1 .. -*- coding: utf-8; mode: rst -*-
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.
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*.
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.
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.
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
47 All video capture and output devices must report the current field
48 order. Some drivers may permit the selection of a different order, to
49 this end applications initialize the ``field`` field of struct
50 :c:type:`v4l2_pix_format` before calling the
51 :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>` ioctl. If this is not desired it
52 should have the value ``V4L2_FIELD_ANY`` (0).
58 .. c:type:: v4l2_field
60 .. tabularcolumns:: |p{6.6cm}|p{2.2cm}|p{8.7cm}|
74 - Applications request this field order when any one of the
75 ``V4L2_FIELD_NONE``, ``V4L2_FIELD_TOP``, ``V4L2_FIELD_BOTTOM``, or
76 ``V4L2_FIELD_INTERLACED`` formats is acceptable. Drivers choose
77 depending on hardware capabilities or e. g. the requested image
78 size, and return the actual field order. Drivers must never return
79 ``V4L2_FIELD_ANY``. If multiple field orders are possible the
80 driver must choose one of the possible field orders during
81 :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>` or
82 :ref:`VIDIOC_TRY_FMT <VIDIOC_G_FMT>`. struct
83 :c:type:`v4l2_buffer` ``field`` can never be
92 - Images are in progressive format, not interlaced. The driver may
93 also indicate this order when it cannot distinguish between
94 ``V4L2_FIELD_TOP`` and ``V4L2_FIELD_BOTTOM``.
102 - Images consist of the top (aka odd) field only.
106 - ``V4L2_FIELD_BOTTOM``
110 - Images consist of the bottom (aka even) field only. Applications
111 may wish to prevent a device from capturing interlaced images
112 because they will have "comb" or "feathering" artefacts around
117 - ``V4L2_FIELD_INTERLACED``
121 - Images contain both fields, interleaved line by line. The temporal
122 order of the fields (whether the top or bottom field is first
123 transmitted) depends on the current video standard. M/NTSC
124 transmits the bottom field first, all other standards the top
129 - ``V4L2_FIELD_SEQ_TB``
133 - Images contain both fields, the top field lines are stored first
134 in memory, immediately followed by the bottom field lines. Fields
135 are always stored in temporal order, the older one first in
136 memory. Image sizes refer to the frame, not fields.
140 - ``V4L2_FIELD_SEQ_BT``
144 - Images contain both fields, the bottom field lines are stored
145 first in memory, immediately followed by the top field lines.
146 Fields are always stored in temporal order, the older one first in
147 memory. Image sizes refer to the frame, not fields.
151 - ``V4L2_FIELD_ALTERNATE``
155 - The two fields of a frame are passed in separate buffers, in
156 temporal order, i. e. the older one first. To indicate the field
157 parity (whether the current field is a top or bottom field) the
158 driver or application, depending on data direction, must set
159 struct :c:type:`v4l2_buffer` ``field`` to
160 ``V4L2_FIELD_TOP`` or ``V4L2_FIELD_BOTTOM``. Any two successive
161 fields pair to build a frame. If fields are successive, without
162 any dropped fields between them (fields can drop individually),
163 can be determined from the struct
164 :c:type:`v4l2_buffer` ``sequence`` field. This
165 format cannot be selected when using the read/write I/O method
166 since there is no way to communicate if a field was a top or
171 - ``V4L2_FIELD_INTERLACED_TB``
175 - Images contain both fields, interleaved line by line, top field
176 first. The top field is transmitted first.
180 - ``V4L2_FIELD_INTERLACED_BT``
184 - Images contain both fields, interleaved line by line, top field
185 first. The bottom field is transmitted first.
191 Field Order, Top Field First Transmitted
192 ========================================
194 .. figure:: field-order_files/fieldseq_tb.*
195 :alt: fieldseq_tb.pdf / fieldseq_tb.gif
201 Field Order, Bottom Field First Transmitted
202 ===========================================
204 .. figure:: field-order_files/fieldseq_bt.*
205 :alt: fieldseq_bt.pdf / fieldseq_bt.gif