Documentation/media/uapi/v4l/field-order.rst

   1 .. -*- coding: utf-8; mode: rst -*-
   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 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).
  53
  54
  55 enum v4l2_field
  56 ===============
  57
  58 .. c:type:: v4l2_field
  59
  60 .. tabularcolumns:: |p{6.6cm}|p{2.2cm}|p{8.7cm}|
  61
  62 .. flat-table::
  63     :header-rows:  0
  64     :stub-columns: 0
  65     :widths:       3 1 4
  66
  67
  68     -  .. row 1
  69
  70        -  ``V4L2_FIELD_ANY``
  71
  72        -  0
  73
  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
  84           ``V4L2_FIELD_ANY``.
  85
  86     -  .. row 2
  87
  88        -  ``V4L2_FIELD_NONE``
  89
  90        -  1
  91
  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``.
  95
  96     -  .. row 3
  97
  98        -  ``V4L2_FIELD_TOP``
  99
 100        -  2
 101
 102        -  Images consist of the top (aka odd) field only.
 103
 104     -  .. row 4
 105
 106        -  ``V4L2_FIELD_BOTTOM``
 107
 108        -  3
 109
 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
 113           moving objects.
 114
 115     -  .. row 5
 116
 117        -  ``V4L2_FIELD_INTERLACED``
 118
 119        -  4
 120
 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
 125           field first.
 126
 127     -  .. row 6
 128
 129        -  ``V4L2_FIELD_SEQ_TB``
 130
 131        -  5
 132
 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.
 137
 138     -  .. row 7
 139
 140        -  ``V4L2_FIELD_SEQ_BT``
 141
 142        -  6
 143
 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.
 148
 149     -  .. row 8
 150
 151        -  ``V4L2_FIELD_ALTERNATE``
 152
 153        -  7
 154
 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
 167           bottom field.
 168
 169     -  .. row 9
 170
 171        -  ``V4L2_FIELD_INTERLACED_TB``
 172
 173        -  8
 174
 175        -  Images contain both fields, interleaved line by line, top field
 176           first. The top field is transmitted first.
 177
 178     -  .. row 10
 179
 180        -  ``V4L2_FIELD_INTERLACED_BT``
 181
 182        -  9
 183
 184        -  Images contain both fields, interleaved line by line, top field
 185           first. The bottom field is transmitted first.
 186
 187
 188
 189 .. _fieldseq-tb:
 190
 191 Field Order, Top Field First Transmitted
 192 ========================================
 193
 194 .. figure::  field-order_files/fieldseq_tb.*
 195     :alt:    fieldseq_tb.pdf / fieldseq_tb.gif
 196     :align:  center
 197
 198
 199 .. _fieldseq-bt:
 200
 201 Field Order, Bottom Field First Transmitted
 202 ===========================================
 203
 204 .. figure::  field-order_files/fieldseq_bt.*
 205     :alt:    fieldseq_bt.pdf / fieldseq_bt.gif
 206     :align:  center
 207