3.5. Buffers

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 v4l2_buffer, argument to the VIDIOC_QUERYBUF, VIDIOC_QBUF and VIDIOC_DQBUF ioctl.

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 Section 1.7.

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.

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. [1]

Table 3-1. struct v4l2_buffer

__u32index Number of the buffer, set by the application. This field is only used for memory mapping I/O and can range from zero to the number of buffers allocated with the VIDIOC_REQBUFS ioctl (struct v4l2_requestbuffers count) minus one.
enum v4l2_buf_typetype Type of the buffer, same as struct v4l2_format type or struct v4l2_requestbuffers type, set by the application.
__u32bytesused 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 type refers to an input stream, applications when an output stream.
__u32flags Flags set by the application or driver, see Table 3-3.
enum v4l2_fieldfield Indicates the field order of the image in the buffer, see Table 3-8. This field is not used when the buffer contains VBI data. Drivers must set it when type refers to an input stream, applications when an output stream.
struct timevaltimestamp 

For input streams this is the system time (as returned by the gettimeofday() 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 timestamp field. This permits applications to monitor the drift between the video and system clock.

struct v4l2_timecodetimecode When type is V4L2_BUF_TYPE_VIDEO_CAPTURE and the V4L2_BUF_FLAG_TIMECODE flag is set in flags, this structure contains a frame timecode. In V4L2_FIELD_ALTERNATE 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 timestamp and sequence fields.
__u32sequence Set by the driver, counting the frames in the sequence.

In V4L2_FIELD_ALTERNATE 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.

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 Section 1.7.

enum v4l2_memorymemory This field must be set by applications and/or drivers in accordance with the selected I/O method.
 __u32offsetWhen memory is V4L2_MEMORY_MMAP 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 mmap() function not useful for applications. See Section 3.2 for details.
 unsigned longuserptrWhen memory is V4L2_MEMORY_USERPTR this is a pointer to the buffer (casted to unsigned long type) in virtual memory, set by the application. See Section 3.3 for details.
__u32length Size of the buffer (not the payload) in bytes.
__u32input 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 V4L2_BUF_FLAG_INPUT flag, and this field to the number of a video input as in struct v4l2_input field index.
__u32reserved A place holder for future extensions and custom (driver defined) buffer types V4L2_BUF_TYPE_PRIVATE and higher.

Table 3-2. enum v4l2_buf_type

V4L2_BUF_TYPE_VIDEO_CAPTURE1Buffer of a video capture stream, see Section 4.1.
V4L2_BUF_TYPE_VIDEO_OUTPUT2Buffer of a video output stream, see Section 4.3.
V4L2_BUF_TYPE_VIDEO_OVERLAY3Buffer for video overlay, see Section 4.2.
V4L2_BUF_TYPE_VBI_CAPTURE4Buffer of a raw VBI capture stream, see Section 4.7.
V4L2_BUF_TYPE_VBI_OUTPUT5Buffer of a raw VBI output stream, see Section 4.7.
V4L2_BUF_TYPE_SLICED_VBI_CAPTURE6Buffer of a sliced VBI capture stream, see Section 4.8.
V4L2_BUF_TYPE_SLICED_VBI_OUTPUT7Buffer of a sliced VBI output stream, see Section 4.8.
V4L2_BUF_TYPE_VIDEO_OUTPUT_OVERLAY8Buffer for video output overlay (OSD), see Section 4.4. Status: Experimental.
V4L2_BUF_TYPE_PRIVATE0x80This and higher values are reserved for custom (driver defined) buffer types.

Table 3-3. Buffer Flags

V4L2_BUF_FLAG_MAPPED0x0001The buffer resides in device memory and has been mapped into the application's address space, see Section 3.2 for details. Drivers set or clear this flag when the VIDIOC_QUERYBUF, VIDIOC_QBUF or VIDIOC_DQBUF ioctl is called. Set by the driver.
V4L2_BUF_FLAG_QUEUED0x0002Internally 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 VIDIOC_QUERYBUF ioctl is called. After (successful) calling the VIDIOC_QBUF ioctl it is always set and after VIDIOC_DQBUF always cleared.
V4L2_BUF_FLAG_DONE0x0004When 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 VIDIOC_QUERYBUF ioctl is called. After calling the VIDIOC_QBUF or VIDIOC_DQBUF it is always cleared. Of course a buffer cannot be on both queues at the same time, the V4L2_BUF_FLAG_QUEUED and V4L2_BUF_FLAG_DONE flag are mutually exclusive. They can be both cleared however, then the buffer is in "dequeued" state, in the application domain to say so.
V4L2_BUF_FLAG_KEYFRAME0x0008Drivers set or clear this flag when calling the VIDIOC_DQBUF ioctl. It may be set by video capture devices when the buffer contains a compressed image which is a key frame (or field), i. e. can be decompressed on its own.
V4L2_BUF_FLAG_PFRAME0x0010Similar to V4L2_BUF_FLAG_KEYFRAME this flags predicted frames or fields which contain only differences to a previous key frame.
V4L2_BUF_FLAG_BFRAME0x0020Similar to V4L2_BUF_FLAG_PFRAME this is a bidirectional predicted frame or field. [ooc tbd]
V4L2_BUF_FLAG_TIMECODE0x0100The timecode field is valid. Drivers set or clear this flag when the VIDIOC_DQBUF ioctl is called.
V4L2_BUF_FLAG_INPUT0x0200The input field is valid. Applications set or clear this flag before calling the VIDIOC_QBUF ioctl.

Table 3-4. enum v4l2_memory

V4L2_MEMORY_MMAP1The buffer is used for memory mapping I/O.
V4L2_MEMORY_USERPTR2The buffer is used for user pointer I/O.

3.5.1. Timecodes

The v4l2_timecode structure is designed to hold a SMPTE 12M or similar timecode. (struct timeval timestamps are stored in struct v4l2_buffer field timestamp.)

Table 3-5. struct v4l2_timecode

__u32typeFrame rate the timecodes are based on, see Table 3-6.
__u32flagsTimecode flags, see Table 3-7.
__u8framesFrame count, 0 ... 23/24/29/49/59, depending on the type of timecode.
__u8secondsSeconds count, 0 ... 59. This is a binary, not BCD number.
__u8minutesMinutes count, 0 ... 59. This is a binary, not BCD number.
__u8hoursHours count, 0 ... 29. This is a binary, not BCD number.
__u8userbits[4]The "user group" bits from the timecode.

Table 3-6. Timecode Types

V4L2_TC_TYPE_24FPS124 frames per second, i. e. film.
V4L2_TC_TYPE_25FPS225 frames per second, i. e. PAL or SECAM video.
V4L2_TC_TYPE_30FPS330 frames per second, i. e. NTSC video.

Table 3-7. Timecode Flags

V4L2_TC_FLAG_DROPFRAME0x0001Indicates "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.
V4L2_TC_FLAG_COLORFRAME0x0002The "color frame" flag.
V4L2_TC_USERBITS_field0x000CField mask for the "binary group flags".
V4L2_TC_USERBITS_USERDEFINED0x0000Unspecified format.
V4L2_TC_USERBITS_8BITCHARS0x00088-bit ISO characters.



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.