Field Order

Progressive와 interlaced field

Progressive video는 한 영상의 모든 line을 순서대로 전송합니다. Interlaced video는 영상을 odd line의 top field와 even line의 bottom field로 나누고 두 field를 번갈아 전송합니다. CRT는 약간의 시간차를 둔 line을 서로 끼워 원래 frame으로 표시합니다. 이 방식은 frame rate와 channel bandwidth를 두 배로 늘리지 않고 film과 비슷한 refresh rate에서 flicker를 줄이기 위해 고안됐습니다.

카메라가 frame 하나를 한 시점에 노출한 뒤 field로 나누어 보내는 것은 아닙니다. 두 field는 실제로 서로 다른 시각에 capture되므로 물체가 그 사이에 움직일 수 있습니다. Motion analysis에서는 어느 field가 더 오래됐는지 나타내는 temporal order가 중요합니다.

Spatial order는 top(odd) field와 bottom(even) field가 frame line에 어떻게 결합되는지를 뜻합니다. Top field의 첫 line은 interlaced frame의 첫 line이고 bottom field의 첫 line은 둘째 line입니다. 그러나 연속 capture된 field라면 top-bottom 또는 bottom-top 어느 연속 쌍도 frame이 될 수 있습니다. Progressive source를 film-to-video로 나눈 경우처럼 두 field가 같은 frame에서 나올 때만 자연스러운 쌍이 생깁니다.

Top field가 반드시 더 오래된 field는 아닙니다. 어느 field가 먼저 capture됐는지는 video standard의 temporal convention이고, top/bottom은 spatial position입니다. V4L은 camera가 media bus에서 capture 순서대로 field를 전송한다고 가정합니다.

모든 capture와 output 장치는 현재 field order를 보고해야 합니다. 다른 순서를 선택하려면 `VIDIOC_S_FMT` 전에 `v4l2_pix_format.field`를 초기화합니다. 특별한 요청이 없으면 `V4L2_FIELD_ANY`(0)를 사용합니다.

.. SPDX-License-Identifier: GFDL-1.1-no-invariants-or-later

.. _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.

In V4L it is assumed that all video cameras transmit fields on the media
bus in the same order they were captured, so if the top field was
captured first (is the older field), the top field is also transmitted
first on the bus.

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
:c:type:`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).

`enum v4l2_field`

`V4L2_FIELD_ANY` 요청에서 여러 순서가 가능하면 driver는 `VIDIOC_S_FMT` 또는 `VIDIOC_TRY_FMT` 중 하나를 선택해 실제 값을 반환합니다. `V4L2_FIELD_ALTERNATE`에서는 연속된 field 두 개가 한 frame을 이루지만 field는 개별적으로 drop될 수 있습니다. Top/bottom 정보를 전달할 방법이 없으므로 read/write I/O 방식에서는 ALTERNATE를 선택할 수 없습니다.

enum v4l2_field
===============

.. c:type:: v4l2_field

.. tabularcolumns:: |p{5.8cm}|p{0.6cm}|p{10.9cm}|

.. cssclass:: longtable

.. flat-table::
    :header-rows:  0
    :stub-columns: 0
    :widths:       3 1 4

    * - ``V4L2_FIELD_ANY``
      - 0
      - Applications request this field order when any field format
	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
	:c:type:`v4l2_buffer` ``field`` can never be
	``V4L2_FIELD_ANY``.
    * - ``V4L2_FIELD_NONE``
      - 1
      - Images are in progressive (frame-based) format, not interlaced
        (field-based).
    * - ``V4L2_FIELD_TOP``
      - 2
      - Images consist of the top (aka odd) field only.
    * - ``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.
    * - ``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 older)
	depends on the current video standard. In M/NTSC the bottom
	field is the older field. In all other standards the top field
	is the older field.
    * - ``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.
    * - ``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.
    * - ``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 :c:type:`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
	:c:type:`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.
    * - ``V4L2_FIELD_INTERLACED_TB``
      - 8
      - Images contain both fields, interleaved line by line, top field
	first. The top field is the older field.
    * - ``V4L2_FIELD_INTERLACED_BT``
      - 9
      - Images contain both fields, interleaved line by line, top field
	first. The bottom field is the older field.

Top-first와 bottom-first 전송 도식

`fieldseq_tb.svg`는 top field가 먼저 capture되어 bus에서도 먼저 전송되는 순서를 보여 줍니다. `fieldseq_bt.svg`는 bottom field가 먼저 capture되고 먼저 전송되는 경우를 보여 줍니다. 두 그림 모두 spatial line 위치와 temporal age를 혼동하지 않는 것이 핵심입니다.

.. _fieldseq-tb:

Field Order, Top Field First Transmitted
========================================

.. kernel-figure:: fieldseq_tb.svg
    :alt:    fieldseq_tb.svg
    :align:  center

    Field Order, Top Field First Transmitted


.. _fieldseq-bt:

Field Order, Bottom Field First Transmitted
===========================================

.. kernel-figure:: fieldseq_bt.svg
    :alt:    fieldseq_bt.svg
    :align:  center

    Field Order, Bottom Field First Transmitted