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- <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>
-&gt; vidctrl /dev/video --input=0 --format=YUYV --size=352x288
-&gt; 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. In the single-planar API case, 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. When using the multi-planar API,
-struct &v4l2-buffer; contains an array of &v4l2-plane; structures, each
-containing its own <structfield>m.offset</structfield> and
-<structfield>length</structfield>. When using the multi-planar API, every
-plane of every buffer has to be mapped separately, so the number of
-calls to &func-mmap; should be equal to number of buffers times number of
-planes in each buffer. 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 in the single-planar API</title>
- <programlisting>
-&v4l2-requestbuffers; reqbuf;
-struct {
- void *start;
- size_t length;
-} *buffers;
-unsigned int i;
-
-memset(&amp;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;, &amp;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 &lt; 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 &lt; reqbuf.count; i++) {
- &v4l2-buffer; buffer;
-
- memset(&amp;buffer, 0, sizeof(buffer));
- buffer.type = reqbuf.type;
- buffer.memory = V4L2_MEMORY_MMAP;
- buffer.index = i;
-
- if (-1 == ioctl (fd, &VIDIOC-QUERYBUF;, &amp;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 &lt; reqbuf.count; i++)
- munmap(buffers[i].start, buffers[i].length);
- </programlisting>
- </example>
-
- <example>
- <title>Mapping buffers in the multi-planar API</title>
- <programlisting>
-&v4l2-requestbuffers; reqbuf;
-/* Our current format uses 3 planes per buffer */
-#define FMT_NUM_PLANES = 3
-
-struct {
- void *start[FMT_NUM_PLANES];
- size_t length[FMT_NUM_PLANES];
-} *buffers;
-unsigned int i, j;
-
-memset(&amp;reqbuf, 0, sizeof(reqbuf));
-reqbuf.type = V4L2_BUF_TYPE_VIDEO_CAPTURE_MPLANE;
-reqbuf.memory = V4L2_MEMORY_MMAP;
-reqbuf.count = 20;
-
-if (ioctl(fd, &VIDIOC-REQBUFS;, &amp;reqbuf) &lt; 0) {
- 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 &lt; 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 &lt; reqbuf.count; i++) {
- &v4l2-buffer; buffer;
- &v4l2-plane; planes[FMT_NUM_PLANES];
-
- memset(&amp;buffer, 0, sizeof(buffer));
- buffer.type = reqbuf.type;
- buffer.memory = V4L2_MEMORY_MMAP;
- buffer.index = i;
- /* length in struct v4l2_buffer in multi-planar API stores the size
- * of planes array. */
- buffer.length = FMT_NUM_PLANES;
- buffer.m.planes = planes;
-
- if (ioctl(fd, &VIDIOC-QUERYBUF;, &amp;buffer) &lt; 0) {
- perror("VIDIOC_QUERYBUF");
- exit(EXIT_FAILURE);
- }
-
- /* Every plane has to be mapped separately */
- for (j = 0; j &lt; FMT_NUM_PLANES; j++) {
- buffers[i].length[j] = buffer.m.planes[j].length; /* remember for munmap() */
-
- buffers[i].start[j] = mmap(NULL, buffer.m.planes[j].length,
- PROT_READ | PROT_WRITE, /* recommended */
- MAP_SHARED, /* recommended */
- fd, buffer.m.planes[j].m.offset);
-
- if (MAP_FAILED == buffers[i].start[j]) {
- /* If you do not exit here you should unmap() and free()
- the buffers and planes mapped so far. */
- perror("mmap");
- exit(EXIT_FAILURE);
- }
- }
-}
-
-/* Cleanup. */
-
-for (i = 0; i &lt; reqbuf.count; i++)
- for (j = 0; j &lt; FMT_NUM_PLANES; j++)
- munmap(buffers[i].start[j], buffers[i].length[j]);
- </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 (planes) 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; (or in &v4l2-plane; in the multi-planar API case).
-The driver must be switched into user pointer I/O mode by calling the
-&VIDIOC-REQBUFS; with the desired buffer type. No buffers (planes) are allocated
-beforehand, 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 (&amp;reqbuf, 0, sizeof (reqbuf));
-reqbuf.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
-reqbuf.memory = V4L2_MEMORY_USERPTR;
-
-if (ioctl (fd, &VIDIOC-REQBUFS;, &amp;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 (plane) 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. In the multi-planar API, the
-data is held in planes, while the buffer structure acts as a container
-for the planes. Only pointers to buffers (planes) 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.
-In the multi-planar API, some plane-specific members of struct
-<structname>v4l2_buffer</structname>, such as pointers and sizes for each
-plane, are stored in struct <structname>v4l2_plane</structname> instead.
-In that case, struct <structname>v4l2_buffer</structname> contains an array of
-plane structures.</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>__u32</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. See <xref
-linkend="v4l2-buf-type" /></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>__u32</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>__u32</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. See <xref linkend="v4l2-memory"
- /></entry>
- </row>
- <row>
- <entry>union</entry>
- <entry><structfield>m</structfield></entry>
- </row>
- <row>
- <entry></entry>
- <entry>__u32</entry>
- <entry><structfield>offset</structfield></entry>
- <entry>For the single-planar API and 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>For the single-planar API and 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></entry>
- <entry>struct v4l2_plane</entry>
- <entry><structfield>*planes</structfield></entry>
- <entry>When using the multi-planar API, contains a userspace pointer
- to an array of &v4l2-plane;. The size of the array should be put
- in the <structfield>length</structfield> field of this
- <structname>v4l2_buffer</structname> structure.</entry>
- </row>
- <row>
- <entry>__u32</entry>
- <entry><structfield>length</structfield></entry>
- <entry></entry>
- <entry>Size of the buffer (not the payload) in bytes for the
- single-planar API. For the multi-planar API should contain the
- number of elements in the <structfield>planes</structfield> array.
