3.5. Buffers

3.5. Buffers
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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, “Video Standards”.

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. ^[19]

Table 3.1. struct v4l2_buffer

__u32	`index`		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_type	`type`		Type of the buffer, same as struct v4l2_format `type` or struct v4l2_requestbuffers `type`, set by the application.
__u32	`bytesused`		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.
__u32	`flags`		Flags set by the application or driver, see Table 3.3, “Buffer Flags”.
enum v4l2_field	`field`		Indicates the field order of the image in the buffer, see Table 3.8, “enum v4l2_field”. 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 timeval	`timestamp`		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_timecode	`timecode`		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.
__u32	`sequence`		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, “Video Standards”.
enum v4l2_memory	`memory`		This field must be set by applications and/or drivers in accordance with the selected I/O method.
union	`m`
	__u32	`offset`	When `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, “Streaming I/O (Memory Mapping)” for details.
	unsigned long	`userptr`	When `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, “Streaming I/O (User Pointers)” for details.
__u32	`length`		Size of the buffer (not the payload) in bytes.
__u32	`input`		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`.
__u32	`reserved`		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_CAPTURE`	1	Buffer of a video capture stream, see Section 4.1, “Video Capture Interface”.
`V4L2_BUF_TYPE_VIDEO_OUTPUT`	2	Buffer of a video output stream, see Section 4.3, “Video Output Interface”.
`V4L2_BUF_TYPE_VIDEO_OVERLAY`	3	Buffer for video overlay, see Section 4.2, “Video Overlay Interface”.
`V4L2_BUF_TYPE_VBI_CAPTURE`	4	Buffer of a raw VBI capture stream, see Section 4.7, “Raw VBI Data Interface”.
`V4L2_BUF_TYPE_VBI_OUTPUT`	5	Buffer of a raw VBI output stream, see Section 4.7, “Raw VBI Data Interface”.
`V4L2_BUF_TYPE_SLICED_VBI_CAPTURE`	6	Buffer of a sliced VBI capture stream, see Section 4.8, “Sliced VBI Data Interface”.
`V4L2_BUF_TYPE_SLICED_VBI_OUTPUT`	7	Buffer of a sliced VBI output stream, see Section 4.8, “Sliced VBI Data Interface”.
`V4L2_BUF_TYPE_VIDEO_OUTPUT_OVERLAY`	8	Buffer for video output overlay (OSD), see Section 4.4, “Video Output Overlay Interface”. Status: Experimental.
`V4L2_BUF_TYPE_PRIVATE`	0x80	This and higher values are reserved for custom (driver defined) buffer types.

Table 3.3. Buffer Flags

`V4L2_BUF_FLAG_MAPPED`	0x0001	The buffer resides in device memory and has been mapped into the application's address space, see Section 3.2, “Streaming I/O (Memory Mapping)” 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_QUEUED`	0x0002	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 `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_DONE`	0x0004	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 `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_KEYFRAME`	0x0008	Drivers 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_PFRAME`	0x0010	Similar to `V4L2_BUF_FLAG_KEYFRAME` this flags predicted frames or fields which contain only differences to a previous key frame.
`V4L2_BUF_FLAG_BFRAME`	0x0020	Similar to `V4L2_BUF_FLAG_PFRAME` this is a bidirectional predicted frame or field. [ooc tbd]
`V4L2_BUF_FLAG_TIMECODE`	0x0100	The `timecode` field is valid. Drivers set or clear this flag when the `VIDIOC_DQBUF` ioctl is called.
`V4L2_BUF_FLAG_INPUT`	0x0200	The `input` field is valid. Applications set or clear this flag before calling the `VIDIOC_QBUF` ioctl.

Table 3.4. enum v4l2_memory

`V4L2_MEMORY_MMAP`	1	The buffer is used for memory mapping I/O.
`V4L2_MEMORY_USERPTR`	2	The buffer is used for user pointer I/O.
`V4L2_MEMORY_OVERLAY`	3	[to do]

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

__u32	`type`	Frame rate the timecodes are based on, see Table 3.6, “Timecode Types”.
__u32	`flags`	Timecode flags, see Table 3.7, “Timecode Flags”.
__u8	`frames`	Frame count, 0 ... 23/24/29/49/59, depending on the type of timecode.
__u8	`seconds`	Seconds count, 0 ... 59. This is a binary, not BCD number.
__u8	`minutes`	Minutes count, 0 ... 59. This is a binary, not BCD number.
__u8	`hours`	Hours count, 0 ... 29. This is a binary, not BCD number.
__u8	`userbits`[4]	The "user group" bits from the timecode.

Table 3.6. Timecode Types

`V4L2_TC_TYPE_24FPS`	1	24 frames per second, i. e. film.
`V4L2_TC_TYPE_25FPS`	2	25 frames per second, i. e. PAL or SECAM video.
`V4L2_TC_TYPE_30FPS`	3	30 frames per second, i. e. NTSC video.
`V4L2_TC_TYPE_50FPS`	4
`V4L2_TC_TYPE_60FPS`	5

Table 3.7. Timecode Flags

`V4L2_TC_FLAG_DROPFRAME`	0x0001	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.
`V4L2_TC_FLAG_COLORFRAME`	0x0002	The "color frame" flag.
`V4L2_TC_USERBITS_field`	0x000C	Field mask for the "binary group flags".
`V4L2_TC_USERBITS_USERDEFINED`	0x0000	Unspecified format.
`V4L2_TC_USERBITS_8BITCHARS`	0x0008	8-bit ISO characters.

^[19]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.

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