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diff --git a/Documentation/DocBook/v4l/io.xml b/Documentation/DocBook/v4l/io.xml new file mode 100644 index 0000000..f92f243 --- /dev/null +++ b/Documentation/DocBook/v4l/io.xml @@ -0,0 +1,1073 @@ + <title>Input/Output</title> + + <para>The V4L2 API defines several different methods to read from or +write to a device. All drivers exchanging data with applications must +support at least one of them.</para> + + <para>The classic I/O method using the <function>read()</function> +and <function>write()</function> function is automatically selected +after opening a V4L2 device. When the driver does not support this +method attempts to read or write will fail at any time.</para> + + <para>Other methods must be negotiated. To select the streaming I/O +method with memory mapped or user buffers applications call the +&VIDIOC-REQBUFS; ioctl. The asynchronous I/O method is not defined +yet.</para> + + <para>Video overlay can be considered another I/O method, although +the application does not directly receive the image data. It is +selected by initiating video overlay with the &VIDIOC-S-FMT; ioctl. +For more information see <xref linkend="overlay" />.</para> + + <para>Generally exactly one I/O method, including overlay, is +associated with each file descriptor. The only exceptions are +applications not exchanging data with a driver ("panel applications", +see <xref linkend="open" />) and drivers permitting simultaneous video capturing +and overlay using the same file descriptor, for compatibility with V4L +and earlier versions of V4L2.</para> + + <para><constant>VIDIOC_S_FMT</constant> and +<constant>VIDIOC_REQBUFS</constant> would permit this to some degree, +but for simplicity drivers need not support switching the I/O method +(after first switching away from read/write) other than by closing +and reopening the device.</para> + + <para>The following sections describe the various I/O methods in +more detail.</para> + + <section id="rw"> + <title>Read/Write</title> + + <para>Input and output devices support the +<function>read()</function> and <function>write()</function> function, +respectively, when the <constant>V4L2_CAP_READWRITE</constant> flag in +the <structfield>capabilities</structfield> field of &v4l2-capability; +returned by the &VIDIOC-QUERYCAP; ioctl is set.</para> + + <para>Drivers may need the CPU to copy the data, but they may also +support DMA to or from user memory, so this I/O method is not +necessarily less efficient than other methods merely exchanging buffer +pointers. It is considered inferior though because no meta-information +like frame counters or timestamps are passed. This information is +necessary to recognize frame dropping and to synchronize with other +data streams. However this is also the simplest I/O method, requiring +little or no setup to exchange data. It permits command line stunts +like this (the <application>vidctrl</application> tool is +fictitious):</para> + + <informalexample> + <screen> +> vidctrl /dev/video --input=0 --format=YUYV --size=352x288 +> dd if=/dev/video of=myimage.422 bs=202752 count=1 +</screen> + </informalexample> + + <para>To read from the device applications use the +&func-read; function, to write the &func-write; function. +Drivers must implement one I/O method if they +exchange data with applications, but it need not be this.<footnote> + <para>It would be desirable if applications could depend on +drivers supporting all I/O interfaces, but as much as the complex +memory mapping I/O can be inadequate for some devices we have no +reason to require this interface, which is most useful for simple +applications capturing still images.</para> + </footnote> When reading or writing is supported, the driver +must also support the &func-select; and &func-poll; +function.<footnote> + <para>At the driver level <function>select()</function> and +<function>poll()</function> are the same, and +<function>select()</function> is too important to be optional.</para> + </footnote></para> + </section> + + <section id="mmap"> + <title>Streaming I/O (Memory Mapping)</title> + + <para>Input and output devices support this I/O method when the +<constant>V4L2_CAP_STREAMING</constant> flag in the +<structfield>capabilities</structfield> field of &v4l2-capability; +returned by the &VIDIOC-QUERYCAP; ioctl is set. There are two +streaming methods, to determine if the memory mapping flavor is +supported applications must call the &VIDIOC-REQBUFS; ioctl.</para> + + <para>Streaming is an I/O method where only pointers to buffers +are exchanged between application and driver, the data itself is not +copied. Memory mapping is primarily intended to map buffers in device +memory into the application's address space. Device memory can be for +example the video memory on a graphics card with a video capture +add-on. However, being the most efficient I/O method available for a +long time, many other drivers support streaming as well, allocating +buffers in DMA-able main memory.</para> + + <para>A driver can support many sets of buffers. Each set is +identified by a unique buffer type value. The sets are independent and +each set can hold a different type of data. To access different sets +at the same time different file descriptors must be used.