1 .. SPDX-License-Identifier: GFDL-1.1-no-invariants-or-later
2
3 .. _field-order:
4
5 ***********
6 Field Order
7 ***********
8
9 We have to distinguish between progressive and interlaced video.
10 Progressive video transmits all lines of a video image sequentially.
11 Interlaced video divides an image into two fields, containing only the
12 odd and even lines of the image, respectively. Alternating the so called
13 odd and even field are transmitted, and due to a small delay between
14 fields a cathode ray TV displays the lines interleaved, yielding the
15 original frame. This curious technique was invented because at refresh
16 rates similar to film the image would fade out too quickly. Transmitting
17 fields reduces the flicker without the necessity of doubling the frame
18 rate and with it the bandwidth required for each channel.
19
20 It is important to understand a video camera does not expose one frame
21 at a time, merely transmitting the frames separated into fields. The
22 fields are in fact captured at two different instances in time. An
23 object on screen may well move between one field and the next. For
24 applications analysing motion it is of paramount importance to recognize
25 which field of a frame is older, the *temporal order*.
26
27 When the driver provides or accepts images field by field rather than
28 interleaved, it is also important applications understand how the fields
29 combine to frames. We distinguish between top (aka odd) and bottom (aka
30 even) fields, the *spatial order*: The first line of the top field is
31 the first line of an interlaced frame, the first line of the bottom
32 field is the second line of that frame.
33
34 However because fields were captured one after the other, arguing
35 whether a frame commences with the top or bottom field is pointless. Any
36 two successive top and bottom, or bottom and top fields yield a valid
37 frame. Only when the source was progressive to begin with, e. g. when
38 transferring film to video, two fields may come from the same frame,
39 creating a natural order.
40
41 Counter to intuition the top field is not necessarily the older field.
42 Whether the older field contains the top or bottom lines is a convention
43 determined by the video standard. Hence the distinction between temporal
44 and spatial order of fields. The diagrams below should make this
45 clearer.
46
47 In V4L it is assumed that all video cameras transmit fields on the media
48 bus in the same order they were captured, so if the top field was
49 captured first (is the older field), the top field is also transmitted
50 first on the bus.
51
52 All video capture and output devices must report the current field
53 order. Some drivers may permit the selection of a different order, to
54 this end applications initialize the ``field`` field of struct
55 :c:type:`v4l2_pix_format` before calling the
56 :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>` ioctl. If this is not desired it
57 should have the value ``V4L2_FIELD_ANY`` (0).
58
59
60 enum v4l2_field
61 ===============
62
63 .. c:type:: v4l2_field
64
65 .. tabularcolumns:: |p{5.8cm}|p{0.6cm}|p{10.9cm}|
66
67 .. cssclass:: longtable
68
69 .. flat-table::
70 :header-rows: 0
71 :stub-columns: 0
72 :widths: 3 1 4
73
74 * - ``V4L2_FIELD_ANY``
75 - 0
76 - Applications request this field order when any field format
77 is acceptable. Drivers choose depending on hardware capabilities or
78 e.g. the requested image size, and return the actual field order.
79 Drivers must never return ``V4L2_FIELD_ANY``.
80 If multiple field orders are possible the
81 driver must choose one of the possible field orders during
82 :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>` or
83 :ref:`VIDIOC_TRY_FMT <VIDIOC_G_FMT>`. struct
84 :c:type:`v4l2_buffer` ``field`` can never be
85 ``V4L2_FIELD_ANY``.
86 * - ``V4L2_FIELD_NONE``
87 - 1
88 - Images are in progressive (frame-based) format, not interlaced
89 (field-based).
90 * - ``V4L2_FIELD_TOP``
91 - 2
92 - Images consist of the top (aka odd) field only.
93 * - ``V4L2_FIELD_BOTTOM``
94 - 3
95 - Images consist of the bottom (aka even) field only. Applications
96 may wish to prevent a device from capturing interlaced images
97 because they will have "comb" or "feathering" artefacts around
98 moving objects.
99 * - ``V4L2_FIELD_INTERLACED``
100 - 4
101 - Images contain both fields, interleaved line by line. The temporal
102 order of the fields (whether the top or bottom field is older)
103 depends on the current video standard. In M/NTSC the bottom
104 field is the older field. In all other standards the top field
105 is the older field.
106 * - ``V4L2_FIELD_SEQ_TB``
107 - 5
108 - Images contain both fields, the top field lines are stored first
109 in memory, immediately followed by the bottom field lines. Fields
110 are always stored in temporal order, the older one first in
111 memory. Image sizes refer to the frame, not fields.
112 * - ``V4L2_FIELD_SEQ_BT``
113 - 6
114 - Images contain both fields, the bottom field lines are stored
115 first in memory, immediately followed by the top field lines.
116 Fields are always stored in temporal order, the older one first in
117 memory. Image sizes refer to the frame, not fields.
118 * - ``V4L2_FIELD_ALTERNATE``
119 - 7
120 - The two fields of a frame are passed in separate buffers, in
121 temporal order, i. e. the older one first. To indicate the field
122 parity (whether the current field is a top or bottom field) the
123 driver or application, depending on data direction, must set
124 struct :c:type:`v4l2_buffer` ``field`` to
125 ``V4L2_FIELD_TOP`` or ``V4L2_FIELD_BOTTOM``. Any two successive
126 fields pair to build a frame. If fields are successive, without
127 any dropped fields between them (fields can drop individually),
128 can be determined from the struct
129 :c:type:`v4l2_buffer` ``sequence`` field. This
130 format cannot be selected when using the read/write I/O method
131 since there is no way to communicate if a field was a top or
132 bottom field.
133 * - ``V4L2_FIELD_INTERLACED_TB``
134 - 8
135 - Images contain both fields, interleaved line by line, top field
136 first. The top field is the older field.
137 * - ``V4L2_FIELD_INTERLACED_BT``
138 - 9
139 - Images contain both fields, interleaved line by line, top field
140 first. The bottom field is the older field.
141
142
143
144 .. _fieldseq-tb:
145
146 Field Order, Top Field First Transmitted
147 ========================================
148
149 .. kernel-figure:: fieldseq_tb.svg
150 :alt: fieldseq_tb.svg
151 :align: center
152
153 Field Order, Top Field First Transmitted
154
155
156 .. _fieldseq-bt:
157
158 Field Order, Bottom Field First Transmitted
159 ===========================================
160
161 .. kernel-figure:: fieldseq_bt.svg
162 :alt: fieldseq_bt.svg
163 :align: center
164
165 Field Order, Bottom Field First Transmitted
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