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+.. -*- coding: utf-8; mode: rst -*-
+
+.. _field-order:
+
+***********
+Field Order
+***********
+
+We have to distinguish between progressive and interlaced video.
+Progressive video transmits all lines of a video image sequentially.
+Interlaced video divides an image into two fields, containing only the
+odd and even lines of the image, respectively. Alternating the so called
+odd and even field are transmitted, and due to a small delay between
+fields a cathode ray TV displays the lines interleaved, yielding the
+original frame. This curious technique was invented because at refresh
+rates similar to film the image would fade out too quickly. Transmitting
+fields reduces the flicker without the necessity of doubling the frame
+rate and with it the bandwidth required for each channel.
+
+It is important to understand a video camera does not expose one frame
+at a time, merely transmitting the frames separated into fields. The
+fields are in fact captured at two different instances in time. An
+object on screen may well move between one field and the next. For
+applications analysing motion it is of paramount importance to recognize
+which field of a frame is older, the *temporal order*.
+
+When the driver provides or accepts images field by field rather than
+interleaved, it is also important applications understand how the fields
+combine to frames. We distinguish between top (aka odd) and bottom (aka
+even) fields, the *spatial order*: The first line of the top field is
+the first line of an interlaced frame, the first line of the bottom
+field is the second line of that frame.
+
+However because fields were captured one after the other, arguing
+whether a frame commences with the top or bottom field is pointless. Any
+two successive top and bottom, or bottom and top fields yield a valid
+frame. Only when the source was progressive to begin with, e. g. when
+transferring film to video, two fields may come from the same frame,
+creating a natural order.
+
+Counter to intuition the top field is not necessarily the older field.
+Whether the older field contains the top or bottom lines is a convention
+determined by the video standard. Hence the distinction between temporal
+and spatial order of fields. The diagrams below should make this
+clearer.
+
+All video capture and output devices must report the current field
+order. Some drivers may permit the selection of a different order, to
+this end applications initialize the ``field`` field of struct
+:ref:`v4l2_pix_format <v4l2-pix-format>` before calling the
+:ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>` ioctl. If this is not desired it
+should have the value ``V4L2_FIELD_ANY`` (0).
+
+
+.. _v4l2-field:
+
+enum v4l2_field
+===============
+
+.. flat-table::
+ :header-rows: 0
+ :stub-columns: 0
+ :widths: 3 1 4
+
+
+ - .. row 1
+
+ - ``V4L2_FIELD_ANY``
+
+ - 0
+
+ - Applications request this field order when any one of the
+ ``V4L2_FIELD_NONE``, ``V4L2_FIELD_TOP``, ``V4L2_FIELD_BOTTOM``, or
+ ``V4L2_FIELD_INTERLACED`` formats is acceptable. Drivers choose
+ depending on hardware capabilities or e. g. the requested image
+ size, and return the actual field order. Drivers must never return
+ ``V4L2_FIELD_ANY``. If multiple field orders are possible the
+ driver must choose one of the possible field orders during
+ :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>` or
+ :ref:`VIDIOC_TRY_FMT <VIDIOC_G_FMT>`. struct
+ :ref:`v4l2_buffer <v4l2-buffer>` ``field`` can never be
+ ``V4L2_FIELD_ANY``.
+
+ - .. row 2
+
+ - ``V4L2_FIELD_NONE``
+
+ - 1
+
+ - Images are in progressive format, not interlaced. The driver may
+ also indicate this order when it cannot distinguish between
+ ``V4L2_FIELD_TOP`` and ``V4L2_FIELD_BOTTOM``.
+
+ - .. row 3
+
+ - ``V4L2_FIELD_TOP``
+
+ - 2
+
+ - Images consist of the top (aka odd) field only.
+
+ - .. row 4
+
+ - ``V4L2_FIELD_BOTTOM``
+
+ - 3
+
+ - Images consist of the bottom (aka even) field only. Applications
+ may wish to prevent a device from capturing interlaced images
+ because they will have "comb" or "feathering" artefacts around
+ moving objects.
+
+ - .. row 5
+
+ - ``V4L2_FIELD_INTERLACED``
+
+ - 4
+
+ - Images contain both fields, interleaved line by line. The temporal
+ order of the fields (whether the top or bottom field is first
+ transmitted) depends on the current video standard. M/NTSC
+ transmits the bottom field first, all other standards the top
+ field first.
+
+ - .. row 6
+
+ - ``V4L2_FIELD_SEQ_TB``
+
+ - 5
+
+ - Images contain both fields, the top field lines are stored first
+ in memory, immediately followed by the bottom field lines. Fields
+ are always stored in temporal order, the older one first in
+ memory. Image sizes refer to the frame, not fields.
+
+ - .. row 7
+
+ - ``V4L2_FIELD_SEQ_BT``
+
+ - 6
+
+ - Images contain both fields, the bottom field lines are stored
+ first in memory, immediately followed by the top field lines.
+ Fields are always stored in temporal order, the older one first in
+ memory. Image sizes refer to the frame, not fields.
+
+ - .. row 8
+
+ - ``V4L2_FIELD_ALTERNATE``
+
+ - 7
+
+ - The two fields of a frame are passed in separate buffers, in
+ temporal order, i. e. the older one first. To indicate the field
+ parity (whether the current field is a top or bottom field) the
+ driver or application, depending on data direction, must set
+ struct :ref:`v4l2_buffer <v4l2-buffer>` ``field`` to
+ ``V4L2_FIELD_TOP`` or ``V4L2_FIELD_BOTTOM``. Any two successive
+ fields pair to build a frame. If fields are successive, without
+ any dropped fields between them (fields can drop individually),
+ can be determined from the struct
+ :ref:`v4l2_buffer <v4l2-buffer>` ``sequence`` field. This
+ format cannot be selected when using the read/write I/O method
+ since there is no way to communicate if a field was a top or
+ bottom field.
+
+ - .. row 9
+
+ - ``V4L2_FIELD_INTERLACED_TB``
+
+ - 8
+
+ - Images contain both fields, interleaved line by line, top field
+ first. The top field is transmitted first.
+
+ - .. row 10
+
+ - ``V4L2_FIELD_INTERLACED_BT``
+
+ - 9
+
+ - Images contain both fields, interleaved line by line, top field
+ first. The bottom field is transmitted first.
+
+
+
+.. _fieldseq-tb:
+
+Field Order, Top Field First Transmitted
+========================================
+
+.. figure:: field-order_files/fieldseq_tb.*
+ :alt: fieldseq_tb.pdf / fieldseq_tb.gif
+ :align: center
+
+
+.. _fieldseq-bt:
+
+Field Order, Bottom Field First Transmitted
+===========================================
+
+.. figure:: field-order_files/fieldseq_bt.*
+ :alt: fieldseq_bt.pdf / fieldseq_bt.gif
+ :align: center
+