<footnote> + <para>One could use one file descriptor and set the buffer +type field accordingly when calling &VIDIOC-QBUF; etc., but it makes +the <function>select()</function> function ambiguous. We also like the +clean approach of one file descriptor per logical stream. Video +overlay for example is also a logical stream, although the CPU is not +needed for continuous operation.</para> + </footnote></para> + + <para>To allocate device buffers applications call the +&VIDIOC-REQBUFS; ioctl with the desired number of buffers and buffer +type, for example <constant>V4L2_BUF_TYPE_VIDEO_CAPTURE</constant>. +This ioctl can also be used to change the number of buffers or to free +the allocated memory, provided none of the buffers are still +mapped.</para> + + <para>Before applications can access the buffers they must map +them into their address space with the &func-mmap; function. The +location of the buffers in device memory can be determined with the +&VIDIOC-QUERYBUF; ioctl. The <structfield>m.offset</structfield> and +<structfield>length</structfield> returned in a &v4l2-buffer; are +passed as sixth and second parameter to the +<function>mmap()</function> function. The offset and length values +must not be modified. Remember the buffers are allocated in physical +memory, as opposed to virtual memory which can be swapped out to disk. +Applications should free the buffers as soon as possible with the +&func-munmap; function.</para> + + <example> + <title>Mapping buffers</title> + + <programlisting> +&v4l2-requestbuffers; reqbuf; +struct { + void *start; + size_t length; +} *buffers; +unsigned int i; + +memset (&reqbuf, 0, sizeof (reqbuf)); +reqbuf.type = V4L2_BUF_TYPE_VIDEO_CAPTURE; +reqbuf.memory = V4L2_MEMORY_MMAP; +reqbuf.count = 20; + +if (-1 == ioctl (fd, &VIDIOC-REQBUFS;, &reqbuf)) { + if (errno == EINVAL) + printf ("Video capturing or mmap-streaming is not supported\n"); + else + perror ("VIDIOC_REQBUFS"); + + exit (EXIT_FAILURE); +} + +/* We want at least five buffers. */ + +if (reqbuf.count < 5) { + /* You may need to free the buffers here. */ + printf ("Not enough buffer memory\n"); + exit (EXIT_FAILURE); +} + +buffers = calloc (reqbuf.count, sizeof (*buffers)); +assert (buffers != NULL); + +for (i = 0; i < reqbuf.count; i++) { + &v4l2-buffer; buffer; + + memset (&buffer, 0, sizeof (buffer)); + buffer.type = reqbuf.type; + buffer.memory = V4L2_MEMORY_MMAP; + buffer.index = i; + + if (-1 == ioctl (fd, &VIDIOC-QUERYBUF;, &buffer)) { + perror ("VIDIOC_QUERYBUF"); + exit (EXIT_FAILURE); + } + + buffers[i].length = buffer.length; /* remember for munmap() */ + + buffers[i].start = mmap (NULL, buffer.length, + PROT_READ | PROT_WRITE, /* recommended */ + MAP_SHARED, /* recommended */ + fd, buffer.m.offset); + + if (MAP_FAILED == buffers[i].start) { + /* If you do not exit here you should unmap() and free() + the buffers mapped so far. */ + perror ("mmap"); + exit (EXIT_FAILURE); + } +} + +/* Cleanup. */ + +for (i = 0; i < reqbuf.count; i++) + munmap (buffers[i].start, buffers[i].length); + </programlisting> + </example> + + <para>Conceptually streaming drivers maintain two buffer queues, an incoming +and an outgoing queue. They separate the synchronous capture or output +operation locked to a video clock from the application which is +subject to random disk or network delays and preemption by +other processes, thereby reducing the probability of data loss. +The queues are organized as FIFOs, buffers will be +output in the order enqueued in the incoming FIFO, and were +captured in the order dequeued from the outgoing FIFO.</para> + + <para>The driver may require a minimum number of buffers enqueued +at all times to function, apart of this no limit exists on the number +of buffers applications can enqueue in advance, or dequeue and +process. They can also enqueue in a different order than buffers have +been dequeued, and the driver can <emphasis>fill</emphasis> enqueued +<emphasis>empty</emphasis> buffers in any order. <footnote> + <para>Random enqueue order permits applications processing +images out of order (such as video codecs) to return buffers earlier, +reducing the probability of data loss. Random fill order allows +drivers to reuse buffers on a LIFO-basis, taking advantage of caches +holding scatter-gather lists and the like.</para> + </footnote> The index number of a buffer (&v4l2-buffer; +<structfield>index</structfield>) plays no role here, it only +identifies the buffer.</para> + + <para>Initially all mapped buffers are in dequeued state, +inaccessible by the driver. For capturing applications it is customary +to first enqueue all mapped buffers, then to start capturing and enter +the read loop. Here the application waits until a filled buffer can be +dequeued, and re-enqueues the buffer when the data is no longer +needed. Output applications fill and enqueue buffers, when enough +buffers are stacked up the output is started with +<constant>VIDIOC_STREAMON</constant>. In the write loop, when +the application runs out of free buffers, it must wait until an empty +buffer can be dequeued and reused.</para> + + <para>To enqueue and dequeue a buffer applications use the +&VIDIOC-QBUF; and &VIDIOC-DQBUF; ioctl. The status of a buffer being +mapped, enqueued, full or empty can be determined at any time using the +&VIDIOC-QUERYBUF; ioctl. Two methods exist to suspend execution of the +application until one or more buffers can be dequeued. By default +<constant>VIDIOC_DQBUF</constant> blocks when no buffer is in the +outgoing queue. When the <constant>O_NONBLOCK</constant> flag was +given to the &func-open; function, <constant>VIDIOC_DQBUF</constant> +returns immediately with an &EAGAIN; when no buffer is available. The +&func-select; or &func-poll; function are always available.