1 == ioctl (fd, &VIDIOC-S-INPUT;, &index)) { |
| 351 | perror ("VIDIOC_S_INPUT"); |
| 352 | exit (EXIT_FAILURE); |
| 353 | } |
| 354 | </programlisting> |
| 355 | </example> |
| 356 | </section> |
| 357 | |
| 358 | <section id="audio"> |
| 359 | <title>Audio Inputs and Outputs</title> |
| 360 | |
| 361 | <para>Audio inputs and outputs are physical connectors of a |
| 362 | device. Video capture devices have inputs, output devices have |
| 363 | outputs, zero or more each. Radio devices have no audio inputs or |
| 364 | outputs. They have exactly one tuner which in fact |
| 365 | <emphasis>is</emphasis> an audio source, but this API associates |
| 366 | tuners with video inputs or outputs only, and radio devices have |
| 367 | none of these.<footnote> |
| 368 | <para>Actually &v4l2-audio; ought to have a |
| 369 | <structfield>tuner</structfield> field like &v4l2-input;, not only |
| 370 | making the API more consistent but also permitting radio devices with |
| 371 | multiple tuners.</para> |
| 372 | </footnote> A connector on a TV card to loop back the received |
| 373 | audio signal to a sound card is not considered an audio output.</para> |
| 374 | |
| 375 | <para>Audio and video inputs and outputs are associated. Selecting |
| 376 | a video source also selects an audio source. This is most evident when |
| 377 | the video and audio source is a tuner. Further audio connectors can |
| 378 | combine with more than one video input or output. Assumed two |
| 379 | composite video inputs and two audio inputs exist, there may be up to |
| 380 | four valid combinations. The relation of video and audio connectors |
| 381 | is defined in the <structfield>audioset</structfield> field of the |
| 382 | respective &v4l2-input; or &v4l2-output;, where each bit represents |
| 383 | the index number, starting at zero, of one audio input or output.</para> |
| 384 | |
| 385 | <para>To learn about the number and attributes of the |
| 386 | available inputs and outputs applications can enumerate them with the |
| 387 | &VIDIOC-ENUMAUDIO; and &VIDIOC-ENUMAUDOUT; ioctl, respectively. The |
| 388 | &v4l2-audio; returned by the <constant>VIDIOC_ENUMAUDIO</constant> ioctl |
| 389 | also contains signal status information applicable when the current |
| 390 | audio input is queried.</para> |
| 391 | |
| 392 | <para>The &VIDIOC-G-AUDIO; and &VIDIOC-G-AUDOUT; ioctl report |
| 393 | the current audio input and output, respectively. Note that, unlike |
| 394 | &VIDIOC-G-INPUT; and &VIDIOC-G-OUTPUT; these ioctls return a structure |
| 395 | as <constant>VIDIOC_ENUMAUDIO</constant> and |
| 396 | <constant>VIDIOC_ENUMAUDOUT</constant> do, not just an index.</para> |
| 397 | |
| 398 | <para>To select an audio input and change its properties |
| 399 | applications call the &VIDIOC-S-AUDIO; ioctl. To select an audio |
| 400 | output (which presently has no changeable properties) applications |
| 401 | call the &VIDIOC-S-AUDOUT; ioctl.</para> |
| 402 | |
| 403 | <para>Drivers must implement all input ioctls when the device |
| 404 | has one or more inputs, all output ioctls when the device has one |
| 405 | or more outputs. When the device has any audio inputs or outputs the |
| 406 | driver must set the <constant>V4L2_CAP_AUDIO</constant> flag in the |
| 407 | &v4l2-capability; returned by the &VIDIOC-QUERYCAP; ioctl.</para> |
| 408 | |
| 409 | <example> |
| 410 | <title>Information about the current audio input</title> |
| 411 | |
| 412 | <programlisting> |
| 413 | &v4l2-audio; audio; |
| 414 | |
| 415 | memset (&audio, 0, sizeof (audio)); |
| 416 | |
| 417 | if (-1 == ioctl (fd, &VIDIOC-G-AUDIO;, &audio)) { |
| 418 | perror ("VIDIOC_G_AUDIO"); |
| 419 | exit (EXIT_FAILURE); |
| 420 | } |
| 421 | |
| 422 | printf ("Current input: %s\n", audio.name); |
| 423 | </programlisting> |
| 424 | </example> |
| 425 | |
| 426 | <example> |
| 427 | <title>Switching to the first audio input</title> |
| 428 | |
| 429 | <programlisting> |
| 430 | &v4l2-audio; audio; |
| 431 | |
| 432 | memset (&audio, 0, sizeof (audio)); /* clear audio.mode, audio.reserved */ |
| 433 | |
| 434 | audio.index = 0; |
| 435 | |
| 436 | if (-1 == ioctl (fd, &VIDIOC-S-AUDIO;, &audio)) { |
| 437 | perror ("VIDIOC_S_AUDIO"); |
| 438 | exit (EXIT_FAILURE); |
| 439 | } |
| 440 | </programlisting> |
| 441 | </example> |
| 442 | </section> |
| 443 | |
| 444 | <section id="tuner"> |
| 445 | <title>Tuners and Modulators</title> |
| 446 | |
| 447 | <section> |
| 448 | <title>Tuners</title> |
| 449 | |
| 450 | <para>Video input devices can have one or more tuners |
| 451 | demodulating a RF signal. Each tuner is associated with one or more |
| 452 | video inputs, depending on the number of RF connectors on the tuner. |
| 453 | The <structfield>type</structfield> field of the respective |
| 454 | &v4l2-input; returned by the &VIDIOC-ENUMINPUT; ioctl is set to |
| 455 | <constant>V4L2_INPUT_TYPE_TUNER</constant> and its |
| 456 | <structfield>tuner</structfield> field contains the index number of |
| 457 | the tuner.</para> |
| 458 | |
| 459 | <para>Radio devices have exactly one tuner with index zero, no |
