158 lines
6.3 KiB
ReStructuredText
158 lines
6.3 KiB
ReStructuredText
|
.. -*- 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
|
||
|
|
: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
|
||
|
|
===============
|
||
|
|
|
||
|
|
.. c:type:: v4l2_field
|
||
|
|
|
||
|
|
.. tabularcolumns:: |p{6.6cm}|p{2.2cm}|p{8.7cm}|
|
||
|
|
|
||
|
|
.. flat-table::
|
||
|
|
:header-rows: 0
|
||
|
|
:stub-columns: 0
|
||
|
|
:widths: 3 1 4
|
||
|
|
|
||
|
|
* - ``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
|
||
|
|
:c:type:`v4l2_buffer` ``field`` can never be
|
||
|
|
``V4L2_FIELD_ANY``.
|
||
|
|
* - ``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``.
|
||
|
|
* - ``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 first
|
||
|
|
transmitted) depends on the current video standard. M/NTSC
|
||
|
|
transmits the bottom field first, all other standards the top
|
||
|
|
field first.
|
||
|
|
* - ``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 transmitted first.
|
||
|
|
* - ``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
|
||
|
|
|