</para> + + <para>To start and stop capturing or output applications call the +&VIDIOC-STREAMON; and &VIDIOC-STREAMOFF; ioctl. Note +<constant>VIDIOC_STREAMOFF</constant> removes all buffers from both +queues as a side effect. Since there is no notion of doing anything +"now" on a multitasking system, if an application needs to synchronize +with another event it should examine the &v4l2-buffer; +<structfield>timestamp</structfield> of captured buffers, or set the +field before enqueuing buffers for output.</para> + + <para>Drivers implementing memory mapping I/O must +support the <constant>VIDIOC_REQBUFS</constant>, +<constant>VIDIOC_QUERYBUF</constant>, +<constant>VIDIOC_QBUF</constant>, <constant>VIDIOC_DQBUF</constant>, +<constant>VIDIOC_STREAMON</constant> and +<constant>VIDIOC_STREAMOFF</constant> ioctl, the +<function>mmap()</function>, <function>munmap()</function>, +<function>select()</function> and <function>poll()</function> +function.<footnote> + <para>At the driver level <function>select()</function> and +<function>poll()</function> are the same, and +<function>select()</function> is too important to be optional. The +rest should be evident.</para> + </footnote></para> + + <para>[capture example]</para> + + </section> + + <section id="userp"> + <title>Streaming I/O (User Pointers)</title> + + <para>Input and output devices support this I/O method when the +<constant>V4L2_CAP_STREAMING</constant> flag in the +<structfield>capabilities</structfield> field of &v4l2-capability; +returned by the &VIDIOC-QUERYCAP; ioctl is set. If the particular user +pointer method (not only memory mapping) is supported must be +determined by calling the &VIDIOC-REQBUFS; ioctl.</para> + + <para>This I/O method combines advantages of the read/write and +memory mapping methods. Buffers are allocated by the application +itself, and can reside for example in virtual or shared memory. Only +pointers to data are exchanged, these pointers and meta-information +are passed in &v4l2-buffer;. The driver must be switched +into user pointer I/O mode by calling the &VIDIOC-REQBUFS; with the +desired buffer type. No buffers are allocated beforehands, +consequently they are not indexed and cannot be queried like mapped +buffers with the <constant>VIDIOC_QUERYBUF</constant> ioctl.</para> + + <example> + <title>Initiating streaming I/O with user pointers</title> + + <programlisting> +&v4l2-requestbuffers; reqbuf; + +memset (&reqbuf, 0, sizeof (reqbuf)); +reqbuf.type = V4L2_BUF_TYPE_VIDEO_CAPTURE; +reqbuf.memory = V4L2_MEMORY_USERPTR; + +if (ioctl (fd, &VIDIOC-REQBUFS;, &reqbuf) == -1) { + if (errno == EINVAL) + printf ("Video capturing or user pointer streaming is not supported\n"); + else + perror ("VIDIOC_REQBUFS"); + + exit (EXIT_FAILURE); +} + </programlisting> + </example> + + <para>Buffer addresses and sizes are passed on the fly with the +&VIDIOC-QBUF; ioctl. Although buffers are commonly cycled, +applications can pass different addresses and sizes at each +<constant>VIDIOC_QBUF</constant> call. If required by the hardware the +driver swaps memory pages within physical memory to create a +continuous area of memory. This happens transparently to the +application in the virtual memory subsystem of the kernel. When buffer +pages have been swapped out to disk they are brought back and finally +locked in physical memory for DMA.<footnote> + <para>We expect that frequently used buffers are typically not +swapped out. Anyway, the process of swapping, locking or generating +scatter-gather lists may be time consuming. The delay can be masked by +the depth of the incoming buffer queue, and perhaps by maintaining +caches assuming a buffer will be soon enqueued again. On the other +hand, to optimize memory usage drivers can limit the number of buffers +locked in advance and recycle the most recently used buffers first. Of +course, the pages of empty buffers in the incoming queue need not be +saved to disk. Output buffers must be saved on the incoming and +outgoing queue because an application may share them with other +processes.</para> + </footnote></para> + + <para>Filled or displayed buffers are dequeued with the +&VIDIOC-DQBUF; ioctl. The driver can unlock the memory pages at any +time between the completion of the DMA and this ioctl. The memory is +also unlocked when &VIDIOC-STREAMOFF; is called, &VIDIOC-REQBUFS;, or +when the device is closed. Applications must take care not to free +buffers without dequeuing. For once, the buffers remain locked until +further, wasting physical memory. Second the driver will not be +notified when the memory is returned to the application's free list +and subsequently reused for other purposes, possibly completing the +requested DMA and overwriting valuable data.</para> + + <para>For capturing applications it is customary to enqueue a +number of empty buffers, to start capturing and enter the read loop. +Here the application waits until a filled buffer can be dequeued, and +re-enqueues the buffer when the data is no longer needed. Output +applications fill and enqueue buffers, when enough buffers are stacked +up output is started. In the write loop, when the application +runs out of free buffers it must wait until an empty buffer can be +dequeued and reused. Two methods exist to suspend execution of the +application until one or more buffers can be dequeued. By default +<constant>VIDIOC_DQBUF</constant> blocks when no buffer is in the +outgoing queue. When the <constant>O_NONBLOCK</constant> flag was +given to the &func-open; function, <constant>VIDIOC_DQBUF</constant> +returns immediately with an &EAGAIN; when no buffer is available. The +&func-select; or &func-poll; function are always available.