| 460 | video inputs.</para> |
| 461 | |
| 462 | <para>To query and change tuner properties applications use the |
| 463 | &VIDIOC-G-TUNER; and &VIDIOC-S-TUNER; ioctl, respectively. The |
| 464 | &v4l2-tuner; returned by <constant>VIDIOC_G_TUNER</constant> also |
| 465 | contains signal status information applicable when the tuner of the |
| 466 | current video input, or a radio tuner is queried. Note that |
| 467 | <constant>VIDIOC_S_TUNER</constant> does not switch the current tuner, |
| 468 | when there is more than one at all. The tuner is solely determined by |
| 469 | the current video input. Drivers must support both ioctls and set the |
| 470 | <constant>V4L2_CAP_TUNER</constant> flag in the &v4l2-capability; |
| 471 | returned by the &VIDIOC-QUERYCAP; ioctl when the device has one or |
| 472 | more tuners.</para> |
| 473 | </section> |
| 474 | |
| 475 | <section> |
| 476 | <title>Modulators</title> |
| 477 | |
| 478 | <para>Video output devices can have one or more modulators, uh, |
| 479 | modulating a video signal for radiation or connection to the antenna |
| 480 | input of a TV set or video recorder. Each modulator is associated with |
| 481 | one or more video outputs, depending on the number of RF connectors on |
| 482 | the modulator. The <structfield>type</structfield> field of the |
| 483 | respective &v4l2-output; returned by the &VIDIOC-ENUMOUTPUT; ioctl is |
| 484 | set to <constant>V4L2_OUTPUT_TYPE_MODULATOR</constant> and its |
| 485 | <structfield>modulator</structfield> field contains the index number |
| 486 | of the modulator. This specification does not define radio output |
| 487 | devices.</para> |
| 488 | |
| 489 | <para>To query and change modulator properties applications use |
| 490 | the &VIDIOC-G-MODULATOR; and &VIDIOC-S-MODULATOR; ioctl. Note that |
| 491 | <constant>VIDIOC_S_MODULATOR</constant> does not switch the current |
| 492 | modulator, when there is more than one at all. The modulator is solely |
| 493 | determined by the current video output. Drivers must support both |
| 494 | ioctls and set the <constant>V4L2_CAP_MODULATOR</constant> flag in |
| 495 | the &v4l2-capability; returned by the &VIDIOC-QUERYCAP; ioctl when the |
| 496 | device has one or more modulators.</para> |
| 497 | </section> |
| 498 | |
| 499 | <section> |
| 500 | <title>Radio Frequency</title> |
| 501 | |
| 502 | <para>To get and set the tuner or modulator radio frequency |
| 503 | applications use the &VIDIOC-G-FREQUENCY; and &VIDIOC-S-FREQUENCY; |
| 504 | ioctl which both take a pointer to a &v4l2-frequency;. These ioctls |
| 505 | are used for TV and radio devices alike. Drivers must support both |
| 506 | ioctls when the tuner or modulator ioctls are supported, or |
| 507 | when the device is a radio device.</para> |
| 508 | </section> |
Mauro Carvalho Chehab | 8e080c2 | 2009-09-13 22:16:04 -0300 | [diff] [blame] | 509 | </section> |
| 510 | |
| 511 | <section id="standard"> |
| 512 | <title>Video Standards</title> |
| 513 | |
| 514 | <para>Video devices typically support one or more different video |
| 515 | standards or variations of standards. Each video input and output may |
| 516 | support another set of standards. This set is reported by the |
| 517 | <structfield>std</structfield> field of &v4l2-input; and |
| 518 | &v4l2-output; returned by the &VIDIOC-ENUMINPUT; and |
| 519 | &VIDIOC-ENUMOUTPUT; ioctl, respectively.</para> |
| 520 | |
| 521 | <para>V4L2 defines one bit for each analog video standard |
| 522 | currently in use worldwide, and sets aside bits for driver defined |
| 523 | standards, ⪚ hybrid standards to watch NTSC video tapes on PAL TVs |
| 524 | and vice versa. Applications can use the predefined bits to select a |
| 525 | particular standard, although presenting the user a menu of supported |
| 526 | standards is preferred. To enumerate and query the attributes of the |
| 527 | supported standards applications use the &VIDIOC-ENUMSTD; ioctl.</para> |
| 528 | |
| 529 | <para>Many of the defined standards are actually just variations |
| 530 | of a few major standards. The hardware may in fact not distinguish |
| 531 | between them, or do so internal and switch automatically. Therefore |
| 532 | enumerated standards also contain sets of one or more standard |
| 533 | bits.</para> |
| 534 | |
| 535 | <para>Assume a hypothetic tuner capable of demodulating B/PAL, |
| 536 | G/PAL and I/PAL signals. The first enumerated standard is a set of B |
| 537 | and G/PAL, switched automatically depending on the selected radio |
| 538 | frequency in UHF or VHF band. Enumeration gives a "PAL-B/G" or "PAL-I" |
| 539 | choice. Similar a Composite input may collapse standards, enumerating |
| 540 | "PAL-B/G/H/I", "NTSC-M" and "SECAM-D/K".<footnote> |
| 541 | <para>Some users are already confused by technical terms PAL, |
| 542 | NTSC and SECAM. There is no point asking them to distinguish between |
| 543 | B, G, D, or K when the software or hardware can do that |
| 544 | automatically.</para> |
| 545 | </footnote></para> |
| 546 | |
| 547 | <para>To query and select the standard used by the current video |
