1.. Permission is granted to copy, distribute and/or modify this
2.. document under the terms of the GNU Free Documentation License,
3.. Version 1.1 or any later version published by the Free Software
4.. Foundation, with no Invariant Sections, no Front-Cover Texts
5.. and no Back-Cover Texts. A copy of the license is included at
6.. Documentation/userspace-api/media/fdl-appendix.rst.
7..
8.. TODO: replace it to GFDL-1.1-or-later WITH no-invariant-sections
9
10.. _format:
11
12************
13Data Formats
14************
15
16
17Data Format Negotiation
18=======================
19
20Different devices exchange different kinds of data with applications,
21for example video images, raw or sliced VBI data, RDS datagrams. Even
22within one kind many different formats are possible, in particular there is an
23abundance of image formats. Although drivers must provide a default and
24the selection persists across closing and reopening a device,
25applications should always negotiate a data format before engaging in
26data exchange. Negotiation means the application asks for a particular
27format and the driver selects and reports the best the hardware can do
28to satisfy the request. Of course applications can also just query the
29current selection.
30
31A single mechanism exists to negotiate all data formats using the
32aggregate struct :c:type:`v4l2_format` and the
33:ref:`VIDIOC_G_FMT <VIDIOC_G_FMT>` and
34:ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>` ioctls. Additionally the
35:ref:`VIDIOC_TRY_FMT <VIDIOC_G_FMT>` ioctl can be used to examine
36what the hardware *could* do, without actually selecting a new data
37format. The data formats supported by the V4L2 API are covered in the
38respective device section in :ref:`devices`. For a closer look at
39image formats see :ref:`pixfmt`.
40
41The :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>` ioctl is a major turning-point in the
42initialization sequence. Prior to this point multiple panel applications
43can access the same device concurrently to select the current input,
44change controls or modify other properties. The first :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>`
45assigns a logical stream (video data, VBI data etc.) exclusively to one
46file descriptor.
47
48Exclusive means no other application, more precisely no other file
49descriptor, can grab this stream or change device properties
50inconsistent with the negotiated parameters. A video standard change for
51example, when the new standard uses a different number of scan lines,
52can invalidate the selected image format. Therefore only the file
53descriptor owning the stream can make invalidating changes. Accordingly
54multiple file descriptors which grabbed different logical streams
55prevent each other from interfering with their settings. When for
56example video overlay is about to start or already in progress,
57simultaneous video capturing may be restricted to the same cropping and
58image size.
59
60When applications omit the :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>` ioctl its locking side
61effects are implied by the next step, the selection of an I/O method
62with the :ref:`VIDIOC_REQBUFS` ioctl or implicit
63with the first :ref:`read() <func-read>` or
64:ref:`write() <func-write>` call.
65
66Generally only one logical stream can be assigned to a file descriptor,
67the exception being drivers permitting simultaneous video capturing and
68overlay using the same file descriptor for compatibility with V4L and
69earlier versions of V4L2. Switching the logical stream or returning into
70"panel mode" is possible by closing and reopening the device. Drivers
71*may* support a switch using :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>`.
72
73All drivers exchanging data with applications must support the
74:ref:`VIDIOC_G_FMT <VIDIOC_G_FMT>` and :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>` ioctl. Implementation of the
75:ref:`VIDIOC_TRY_FMT <VIDIOC_G_FMT>` is highly recommended but optional.
76
77
78Image Format Enumeration
79========================
80
81Apart of the generic format negotiation functions a special ioctl to
82enumerate all image formats supported by video capture, overlay or
83output devices is available. [#f1]_
84
85The :ref:`VIDIOC_ENUM_FMT` ioctl must be supported
86by all drivers exchanging image data with applications.
87
88.. important::
89
90    Drivers are not supposed to convert image formats in kernel space.
91    They must enumerate only formats directly supported by the hardware.
92    If necessary driver writers should publish an example conversion
93    routine or library for integration into applications.
94
95.. [#f1]
96   Enumerating formats an application has no a-priori knowledge of
97   (otherwise it could explicitly ask for them and need not enumerate)
98   seems useless, but there are applications serving as proxy between
99   drivers and the actual video applications for which this is useful.
100