</para> + + <para>To start and stop capturing or output applications call the +&VIDIOC-STREAMON; and &VIDIOC-STREAMOFF; ioctl. Note +<constant>VIDIOC_STREAMOFF</constant> removes all buffers from both +queues and unlocks all buffers as a side effect. Since there is no +notion of doing anything "now" on a multitasking system, if an +application needs to synchronize with another event it should examine +the &v4l2-buffer; <structfield>timestamp</structfield> of captured +buffers, or set the field before enqueuing buffers for output.</para> + + <para>Drivers implementing user pointer I/O must +support the <constant>VIDIOC_REQBUFS</constant>, +<constant>VIDIOC_QBUF</constant>, <constant>VIDIOC_DQBUF</constant>, +<constant>VIDIOC_STREAMON</constant> and +<constant>VIDIOC_STREAMOFF</constant> ioctl, the +<function>select()</function> and <function>poll()</function> function.<footnote> + <para>At the driver level <function>select()</function> and +<function>poll()</function> are the same, and +<function>select()</function> is too important to be optional. The +rest should be evident.</para> + </footnote></para> + </section> + + <section id="async"> + <title>Asynchronous I/O</title> + + <para>This method is not defined yet.</para> + </section> + + <section id="buffer"> + <title>Buffers</title> + + <para>A buffer contains data exchanged by application and +driver using one of the Streaming I/O methods. Only pointers to +buffers are exchanged, the data itself is not copied. These pointers, +together with meta-information like timestamps or field parity, are +stored in a struct <structname>v4l2_buffer</structname>, argument to +the &VIDIOC-QUERYBUF;, &VIDIOC-QBUF; and &VIDIOC-DQBUF; ioctl.</para> + + <para>Nominally timestamps refer to the first data byte transmitted. +In practice however the wide range of hardware covered by the V4L2 API +limits timestamp accuracy. Often an interrupt routine will +sample the system clock shortly after the field or frame was stored +completely in memory. So applications must expect a constant +difference up to one field or frame period plus a small (few scan +lines) random error. The delay and error can be much +larger due to compression or transmission over an external bus when +the frames are not properly stamped by the sender. This is frequently +the case with USB cameras. Here timestamps refer to the instant the +field or frame was received by the driver, not the capture time. These +devices identify by not enumerating any video standards, see <xref +linkend="standard" />.</para> + + <para>Similar limitations apply to output timestamps. Typically +the video hardware locks to a clock controlling the video timing, the +horizontal and vertical synchronization pulses. At some point in the +line sequence, possibly the vertical blanking, an interrupt routine +samples the system clock, compares against the timestamp and programs +the hardware to repeat the previous field or frame, or to display the +buffer contents.</para> + + <para>Apart of limitations of the video device and natural +inaccuracies of all clocks, it should be noted system time itself is +not perfectly stable. It can be affected by power saving cycles, +warped to insert leap seconds, or even turned back or forth by the +system administrator affecting long term measurements. <footnote> + <para>Since no other Linux multimedia +API supports unadjusted time it would be foolish to introduce here. We +must use a universally supported clock to synchronize different media, +hence time of day.</para> + </footnote></para> + + <table frame="none" pgwide="1" id="v4l2-buffer"> + <title>struct <structname>v4l2_buffer</structname></title> + <tgroup cols="4"> + &cs-ustr; + <tbody valign="top"> + <row> + <entry>__u32</entry> + <entry><structfield>index</structfield></entry> + <entry></entry> + <entry>Number of the buffer, set by the application. This +field is only used for <link linkend="mmap">memory mapping</link> I/O +and can range from zero to the number of buffers allocated +with the &VIDIOC-REQBUFS; ioctl (&v4l2-requestbuffers; <structfield>count</structfield>) minus one.</entry> + </row> + <row> + <entry>&v4l2-buf-type;</entry> + <entry><structfield>type</structfield></entry> + <entry></entry> + <entry>Type of the buffer, same as &v4l2-format; +<structfield>type</structfield> or &v4l2-requestbuffers; +<structfield>type</structfield>, set by the application.</entry> + </row> + <row> + <entry>__u32</entry> + <entry><structfield>bytesused</structfield></entry> + <entry></entry> + <entry>The number of bytes occupied by the data in the +buffer. It depends on the negotiated data format and may change with +each buffer for compressed variable size data like JPEG images. +Drivers must set this field when <structfield>type</structfield> +refers to an input stream, applications when an output stream.</entry> + </row> + <row> + <entry>__u32</entry> + <entry><structfield>flags</structfield></entry> + <entry></entry> + <entry>Flags set by the application or driver, see <xref +linkend="buffer-flags" />.