| 548 | input or output applications call the &VIDIOC-G-STD; and |
| 549 | &VIDIOC-S-STD; ioctl, respectively. The <emphasis>received</emphasis> |
| 550 | 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> |
| 551 | <para>An alternative to the current scheme is to use pointers |
| 552 | to indices as arguments of <constant>VIDIOC_G_STD</constant> and |
| 553 | <constant>VIDIOC_S_STD</constant>, the &v4l2-input; and |
| 554 | &v4l2-output; <structfield>std</structfield> field would be a set of |
| 555 | indices like <structfield>audioset</structfield>.</para> |
| 556 | <para>Indices are consistent with the rest of the API |
| 557 | and identify the standard unambiguously. In the present scheme of |
| 558 | things an enumerated standard is looked up by &v4l2-std-id;. Now the |
| 559 | standards supported by the inputs of a device can overlap. Just |
| 560 | assume the tuner and composite input in the example above both |
| 561 | exist on a device. An enumeration of "PAL-B/G", "PAL-H/I" suggests |
| 562 | a choice which does not exist. We cannot merge or omit sets, because |
| 563 | applications would be unable to find the standards reported by |
| 564 | <constant>VIDIOC_G_STD</constant>. That leaves separate enumerations |
| 565 | for each input. Also selecting a standard by &v4l2-std-id; can be |
| 566 | ambiguous. Advantage of this method is that applications need not |
| 567 | identify the standard indirectly, after enumerating.</para><para>So in |
| 568 | summary, the lookup itself is unavoidable. The difference is only |
| 569 | whether the lookup is necessary to find an enumerated standard or to |
| 570 | switch to a standard by &v4l2-std-id;.</para> |
| 571 | </footnote> Drivers must implement all video standard ioctls |
| 572 | when the device has one or more video inputs or outputs.</para> |
| 573 | |
| 574 | <para>Special rules apply to USB cameras where the notion of video |
| 575 | standards makes little sense. More generally any capture device, |
| 576 | output devices accordingly, which is <itemizedlist> |
| 577 | <listitem> |
| 578 | <para>incapable of capturing fields or frames at the nominal |
| 579 | rate of the video standard, or</para> |
| 580 | </listitem> |
| 581 | <listitem> |
| 582 | <para>where <link linkend="buffer">timestamps</link> refer |
| 583 | to the instant the field or frame was received by the driver, not the |
| 584 | capture time, or</para> |
| 585 | </listitem> |
| 586 | <listitem> |
| 587 | <para>where <link linkend="buffer">sequence numbers</link> |
| 588 | refer to the frames received by the driver, not the captured |
| 589 | frames.</para> |
| 590 | </listitem> |
| 591 | </itemizedlist> Here the driver shall set the |
| 592 | <structfield>std</structfield> field of &v4l2-input; and &v4l2-output; |
| 593 | to zero, the <constant>VIDIOC_G_STD</constant>, |
| 594 | <constant>VIDIOC_S_STD</constant>, |
| 595 | <constant>VIDIOC_QUERYSTD</constant> and |
| 596 | <constant>VIDIOC_ENUMSTD</constant> ioctls shall return the |
| 597 | &EINVAL;.<footnote> |
| 598 | <para>See <xref linkend="buffer" /> for a rationale. Probably |
| 599 | even USB cameras follow some well known video standard. It might have |
| 600 | been better to explicitly indicate elsewhere if a device cannot live |
| 601 | up to normal expectations, instead of this exception.</para> |
| 602 | </footnote></para> |
| 603 | |
| 604 | <example> |
| 605 | <title>Information about the current video standard</title> |
| 606 | |
| 607 | <programlisting> |
| 608 | &v4l2-std-id; std_id; |
| 609 | &v4l2-standard; standard; |
| 610 | |
| 611 | if (-1 == ioctl (fd, &VIDIOC-G-STD;, &std_id)) { |
| 612 | /* Note when VIDIOC_ENUMSTD always returns EINVAL this |
| 613 | is no video device or it falls under the USB exception, |
| 614 | and VIDIOC_G_STD returning EINVAL is no error. */ |
| 615 | |
| 616 | perror ("VIDIOC_G_STD"); |
| 617 | exit (EXIT_FAILURE); |
| 618 | } |
| 619 | |
| 620 | memset (&standard, 0, sizeof (standard)); |
| 621 | standard.index = 0; |
| 622 | |
| 623 | while (0 == ioctl (fd, &VIDIOC-ENUMSTD;, &standard)) { |
| 624 | if (standard.id & std_id) { |
| 625 | printf ("Current video standard: %s\n", standard.name); |
| 626 | exit (EXIT_SUCCESS); |
| 627 | } |
| 628 | |
| 629 | standard.index++; |
| 630 | } |
| 631 | |
| 632 | /* EINVAL indicates the end of the enumeration, which cannot be |
| 633 | empty unless this device falls under the USB exception. */ |
| 634 | |
| 635 | if (errno == EINVAL || standard.index == 0) { |
| 636 | perror ("VIDIOC_ENUMSTD"); |
| 637 | exit (EXIT_FAILURE); |
| 638 | } |
| 639 | </programlisting> |
| 640 | </example> |
| 641 | |
| 642 | <example> |
| 643 | <title>Listing the video standards supported by the current |
| 644 | input</title> |
| 645 | |
| 646 | <programlisting> |
| 647 | &v4l2-input; input; |
| 648 | &v4l2-standard; standard; |
| 649 | |
| 650 | memset (&input, 0, sizeof (input)); |
| 651 | |
| 652 | if (-1 == ioctl (fd, &VIDIOC-G-INPUT;, &input.index)) { |
| 653 | perror ("VIDIOC_G_INPUT"); |
| 654 | exit (EXIT_FAILURE); |
| 655 | } |