- </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. Applications
-should set this to 0.</entry>
- </row>
- </tbody>
- </tgroup>
- </table>
-
- <table frame="none" pgwide="1" id="v4l2-plane">
- <title>struct <structname>v4l2_plane</structname></title>
- <tgroup cols="4">
- &cs-ustr;
- <tbody valign="top">
- <row>
- <entry>__u32</entry>
- <entry><structfield>bytesused</structfield></entry>
- <entry></entry>
- <entry>The number of bytes occupied by data in the plane
- (its payload).</entry>
- </row>
- <row>
- <entry>__u32</entry>
- <entry><structfield>length</structfield></entry>
- <entry></entry>
- <entry>Size in bytes of the plane (not its payload).</entry>
- </row>
- <row>
- <entry>union</entry>
- <entry><structfield>m</structfield></entry>
- <entry></entry>
- <entry></entry>
- </row>
- <row>
- <entry></entry>
- <entry>__u32</entry>
- <entry><structfield>mem_offset</structfield></entry>
- <entry>When the memory type in the containing &v4l2-buffer; is
- <constant>V4L2_MEMORY_MMAP</constant>, this is the value that
- should be passed to &func-mmap;, similar to the
- <structfield>offset</structfield> field in &v4l2-buffer;.</entry>
- </row>
- <row>
- <entry></entry>
- <entry>__unsigned long</entry>
- <entry><structfield>userptr</structfield></entry>
- <entry>When the memory type in the containing &v4l2-buffer; is
- <constant>V4L2_MEMORY_USERPTR</constant>, this is a userspace
- pointer to the memory allocated for this plane by an application.
- </entry>
- </row>
- <row>
- <entry>__u32</entry>
- <entry><structfield>data_offset</structfield></entry>
- <entry></entry>
- <entry>Offset in bytes to video data in the plane, if applicable.
- </entry>
- </row>
- <row>
- <entry>__u32</entry>
- <entry><structfield>reserved[11]</structfield></entry>
- <entry></entry>
- <entry>Reserved for future use. Should be zeroed by an
- application.</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 single-planar video capture stream, see <xref
- linkend="capture" />.</entry>
- </row>
- <row>
- <entry><constant>V4L2_BUF_TYPE_VIDEO_CAPTURE_MPLANE</constant>
- </entry>
- <entry>9</entry>
- <entry>Buffer of a multi-planar 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 single-planar video output stream, see <xref
- linkend="output" />.</entry>
- </row>
- <row>
- <entry><constant>V4L2_BUF_TYPE_VIDEO_OUTPUT_MPLANE</constant>
- </entry>
- <entry>10</entry>
- <entry>Buffer of a multi-planar 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_ERROR</constant></entry>
- <entry>0x0040</entry>
- <entry>When this flag is set, the buffer has been dequeued
- successfully, although the data might have been corrupted.
- This is recoverable, streaming may continue as normal and
- the buffer may be reused normally.
- Drivers set this flag when the <constant>VIDIOC_DQBUF</constant>
- ioctl is called.</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>
- <row>
- <entry><constant>V4L2_BUF_FLAG_PREPARED</constant></entry>
- <entry>0x0400</entry>
- <entry>The buffer has been prepared for I/O and can be queued by the
-application. Drivers set or clear this flag when the
-<link linkend="vidioc-querybuf">VIDIOC_QUERYBUF</link>, <link
- linkend="vidioc-qbuf">VIDIOC_PREPARE_BUF</link>, <link
- linkend="vidioc-qbuf">VIDIOC_QBUF</link> or <link
- linkend="vidioc-qbuf">VIDIOC_DQBUF</link> ioctl is called.</entry>
- </row>
- <row>
- <entry><constant>V4L2_BUF_FLAG_NO_CACHE_INVALIDATE</constant></entry>
- <entry>0x0400</entry>
- <entry>Caches do not have to be invalidated for this buffer.
-Typically applications shall use this flag if the data captured in the buffer
-is not going to be touched by the CPU, instead the buffer will, probably, be
-passed on to a DMA-capable hardware unit for further processing or output.
-</entry>
- </row>
- <row>
- <entry><constant>V4L2_BUF_FLAG_NO_CACHE_CLEAN</constant></entry>
- <entry>0x0800</entry>
- <entry>Caches do not have to be cleaned for this buffer.
-Typically applications shall use this flag for output buffers if the data
-in this buffer has not been created by the CPU but by some DMA-capable unit,
-in which case caches have not been used.</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.&nbsp;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 (aka odd) and
-bottom (aka even) 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, &eg; 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.&nbsp;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 (aka odd) field only.</entry>
- </row>
- <row>
- <entry><constant>V4L2_FIELD_BOTTOM</constant></entry>
- <entry>3</entry>
- <entry>Images consist of the bottom (aka even) 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>