</entry> + </row> + <row> + <entry>&v4l2-field;</entry> + <entry><structfield>field</structfield></entry> + <entry></entry> + <entry>Indicates the field order of the image in the +buffer, see <xref linkend="v4l2-field" />. This field is not used when +the buffer contains VBI data. Drivers must set it when +<structfield>type</structfield> refers to an input stream, +applications when an output stream.</entry> + </row> + <row> + <entry>struct timeval</entry> + <entry><structfield>timestamp</structfield></entry> + <entry></entry> + <entry><para>For input streams this is the +system time (as returned by the <function>gettimeofday()</function> +function) when the first data byte was captured. For output streams +the data will not be displayed before this time, secondary to the +nominal frame rate determined by the current video standard in +enqueued order. Applications can for example zero this field to +display frames as soon as possible. The driver stores the time at +which the first data byte was actually sent out in the +<structfield>timestamp</structfield> field. This permits +applications to monitor the drift between the video and system +clock.</para></entry> + </row> + <row> + <entry>&v4l2-timecode;</entry> + <entry><structfield>timecode</structfield></entry> + <entry></entry> + <entry>When <structfield>type</structfield> is +<constant>V4L2_BUF_TYPE_VIDEO_CAPTURE</constant> and the +<constant>V4L2_BUF_FLAG_TIMECODE</constant> flag is set in +<structfield>flags</structfield>, this structure contains a frame +timecode. In <link linkend="v4l2-field">V4L2_FIELD_ALTERNATE</link> +mode the top and bottom field contain the same timecode. +Timecodes are intended to help video editing and are typically recorded on +video tapes, but also embedded in compressed formats like MPEG. This +field is independent of the <structfield>timestamp</structfield> and +<structfield>sequence</structfield> fields.</entry> + </row> + <row> + <entry>__u32</entry> + <entry><structfield>sequence</structfield></entry> + <entry></entry> + <entry>Set by the driver, counting the frames in the +sequence.</entry> + </row> + <row> + <entry spanname="hspan"><para>In <link +linkend="v4l2-field">V4L2_FIELD_ALTERNATE</link> mode the top and +bottom field have the same sequence number. The count starts at zero +and includes dropped or repeated frames. A dropped frame was received +by an input device but could not be stored due to lack of free buffer +space. A repeated frame was displayed again by an output device +because the application did not pass new data in +time.</para><para>Note this may count the frames received +e.g. over USB, without taking into account the frames dropped by the +remote hardware due to limited compression throughput or bus +bandwidth. These devices identify by not enumerating any video +standards, see <xref linkend="standard" />.</para></entry> + </row> + <row> + <entry>&v4l2-memory;</entry> + <entry><structfield>memory</structfield></entry> + <entry></entry> + <entry>This field must be set by applications and/or drivers +in accordance with the selected I/O method.</entry> + </row> + <row> + <entry>union</entry> + <entry><structfield>m</structfield></entry> + </row> + <row> + <entry></entry> + <entry>__u32</entry> + <entry><structfield>offset</structfield></entry> + <entry>When <structfield>memory</structfield> is +<constant>V4L2_MEMORY_MMAP</constant> this is the offset of the buffer +from the start of the device memory. The value is returned by the +driver and apart of serving as parameter to the &func-mmap; function +not useful for applications. See <xref linkend="mmap" /> for details.</entry> + </row> + <row> + <entry></entry> + <entry>unsigned long</entry> + <entry><structfield>userptr</structfield></entry> + <entry>When <structfield>memory</structfield> is +<constant>V4L2_MEMORY_USERPTR</constant> this is a pointer to the +buffer (casted to unsigned long type) in virtual memory, set by the +application. See <xref linkend="userp" /> for details.</entry> + </row> + <row> + <entry>__u32</entry> + <entry><structfield>length</structfield></entry> + <entry></entry> + <entry>Size of the buffer (not the payload) in bytes.</entry> + </row> + <row> + <entry>__u32</entry> + <entry><structfield>input</structfield></entry> + <entry></entry> + <entry>Some video capture drivers support rapid and +synchronous video input changes, a function useful for example in +video surveillance applications. For this purpose applications set the +<constant>V4L2_BUF_FLAG_INPUT</constant> flag, and this field to the +number of a video input as in &v4l2-input; field +<structfield>index</structfield>.</entry> + </row> + <row> + <entry>__u32</entry> + <entry><structfield>reserved</structfield></entry> + <entry></entry> + <entry>A place holder for future extensions and custom +(driver defined) buffer types +<constant>V4L2_BUF_TYPE_PRIVATE</constant> and higher.</entry> + </row> + </tbody> + </tgroup> + </table> + + <table frame="none" pgwide="1" id="v4l2-buf-type"> + <title>enum v4l2_buf_type</title> + <tgroup cols="3"> + &cs-def; + <tbody valign="top"> + <row> + <entry><constant>V4L2_BUF_TYPE_VIDEO_CAPTURE</constant></entry> + <entry>1</entry> + <entry>Buffer of a video capture stream, see <xref + linkend="capture" />.</entry> + </row> + <row> + <entry><constant>V4L2_BUF_TYPE_VIDEO_OUTPUT</constant></entry> + <entry>2</entry> + <entry>Buffer of a video output stream, see <xref + linkend="output" />.</entry> + </row> + <row> + <entry><constant>V4L2_BUF_TYPE_VIDEO_OVERLAY</constant></entry> + <entry>3</entry> + <entry>Buffer for video overlay, see <xref linkend="overlay" />.