| 656 | |
| 657 | if (-1 == ioctl (fd, &VIDIOC-ENUMINPUT;, &input)) { |
| 658 | perror ("VIDIOC_ENUM_INPUT"); |
| 659 | exit (EXIT_FAILURE); |
| 660 | } |
| 661 | |
| 662 | printf ("Current input %s supports:\n", input.name); |
| 663 | |
| 664 | memset (&standard, 0, sizeof (standard)); |
| 665 | standard.index = 0; |
| 666 | |
| 667 | while (0 == ioctl (fd, &VIDIOC-ENUMSTD;, &standard)) { |
| 668 | if (standard.id & input.std) |
| 669 | printf ("%s\n", standard.name); |
| 670 | |
| 671 | standard.index++; |
| 672 | } |
| 673 | |
| 674 | /* EINVAL indicates the end of the enumeration, which cannot be |
| 675 | empty unless this device falls under the USB exception. */ |
| 676 | |
| 677 | if (errno != EINVAL || standard.index == 0) { |
| 678 | perror ("VIDIOC_ENUMSTD"); |
| 679 | exit (EXIT_FAILURE); |
| 680 | } |
| 681 | </programlisting> |
| 682 | </example> |
| 683 | |
| 684 | <example> |
| 685 | <title>Selecting a new video standard</title> |
| 686 | |
| 687 | <programlisting> |
| 688 | &v4l2-input; input; |
| 689 | &v4l2-std-id; std_id; |
| 690 | |
| 691 | memset (&input, 0, sizeof (input)); |
| 692 | |
| 693 | if (-1 == ioctl (fd, &VIDIOC-G-INPUT;, &input.index)) { |
| 694 | perror ("VIDIOC_G_INPUT"); |
| 695 | exit (EXIT_FAILURE); |
| 696 | } |
| 697 | |
| 698 | if (-1 == ioctl (fd, &VIDIOC-ENUMINPUT;, &input)) { |
| 699 | perror ("VIDIOC_ENUM_INPUT"); |
| 700 | exit (EXIT_FAILURE); |
| 701 | } |
| 702 | |
| 703 | if (0 == (input.std & V4L2_STD_PAL_BG)) { |
| 704 | fprintf (stderr, "Oops. B/G PAL is not supported.\n"); |
| 705 | exit (EXIT_FAILURE); |
| 706 | } |
| 707 | |
| 708 | /* Note this is also supposed to work when only B |
| 709 | <emphasis>or</emphasis> G/PAL is supported. */ |
| 710 | |
| 711 | std_id = V4L2_STD_PAL_BG; |
| 712 | |
| 713 | if (-1 == ioctl (fd, &VIDIOC-S-STD;, &std_id)) { |
| 714 | perror ("VIDIOC_S_STD"); |
| 715 | exit (EXIT_FAILURE); |
| 716 | } |
| 717 | </programlisting> |
| 718 | </example> |
Muralidharan Karicheri | 007701e | 2009-12-03 01:13:17 -0300 | [diff] [blame] | 719 | <section id="dv-timings"> |
| 720 | <title>Digital Video (DV) Timings</title> |
| 721 | <para> |
| 722 | The video standards discussed so far has been dealing with Analog TV and the |
| 723 | corresponding video timings. Today there are many more different hardware interfaces |
| 724 | such as High Definition TV interfaces (HDMI), VGA, DVI connectors etc., that carry |
| 725 | video signals and there is a need to extend the API to select the video timings |
| 726 | for these interfaces. Since it is not possible to extend the &v4l2-std-id; due to |
| 727 | the limited bits available, a new set of IOCTLs is added to set/get video timings at |
| 728 | the input and output: </para><itemizedlist> |
| 729 | <listitem> |
| 730 | <para>DV Presets: Digital Video (DV) presets. These are IDs representing a |
| 731 | video timing at the input/output. Presets are pre-defined timings implemented |
| 732 | by the hardware according to video standards. A __u32 data type is used to represent |
| 733 | a preset unlike the bit mask that is used in &v4l2-std-id; allowing future extensions |
| 734 | to support as many different presets as needed.</para> |
| 735 | </listitem> |
| 736 | <listitem> |
| 737 | <para>Custom DV Timings: This will allow applications to define more detailed |
| 738 | custom video timings for the interface. This includes parameters such as width, height, |
| 739 | polarities, frontporch, backporch etc. |
| 740 | </para> |
| 741 | </listitem> |
| 742 | </itemizedlist> |
| 743 | <para>To enumerate and query the attributes of DV presets supported by a device, |
| 744 | applications use the &VIDIOC-ENUM-DV-PRESETS; ioctl. To get the current DV preset, |
| 745 | applications use the &VIDIOC-G-DV-PRESET; ioctl and to set a preset they use the |
| 746 | &VIDIOC-S-DV-PRESET; ioctl.</para> |
| 747 | <para>To set custom DV timings for the device, applications use the |
| 748 | &VIDIOC-S-DV-TIMINGS; ioctl and to get current custom DV timings they use the |
| 749 | &VIDIOC-G-DV-TIMINGS; ioctl.</para> |
| 750 | <para>Applications can make use of the <xref linkend="input-capabilities" /> and |
| 751 | <xref linkend="output-capabilities"/> flags to decide what ioctls are available to set the |
| 752 | video timings for the device.</para> |
| 753 | </section> |
Mauro Carvalho Chehab | 8e080c2 | 2009-09-13 22:16:04 -0300 | [diff] [blame] | 754 | </section> |
| 755 | |
| 756 | &sub-controls; |
| 757 | |
| 758 | <section id="format"> |
| 759 | <title>Data Formats</title> |
| 760 | |
| 761 | <section> |
| 762 | <title>Data Format Negotiation</title> |
| 763 | |
| 764 | <para>Different devices exchange different kinds of data with |
| 765 | applications, for example video images, raw or sliced VBI data, RDS |
| 766 | datagrams. Even within one kind many different formats are possible, |
| 767 | in particular an abundance of image formats. Although drivers must |
| 768 | provide a default and the selection persists across closing and |
| 769 | reopening a device, applications should always negotiate a data format |
| 770 | before engaging in data exchange. Negotiation means the application |
| 771 | asks for a particular format and the driver selects and reports the |
| 772 | best the hardware can do to satisfy the request. Of course |
| 773 | applications can also just query the current selection.</para> |
| 774 | |
| 775 | <para>A single mechanism exists to negotiate all data formats |
| 776 | using the aggregate &v4l2-format; and the &VIDIOC-G-FMT; and |
| 777 | &VIDIOC-S-FMT; ioctls. Additionally the &VIDIOC-TRY-FMT; ioctl can be |
| 778 | used to examine what the hardware <emphasis>could</emphasis> do, |
| 779 | without actually selecting a new data format. The data formats |
| 780 | supported by the V4L2 API are covered in the respective device section |
| 781 | in <xref linkend="devices" />. For a closer look at image formats see |
| 782 | <xref linkend="pixfmt" />.</para> |
| 783 | |
| 784 | <para>The <constant>VIDIOC_S_FMT</constant> ioctl is a major |
| 785 | turning-point in the initialization sequence. Prior to this point |
| 786 | multiple panel applications can access the same device concurrently to |
| 787 | select the current input, change controls or modify other properties. |
| 788 | The first <constant>VIDIOC_S_FMT</constant> assigns a logical stream |
| 789 | (video data, VBI data etc.) exclusively to one file descriptor.</para> |
| 790 | |
| 791 | <para>Exclusive means no other application, more precisely no |
| 792 | other file descriptor, can grab this stream or change device |
| 793 | properties inconsistent with the negotiated parameters. A video |
| 794 | standard change for example, when the new standard uses a different |
| 795 | number of scan lines, can invalidate the selected image format. |
| 796 | Therefore only the file descriptor owning the stream can make |
| 797 | invalidating changes. Accordingly multiple file descriptors which |
| 798 | grabbed different logical streams prevent each other from interfering |
| 799 | with their settings. When for example video overlay is about to start |
| 800 | or already in progress, simultaneous video capturing may be restricted |
| 801 | to the same cropping and image size.</para> |
| 802 | |
| 803 | <para>When applications omit the |
| 804 | <constant>VIDIOC_S_FMT</constant> ioctl its locking side effects are |
| 805 | implied by the next step, the selection of an I/O method with the |
| 806 | &VIDIOC-REQBUFS; ioctl or implicit with the first &func-read; or |
| 807 | &func-write; call.</para> |
| 808 | |
| 809 | <para>Generally only one logical stream can be assigned to a |
| 810 | file descriptor, the exception being drivers permitting simultaneous |
| 811 | video capturing and overlay using the same file descriptor for |
| 812 | compatibility with V4L and earlier versions of V4L2. Switching the |
| 813 | logical stream or returning into "panel mode" is possible by closing |
| 814 | and reopening the device. Drivers <emphasis>may</emphasis> support a |
| 815 | switch using <constant>VIDIOC_S_FMT</constant>.</para> |
| 816 | |
| 817 | <para>All drivers exchanging data with |
| 818 | applications must support the <constant>VIDIOC_G_FMT</constant> and |
| 819 | <constant>VIDIOC_S_FMT</constant> ioctl. Implementation of the |
| 820 | <constant>VIDIOC_TRY_FMT</constant> is highly recommended but |
| 821 | optional.</para> |
| 822 | </section> |
| 823 | |
| 824 | <section> |
| 825 | <title>Image Format Enumeration</title> |
| 826 | |
| 827 | <para>Apart of the generic format negotiation functions |
| 828 | a special ioctl to enumerate all image formats supported by video |
| 829 | capture, overlay or output devices is available.<footnote> |
| 830 | <para>Enumerating formats an application has no a-priori |
Mauro Carvalho Chehab | 9aa0885 | 2009-09-15 20:27:18 -0300 | [diff] [blame] | 831 | knowledge of (otherwise it could explicitly ask for them and need not |
Mauro Carvalho Chehab | 8e080c2 | 2009-09-13 22:16:04 -0300 | [diff] [blame] | 832 | enumerate) seems useless, but there are applications serving as proxy |
| 833 | between drivers and the actual video applications for which this is |
| 834 | useful.</para> |
| 835 | </footnote></para> |
| 836 | |
| 837 | <para>The &VIDIOC-ENUM-FMT; ioctl must be supported |
| 838 | by all drivers exchanging image data with applications.</para> |
| 839 | |
| 840 | <important> |
| 841 | <para>Drivers are not supposed to convert image formats in |
| 842 | kernel space. They must enumerate only formats directly supported by |
| 843 | the hardware. If necessary driver writers should publish an example |
| 844 | conversion routine or library for integration into applications.</para> |
| 845 | </important> |
| 846 | </section> |
| 847 | </section> |
| 848 | |
Pawel Osciak | 53b5d57 | 2011-01-07 01:41:33 -0300 | [diff] [blame] | 849 | &sub-planar-apis; |
| 850 | |
Mauro Carvalho Chehab | 8e080c2 | 2009-09-13 22:16:04 -0300 | [diff] [blame] | 851 | <section id="crop"> |
| 852 | <title>Image Cropping, Insertion and Scaling</title> |
| 853 | |
| 854 | <para>Some video capture devices can sample a subsection of the |