</entry> + </row> + <row> + <entry><constant>V4L2_BUF_TYPE_VBI_CAPTURE</constant></entry> + <entry>4</entry> + <entry>Buffer of a raw VBI capture stream, see <xref + linkend="raw-vbi" />.</entry> + </row> + <row> + <entry><constant>V4L2_BUF_TYPE_VBI_OUTPUT</constant></entry> + <entry>5</entry> + <entry>Buffer of a raw VBI output stream, see <xref + linkend="raw-vbi" />.</entry> + </row> + <row> + <entry><constant>V4L2_BUF_TYPE_SLICED_VBI_CAPTURE</constant></entry> + <entry>6</entry> + <entry>Buffer of a sliced VBI capture stream, see <xref + linkend="sliced" />.</entry> + </row> + <row> + <entry><constant>V4L2_BUF_TYPE_SLICED_VBI_OUTPUT</constant></entry> + <entry>7</entry> + <entry>Buffer of a sliced VBI output stream, see <xref + linkend="sliced" />.</entry> + </row> + <row> + <entry><constant>V4L2_BUF_TYPE_VIDEO_OUTPUT_OVERLAY</constant></entry> + <entry>8</entry> + <entry>Buffer for video output overlay (OSD), see <xref + linkend="osd" />. Status: <link +linkend="experimental">Experimental</link>.</entry> + </row> + <row> + <entry><constant>V4L2_BUF_TYPE_PRIVATE</constant></entry> + <entry>0x80</entry> + <entry>This and higher values are reserved for custom +(driver defined) buffer types.</entry> + </row> + </tbody> + </tgroup> + </table> + + <table frame="none" pgwide="1" id="buffer-flags"> + <title>Buffer Flags</title> + <tgroup cols="3"> + &cs-def; + <tbody valign="top"> + <row> + <entry><constant>V4L2_BUF_FLAG_MAPPED</constant></entry> + <entry>0x0001</entry> + <entry>The buffer resides in device memory and has been mapped +into the application's address space, see <xref linkend="mmap" /> for details. +Drivers set or clear this flag when the +<link linkend="vidioc-querybuf">VIDIOC_QUERYBUF</link>, <link + linkend="vidioc-qbuf">VIDIOC_QBUF</link> or <link + linkend="vidioc-qbuf">VIDIOC_DQBUF</link> ioctl is called. Set by the driver.</entry> + </row> + <row> + <entry><constant>V4L2_BUF_FLAG_QUEUED</constant></entry> + <entry>0x0002</entry> + <entry>Internally drivers maintain two buffer queues, an +incoming and outgoing queue. When this flag is set, the buffer is +currently on the incoming queue. It automatically moves to the +outgoing queue after the buffer has been filled (capture devices) or +displayed (output devices). Drivers set or clear this flag when the +<constant>VIDIOC_QUERYBUF</constant> ioctl is called. After +(successful) calling the <constant>VIDIOC_QBUF </constant>ioctl it is +always set and after <constant>VIDIOC_DQBUF</constant> always +cleared.</entry> + </row> + <row> + <entry><constant>V4L2_BUF_FLAG_DONE</constant></entry> + <entry>0x0004</entry> + <entry>When this flag is set, the buffer is currently on +the outgoing queue, ready to be dequeued from the driver. Drivers set +or clear this flag when the <constant>VIDIOC_QUERYBUF</constant> ioctl +is called. After calling the <constant>VIDIOC_QBUF</constant> or +<constant>VIDIOC_DQBUF</constant> it is always cleared. Of course a +buffer cannot be on both queues at the same time, the +<constant>V4L2_BUF_FLAG_QUEUED</constant> and +<constant>V4L2_BUF_FLAG_DONE</constant> flag are mutually exclusive. +They can be both cleared however, then the buffer is in "dequeued" +state, in the application domain to say so.</entry> + </row> + <row> + <entry><constant>V4L2_BUF_FLAG_KEYFRAME</constant></entry> + <entry>0x0008</entry> + <entry>Drivers set or clear this flag when calling the +<constant>VIDIOC_DQBUF</constant> ioctl. It may be set by video +capture devices when the buffer contains a compressed image which is a +key frame (or field), &ie; can be decompressed on its own.</entry> + </row> + <row> + <entry><constant>V4L2_BUF_FLAG_PFRAME</constant></entry> + <entry>0x0010</entry> + <entry>Similar to <constant>V4L2_BUF_FLAG_KEYFRAME</constant> +this flags predicted frames or fields which contain only differences to a +previous key frame.</entry> + </row> + <row> + <entry><constant>V4L2_BUF_FLAG_BFRAME</constant></entry> + <entry>0x0020</entry> + <entry>Similar to <constant>V4L2_BUF_FLAG_PFRAME</constant> + this is a bidirectional predicted frame or field. [ooc tbd]</entry> + </row> + <row> + <entry><constant>V4L2_BUF_FLAG_TIMECODE</constant></entry> + <entry>0x0100</entry> + <entry>The <structfield>timecode</structfield> field is valid. +Drivers set or clear this flag when the <constant>VIDIOC_DQBUF</constant> +ioctl is called.</entry> + </row> + <row> + <entry><constant>V4L2_BUF_FLAG_INPUT</constant></entry> + <entry>0x0200</entry> + <entry>The <structfield>input</structfield> field is valid. +Applications set or clear this flag before calling the +<constant>VIDIOC_QBUF</constant> ioctl.</entry> + </row> + </tbody> + </tgroup> + </table> + + <table pgwide="1" frame="none" id="v4l2-memory"> + <title>enum v4l2_memory</title> + <tgroup cols="3"> + &cs-def; + <tbody valign="top"> + <row> + <entry><constant>V4L2_MEMORY_MMAP</constant></entry> + <entry>1</entry> + <entry>The buffer is used for <link linkend="mmap">memory +mapping</link> I/O.</entry> + </row> + <row> + <entry><constant>V4L2_MEMORY_USERPTR</constant></entry> + <entry>2</entry> + <entry>The buffer is used for <link linkend="userp">user +pointer</link> I/O.</entry> + </row> + <row> + <entry><constant>V4L2_MEMORY_OVERLAY</constant></entry> + <entry>3</entry> + <entry>[to do]</entry> + </row> + </tbody> + </tgroup> + </table> + + <section> + <title>Timecodes</title> + + <para>The <structname>v4l2_timecode</structname> structure is +designed to hold a <xref linkend="smpte12m" /> or similar timecode. +(struct <structname>timeval</structname> timestamps are stored in +&v4l2-buffer; field <structfield>timestamp</structfield>.)</para> + + <table frame="none" pgwide="1" id="v4l2-timecode"> + <title>struct <structname>v4l2_timecode</structname></title> + <tgroup cols="3"> + &cs-str; + <tbody valign="top"> + <row> + <entry>__u32</entry> + <entry><structfield>type</structfield></entry> + <entry>Frame rate the timecodes are based on, see <xref + linkend="timecode-type" />.</entry> + </row> + <row> + <entry>__u32</entry> + <entry><structfield>flags</structfield></entry> + <entry>Timecode flags, see <xref linkend="timecode-flags" />.</entry> + </row> + <row> + <entry>__u8</entry> + <entry><structfield>frames</structfield></entry> + <entry>Frame count, 0 ... 23/24/29/49/59, depending on the + type of timecode.</entry> + </row> + <row> + <entry>__u8</entry> + <entry><structfield>seconds</structfield></entry> + <entry>Seconds count, 0 ... 59. This is a binary, not BCD number.</entry> + </row> + <row> + <entry>__u8</entry> + <entry><structfield>minutes</structfield></entry> + <entry>Minutes count, 0 ... 59. This is a binary, not BCD number.</entry> + </row> + <row> + <entry>__u8</entry> + <entry><structfield>hours</structfield></entry> + <entry>Hours count, 0 ... 29. This is a binary, not BCD number.</entry> + </row> + <row> + <entry>__u8</entry> + <entry><structfield>userbits</structfield>[4]</entry> + <entry>The "user group" bits from the timecode.</entry> + </row> + </tbody> + </tgroup> + </table> + + <table frame="none" pgwide="1" id="timecode-type"> + <title>Timecode Types</title> + <tgroup cols="3"> + &cs-def; + <tbody valign="top"> + <row> + <entry><constant>V4L2_TC_TYPE_24FPS</constant></entry> + <entry>1</entry> + <entry>24 frames per second, i. e. film.</entry> + </row> + <row> + <entry><constant>V4L2_TC_TYPE_25FPS</constant></entry> + <entry>2</entry> + <entry>25 frames per second, &ie; PAL or SECAM video.</entry> + </row> + <row> + <entry><constant>V4L2_TC_TYPE_30FPS</constant></entry> + <entry>3</entry> + <entry>30 frames per second, &ie; NTSC video.</entry> + </row> + <row> + <entry><constant>V4L2_TC_TYPE_50FPS</constant></entry> + <entry>4</entry> + <entry></entry> + </row> + <row> + <entry><constant>V4L2_TC_TYPE_60FPS</constant></entry> + <entry>5</entry> + <entry></entry> + </row> + </tbody> + </tgroup> + </table> + + <table frame="none" pgwide="1" id="timecode-flags"> + <title>Timecode Flags</title> + <tgroup cols="3"> + &cs-def; + <tbody valign="top"> + <row> + <entry><constant>V4L2_TC_FLAG_DROPFRAME</constant></entry> + <entry>0x0001</entry> + <entry>Indicates "drop frame" semantics for counting frames +in 29.97 fps material. When set, frame numbers 0 and 1 at the start of +each minute, except minutes 0, 10, 20, 30, 40, 50 are omitted from the +count.</entry> + </row> + <row> + <entry><constant>V4L2_TC_FLAG_COLORFRAME</constant></entry> + <entry>0x0002</entry> + <entry>The "color frame" flag.</entry> + </row> + <row> + <entry><constant>V4L2_TC_USERBITS_field</constant></entry> + <entry>0x000C</entry> + <entry>Field mask for the "binary group flags".</entry> + </row> + <row> + <entry><constant>V4L2_TC_USERBITS_USERDEFINED</constant></entry> + <entry>0x0000</entry> + <entry>Unspecified format.</entry> + </row> + <row> + <entry><constant>V4L2_TC_USERBITS_8BITCHARS</constant></entry> + <entry>0x0008</entry> + <entry>8-bit ISO characters.</entry> + </row> + </tbody> + </tgroup> + </table> + </section> + </section> + + <section id="field-order"> + <title>Field Order</title> + + <para>We have to distinguish between progressive and interlaced +video. Progressive video transmits all lines of a video image +sequentially. Interlaced video divides an image into two fields, +containing only the odd and even lines of the image, respectively. +Alternating the so called odd and even field are transmitted, and due +to a small delay between fields a cathode ray TV displays the lines +interleaved, yielding the original frame. This curious technique was +invented because at refresh rates similar to film the image would +fade out too quickly. Transmitting fields reduces the flicker without +the necessity of doubling the frame rate and with it the bandwidth +required for each channel.</para> + + <para>It is important to understand a video camera does not expose +one frame at a time, merely transmitting the frames separated into +fields. The fields are in fact captured at two different instances in +time. An object on screen may well move between one field and the +next. For applications analysing motion it is of paramount importance +to recognize which field of a frame is older, the <emphasis>temporal +order</emphasis>.</para> + + <para>When the driver provides or accepts images field by field +rather than interleaved, it is also important applications understand +how the fields combine to frames. We distinguish between top and +bottom fields, the <emphasis>spatial order</emphasis>: The first line +of the top field is the first line of an interlaced frame, the first +line of the bottom field is the second line of that frame.</para> + + <para>However because fields were captured one after the other, +arguing whether a frame commences with the top or bottom field is +pointless. Any two successive top and bottom, or bottom and top fields +yield a valid frame. Only when the source was progressive to begin +with, ⪚ when transferring film to video, two fields may come from +the same frame, creating a natural order.