| 855 | picture and shrink or enlarge it to an image of arbitrary size. We |
| 856 | call these abilities cropping and scaling. Some video output devices |
| 857 | can scale an image up or down and insert it at an arbitrary scan line |
| 858 | and horizontal offset into a video signal.</para> |
| 859 | |
| 860 | <para>Applications can use the following API to select an area in |
| 861 | the video signal, query the default area and the hardware limits. |
| 862 | <emphasis>Despite their name, the &VIDIOC-CROPCAP;, &VIDIOC-G-CROP; |
| 863 | and &VIDIOC-S-CROP; ioctls apply to input as well as output |
| 864 | devices.</emphasis></para> |
| 865 | |
| 866 | <para>Scaling requires a source and a target. On a video capture |
| 867 | or overlay device the source is the video signal, and the cropping |
| 868 | ioctls determine the area actually sampled. The target are images |
| 869 | read by the application or overlaid onto the graphics screen. Their |
| 870 | size (and position for an overlay) is negotiated with the |
| 871 | &VIDIOC-G-FMT; and &VIDIOC-S-FMT; ioctls.</para> |
| 872 | |
| 873 | <para>On a video output device the source are the images passed in |
| 874 | by the application, and their size is again negotiated with the |
| 875 | <constant>VIDIOC_G/S_FMT</constant> ioctls, or may be encoded in a |
| 876 | compressed video stream. The target is the video signal, and the |
| 877 | cropping ioctls determine the area where the images are |
| 878 | inserted.</para> |
| 879 | |
| 880 | <para>Source and target rectangles are defined even if the device |
| 881 | does not support scaling or the <constant>VIDIOC_G/S_CROP</constant> |
| 882 | ioctls. Their size (and position where applicable) will be fixed in |
| 883 | this case. <emphasis>All capture and output device must support the |
| 884 | <constant>VIDIOC_CROPCAP</constant> ioctl such that applications can |
| 885 | determine if scaling takes place.</emphasis></para> |
| 886 | |
| 887 | <section> |
| 888 | <title>Cropping Structures</title> |
| 889 | |
| 890 | <figure id="crop-scale"> |
| 891 | <title>Image Cropping, Insertion and Scaling</title> |
| 892 | <mediaobject> |
| 893 | <imageobject> |
| 894 | <imagedata fileref="crop.pdf" format="PS" /> |
| 895 | </imageobject> |
| 896 | <imageobject> |
| 897 | <imagedata fileref="crop.gif" format="GIF" /> |
| 898 | </imageobject> |
| 899 | <textobject> |
| 900 | <phrase>The cropping, insertion and scaling process</phrase> |
| 901 | </textobject> |
| 902 | </mediaobject> |
| 903 | </figure> |
| 904 | |
| 905 | <para>For capture devices the coordinates of the top left |
| 906 | corner, width and height of the area which can be sampled is given by |
| 907 | the <structfield>bounds</structfield> substructure of the |
| 908 | &v4l2-cropcap; returned by the <constant>VIDIOC_CROPCAP</constant> |
| 909 | ioctl. To support a wide range of hardware this specification does not |
| 910 | define an origin or units. However by convention drivers should |
| 911 | horizontally count unscaled samples relative to 0H (the leading edge |
| 912 | of the horizontal sync pulse, see <xref linkend="vbi-hsync" />). |
| 913 | Vertically ITU-R line |
| 914 | numbers of the first field (<xref linkend="vbi-525" />, <xref |
| 915 | linkend="vbi-625" />), multiplied by two if the driver can capture both |
| 916 | fields.</para> |
| 917 | |
| 918 | <para>The top left corner, width and height of the source |
| 919 | rectangle, that is the area actually sampled, is given by &v4l2-crop; |
| 920 | using the same coordinate system as &v4l2-cropcap;. Applications can |
| 921 | use the <constant>VIDIOC_G_CROP</constant> and |
| 922 | <constant>VIDIOC_S_CROP</constant> ioctls to get and set this |
| 923 | rectangle. It must lie completely within the capture boundaries and |
| 924 | the driver may further adjust the requested size and/or position |
| 925 | according to hardware limitations.</para> |
| 926 | |
| 927 | <para>Each capture device has a default source rectangle, given |
| 928 | by the <structfield>defrect</structfield> substructure of |
| 929 | &v4l2-cropcap;. The center of this rectangle shall align with the |
| 930 | center of the active picture area of the video signal, and cover what |
| 931 | the driver writer considers the complete picture. Drivers shall reset |
| 932 | the source rectangle to the default when the driver is first loaded, |
| 933 | but not later.</para> |
| 934 | |
| 935 | <para>For output devices these structures and ioctls are used |
| 936 | accordingly, defining the <emphasis>target</emphasis> rectangle where |
| 937 | the images will be inserted into the video signal.</para> |
| 938 | |
| 939 | </section> |
| 940 | |
| 941 | <section> |
| 942 | <title>Scaling Adjustments</title> |
| 943 | |
| 944 | <para>Video hardware can have various cropping, insertion and |
| 945 | scaling limitations. It may only scale up or down, support only |
| 946 | discrete scaling factors, or have different scaling abilities in |
| 947 | horizontal and vertical direction. Also it may not support scaling at |
| 948 | all. At the same time the &v4l2-crop; rectangle may have to be |
| 949 | aligned, and both the source and target rectangles may have arbitrary |
| 950 | upper and lower size limits. In particular the maximum |
| 951 | <structfield>width</structfield> and <structfield>height</structfield> |
| 952 | in &v4l2-crop; may be smaller than the |
| 953 | &v4l2-cropcap;.<structfield>bounds</structfield> area. Therefore, as |
| 954 | usual, drivers are expected to adjust the requested parameters and |
| 955 | return the actual values selected.</para> |
| 956 | |
| 957 | <para>Applications can change the source or the target rectangle |
| 958 | first, as they may prefer a particular image size or a certain area in |
| 959 | the video signal. If the driver has to adjust both to satisfy hardware |
| 960 | limitations, the last requested rectangle shall take priority, and the |
| 961 | driver should preferably adjust the opposite one. The &VIDIOC-TRY-FMT; |
| 962 | ioctl however shall not change the driver state and therefore only |
| 963 | adjust the requested rectangle.</para> |
| 964 | |
| 965 | <para>Suppose scaling on a video capture device is restricted to |
| 966 | a factor 1:1 or 2:1 in either direction and the target image size must |
| 967 | be a multiple of 16 × 16 pixels. The source cropping |
| 968 | rectangle is set to defaults, which are also the upper limit in this |
| 969 | example, of 640 × 400 pixels at offset 0, 0. An |
| 970 | application requests an image size of 300 × 225 |
| 971 | pixels, assuming video will be scaled down from the "full picture" |
| 972 | accordingly. The driver sets the image size to the closest possible |
| 973 | values 304 × 224, then chooses the cropping rectangle |
| 974 | closest to the requested size, that is 608 × 224 |
| 975 | (224 × 2:1 would exceed the limit 400). The offset |
| 976 | 0, 0 is still valid, thus unmodified. Given the default cropping |
| 977 | rectangle reported by <constant>VIDIOC_CROPCAP</constant> the |
| 978 | application can easily propose another offset to center the cropping |
| 979 | rectangle.</para> |
| 980 | |
| 981 | <para>Now the application may insist on covering an area using a |
| 982 | picture aspect ratio closer to the original request, so it asks for a |
| 983 | cropping rectangle of 608 × 456 pixels. The present |
| 984 | scaling factors limit cropping to 640 × 384, so the |
| 985 | driver returns the cropping size 608 × 384 and adjusts |
| 986 | the image size to closest possible 304 × 192.</para> |
| 987 | |
| 988 | </section> |
| 989 | |
| 990 | <section> |
| 991 | <title>Examples</title> |
| 992 | |
| 993 | <para>Source and target rectangles shall remain unchanged across |
| 994 | closing and reopening a device, such that piping data into or out of a |
| 995 | device will work without special preparations. More advanced |
| 996 | applications should ensure the parameters are suitable before starting |
| 997 | I/O.</para> |
| 998 | |
| 999 | <example> |
| 1000 | <title>Resetting the cropping parameters</title> |
| 1001 | |
| 1002 | <para>(A video capture device is assumed; change |
| 1003 | <constant>V4L2_BUF_TYPE_VIDEO_CAPTURE</constant> for other |
| 1004 | devices.)</para> |
| 1005 | |
| 1006 | <programlisting> |
| 1007 | &v4l2-cropcap; cropcap; |
| 1008 | &v4l2-crop; crop; |
| 1009 | |
| 1010 | memset (&cropcap, 0, sizeof (cropcap)); |
| 1011 | cropcap.type = V4L2_BUF_TYPE_VIDEO_CAPTURE; |
| 1012 | |
| 1013 | if (-1 == ioctl (fd, &VIDIOC-CROPCAP;, &cropcap)) { |
| 1014 | perror ("VIDIOC_CROPCAP"); |
| 1015 | exit (EXIT_FAILURE); |
| 1016 | } |
| 1017 | |
| 1018 | memset (&crop, 0, sizeof (crop)); |
| 1019 | crop.type = V4L2_BUF_TYPE_VIDEO_CAPTURE; |
| 1020 | crop.c = cropcap.defrect; |
| 1021 | |
| 1022 | /* Ignore if cropping is not supported (EINVAL). */ |
| 1023 | |
| 1024 | if (-1 == ioctl (fd, &VIDIOC-S-CROP;, &crop) |
| 1025 | && errno != EINVAL) { |
| 1026 | perror ("VIDIOC_S_CROP"); |
| 1027 | exit (EXIT_FAILURE); |
| 1028 | } |
| 1029 | </programlisting> |
| 1030 | </example> |
| 1031 | |
| 1032 | <example> |
| 1033 | <title>Simple downscaling</title> |
| 1034 | |
| 1035 | <para>(A video capture device is assumed.)</para> |
| 1036 | |
| 1037 | <programlisting> |
| 1038 | &v4l2-cropcap; cropcap; |
| 1039 | &v4l2-format; format; |
| 1040 | |
| 1041 | reset_cropping_parameters (); |
| 1042 | |
| 1043 | /* Scale down to 1/4 size of full picture. */ |
| 1044 | |
| 1045 | memset (&format, 0, sizeof (format)); /* defaults */ |
| 1046 | |
| 1047 | format.type = V4L2_BUF_TYPE_VIDEO_CAPTURE; |
| 1048 | |
| 1049 | format.fmt.pix.width = cropcap.defrect.width >> 1; |
| 1050 | format.fmt.pix.height = cropcap.defrect.height >> 1; |
| 1051 | format.fmt.pix.pixelformat = V4L2_PIX_FMT_YUYV; |
| 1052 | |
| 1053 | if (-1 == ioctl (fd, &VIDIOC-S-FMT;, &format)) { |
| 1054 | perror ("VIDIOC_S_FORMAT"); |
| 1055 | exit (EXIT_FAILURE); |
| 1056 | } |
| 1057 | |
| 1058 | /* We could check the actual image size now, the actual scaling factor |
| 1059 | or if the driver can scale at all. */ |
| 1060 | </programlisting> |
| 1061 | </example> |
| 1062 | |
| 1063 | <example> |