</para> + + <para>Counter to intuition the top field is not necessarily the +older field. Whether the older field contains the top or bottom lines +is a convention determined by the video standard. Hence the +distinction between temporal and spatial order of fields. The diagrams +below should make this clearer.</para> + + <para>All video capture and output devices must report the current +field order. Some drivers may permit the selection of a different +order, to this end applications initialize the +<structfield>field</structfield> field of &v4l2-pix-format; before +calling the &VIDIOC-S-FMT; ioctl. If this is not desired it should +have the value <constant>V4L2_FIELD_ANY</constant> (0).</para> + + <table frame="none" pgwide="1" id="v4l2-field"> + <title>enum v4l2_field</title> + <tgroup cols="3"> + &cs-def; + <tbody valign="top"> + <row> + <entry><constant>V4L2_FIELD_ANY</constant></entry> + <entry>0</entry> + <entry>Applications request this field order when any +one of the <constant>V4L2_FIELD_NONE</constant>, +<constant>V4L2_FIELD_TOP</constant>, +<constant>V4L2_FIELD_BOTTOM</constant>, or +<constant>V4L2_FIELD_INTERLACED</constant> formats is acceptable. +Drivers choose depending on hardware capabilities or e. g. the +requested image size, and return the actual field order. &v4l2-buffer; +<structfield>field</structfield> can never be +<constant>V4L2_FIELD_ANY</constant>.</entry> + </row> + <row> + <entry><constant>V4L2_FIELD_NONE</constant></entry> + <entry>1</entry> + <entry>Images are in progressive format, not interlaced. +The driver may also indicate this order when it cannot distinguish +between <constant>V4L2_FIELD_TOP</constant> and +<constant>V4L2_FIELD_BOTTOM</constant>.</entry> + </row> + <row> + <entry><constant>V4L2_FIELD_TOP</constant></entry> + <entry>2</entry> + <entry>Images consist of the top field only.</entry> + </row> + <row> + <entry><constant>V4L2_FIELD_BOTTOM</constant></entry> + <entry>3</entry> + <entry>Images consist of the bottom field only. +Applications may wish to prevent a device from capturing interlaced +images because they will have "comb" or "feathering" artefacts around +moving objects.</entry> + </row> + <row> + <entry><constant>V4L2_FIELD_INTERLACED</constant></entry> + <entry>4</entry> + <entry>Images contain both fields, interleaved line by +line. The temporal order of the fields (whether the top or bottom +field is first transmitted) depends on the current video standard. +M/NTSC transmits the bottom field first, all other standards the top +field first.</entry> + </row> + <row> + <entry><constant>V4L2_FIELD_SEQ_TB</constant></entry> + <entry>5</entry> + <entry>Images contain both fields, the top field lines +are stored first in memory, immediately followed by the bottom field +lines. Fields are always stored in temporal order, the older one first +in memory. Image sizes refer to the frame, not fields.</entry> + </row> + <row> + <entry><constant>V4L2_FIELD_SEQ_BT</constant></entry> + <entry>6</entry> + <entry>Images contain both fields, the bottom field +lines are stored first in memory, immediately followed by the top +field lines. Fields are always stored in temporal order, the older one +first in memory. Image sizes refer to the frame, not fields.</entry> + </row> + <row> + <entry><constant>V4L2_FIELD_ALTERNATE</constant></entry> + <entry>7</entry> + <entry>The two fields of a frame are passed in separate +buffers, in temporal order, &ie; the older one first. To indicate the field +parity (whether the current field is a top or bottom field) the driver +or application, depending on data direction, must set &v4l2-buffer; +<structfield>field</structfield> to +<constant>V4L2_FIELD_TOP</constant> or +<constant>V4L2_FIELD_BOTTOM</constant>. Any two successive fields pair +to build a frame. If fields are successive, without any dropped fields +between them (fields can drop individually), can be determined from +the &v4l2-buffer; <structfield>sequence</structfield> field. Image +sizes refer to the frame, not fields. This format cannot be selected +when using the read/write I/O method.<!-- Where it's indistinguishable +from V4L2_FIELD_SEQ_*. --></entry> + </row> + <row> + <entry><constant>V4L2_FIELD_INTERLACED_TB</constant></entry> + <entry>8</entry> + <entry>Images contain both fields, interleaved line by +line, top field first. The top field is transmitted first.</entry> + </row> + <row> + <entry><constant>V4L2_FIELD_INTERLACED_BT</constant></entry> + <entry>9</entry> + <entry>Images contain both fields, interleaved line by +line, top field first. The bottom field is transmitted first.</entry> + </row> + </tbody> + </tgroup> + </table> + + <figure id="fieldseq-tb"> + <title>Field Order, Top Field First Transmitted</title> + <mediaobject> + <imageobject> + <imagedata fileref="fieldseq_tb.pdf" format="PS" /> + </imageobject> + <imageobject> + <imagedata fileref="fieldseq_tb.gif" format="GIF" /> + </imageobject> + </mediaobject> + </figure> + + <figure id="fieldseq-bt"> + <title>Field Order, Bottom Field First Transmitted</title> + <mediaobject> + <imageobject> + <imagedata fileref="fieldseq_bt.pdf" format="PS" /> + </imageobject> + <imageobject> + <imagedata fileref="fieldseq_bt.gif" format="GIF" /> + </imageobject> + </mediaobject> + </figure> + </section> + + <!-- +Local Variables: +mode: sgml +sgml-parent-document: "v4l2.sgml" +indent-tabs-mode: nil +End: + --> |