| 1064 | <title>Selecting an output area</title> |
| 1065 | |
| 1066 | <programlisting> |
| 1067 | &v4l2-cropcap; cropcap; |
| 1068 | &v4l2-crop; crop; |
| 1069 | |
| 1070 | memset (&cropcap, 0, sizeof (cropcap)); |
| 1071 | cropcap.type = V4L2_BUF_TYPE_VIDEO_OUTPUT; |
| 1072 | |
| 1073 | if (-1 == ioctl (fd, VIDIOC_CROPCAP;, &cropcap)) { |
| 1074 | perror ("VIDIOC_CROPCAP"); |
| 1075 | exit (EXIT_FAILURE); |
| 1076 | } |
| 1077 | |
| 1078 | memset (&crop, 0, sizeof (crop)); |
| 1079 | |
| 1080 | crop.type = V4L2_BUF_TYPE_VIDEO_OUTPUT; |
| 1081 | crop.c = cropcap.defrect; |
| 1082 | |
| 1083 | /* Scale the width and height to 50 % of their original size |
| 1084 | and center the output. */ |
| 1085 | |
| 1086 | crop.c.width /= 2; |
| 1087 | crop.c.height /= 2; |
| 1088 | crop.c.left += crop.c.width / 2; |
| 1089 | crop.c.top += crop.c.height / 2; |
| 1090 | |
| 1091 | /* Ignore if cropping is not supported (EINVAL). */ |
| 1092 | |
| 1093 | if (-1 == ioctl (fd, VIDIOC_S_CROP, &crop) |
| 1094 | && errno != EINVAL) { |
| 1095 | perror ("VIDIOC_S_CROP"); |
| 1096 | exit (EXIT_FAILURE); |
| 1097 | } |
| 1098 | </programlisting> |
| 1099 | </example> |
| 1100 | |
| 1101 | <example> |
| 1102 | <title>Current scaling factor and pixel aspect</title> |
| 1103 | |
| 1104 | <para>(A video capture device is assumed.)</para> |
| 1105 | |
| 1106 | <programlisting> |
| 1107 | &v4l2-cropcap; cropcap; |
| 1108 | &v4l2-crop; crop; |
| 1109 | &v4l2-format; format; |
| 1110 | double hscale, vscale; |
| 1111 | double aspect; |
| 1112 | int dwidth, dheight; |
| 1113 | |
| 1114 | memset (&cropcap, 0, sizeof (cropcap)); |
| 1115 | cropcap.type = V4L2_BUF_TYPE_VIDEO_CAPTURE; |
| 1116 | |
| 1117 | if (-1 == ioctl (fd, &VIDIOC-CROPCAP;, &cropcap)) { |
| 1118 | perror ("VIDIOC_CROPCAP"); |
| 1119 | exit (EXIT_FAILURE); |
| 1120 | } |
| 1121 | |
| 1122 | memset (&crop, 0, sizeof (crop)); |
| 1123 | crop.type = V4L2_BUF_TYPE_VIDEO_CAPTURE; |
| 1124 | |
| 1125 | if (-1 == ioctl (fd, &VIDIOC-G-CROP;, &crop)) { |
| 1126 | if (errno != EINVAL) { |
| 1127 | perror ("VIDIOC_G_CROP"); |
| 1128 | exit (EXIT_FAILURE); |
| 1129 | } |
| 1130 | |
| 1131 | /* Cropping not supported. */ |
| 1132 | crop.c = cropcap.defrect; |
| 1133 | } |
| 1134 | |
| 1135 | memset (&format, 0, sizeof (format)); |
| 1136 | format.fmt.type = V4L2_BUF_TYPE_VIDEO_CAPTURE; |
| 1137 | |
| 1138 | if (-1 == ioctl (fd, &VIDIOC-G-FMT;, &format)) { |
| 1139 | perror ("VIDIOC_G_FMT"); |
| 1140 | exit (EXIT_FAILURE); |
| 1141 | } |
| 1142 | |
| 1143 | /* The scaling applied by the driver. */ |
| 1144 | |
| 1145 | hscale = format.fmt.pix.width / (double) crop.c.width; |
| 1146 | vscale = format.fmt.pix.height / (double) crop.c.height; |
| 1147 | |
| 1148 | aspect = cropcap.pixelaspect.numerator / |
| 1149 | (double) cropcap.pixelaspect.denominator; |
| 1150 | aspect = aspect * hscale / vscale; |
| 1151 | |
| 1152 | /* Devices following ITU-R BT.601 do not capture |
| 1153 | square pixels. For playback on a computer monitor |
| 1154 | we should scale the images to this size. */ |
| 1155 | |
| 1156 | dwidth = format.fmt.pix.width / aspect; |
| 1157 | dheight = format.fmt.pix.height; |
| 1158 | </programlisting> |
| 1159 | </example> |
| 1160 | </section> |
| 1161 | </section> |
| 1162 | |
| 1163 | <section id="streaming-par"> |
| 1164 | <title>Streaming Parameters</title> |
| 1165 | |
| 1166 | <para>Streaming parameters are intended to optimize the video |
| 1167 | capture process as well as I/O. Presently applications can request a |
| 1168 | high quality capture mode with the &VIDIOC-S-PARM; ioctl.</para> |
| 1169 | |
| 1170 | <para>The current video standard determines a nominal number of |
| 1171 | frames per second. If less than this number of frames is to be |
| 1172 | captured or output, applications can request frame skipping or |
| 1173 | duplicating on the driver side. This is especially useful when using |
| 1174 | the &func-read; or &func-write;, which are not augmented by timestamps |
Daniel Mack | 3ad2f3f | 2010-02-03 08:01:28 +0800 | [diff] [blame] | 1175 | or sequence counters, and to avoid unnecessary data copying.</para> |
Mauro Carvalho Chehab | 8e080c2 | 2009-09-13 22:16:04 -0300 | [diff] [blame] | 1176 | |
| 1177 | <para>Finally these ioctls can be used to determine the number of |
| 1178 | buffers used internally by a driver in read/write mode. For |
| 1179 | implications see the section discussing the &func-read; |
| 1180 | function.</para> |
| 1181 | |
| 1182 | <para>To get and set the streaming parameters applications call |
| 1183 | the &VIDIOC-G-PARM; and &VIDIOC-S-PARM; ioctl, respectively. They take |
| 1184 | a pointer to a &v4l2-streamparm;, which contains a union holding |
| 1185 | separate parameters for input and output devices.</para> |
| 1186 | |
| 1187 | <para>These ioctls are optional, drivers need not implement |
| 1188 | them. If so, they return the &EINVAL;.</para> |
| 1189 | </section> |
| 1190 | |
| 1191 | <!-- |
| 1192 | Local Variables: |
| 1193 | mode: sgml |
| 1194 | sgml-parent-document: "v4l2.sgml" |
| 1195 | indent-tabs-mode: nil |
| 1196 | End: |
| 1197 | --> |