158 lines
6.3 KiB
ReStructuredText
158 lines
6.3 KiB
ReStructuredText
.. -*- coding: utf-8; mode: rst -*-
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.. _field-order:
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***********
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Field Order
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***********
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We have to distinguish between progressive and interlaced video.
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Progressive video transmits all lines of a video image sequentially.
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Interlaced video divides an image into two fields, containing only the
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odd and even lines of the image, respectively. Alternating the so called
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odd and even field are transmitted, and due to a small delay between
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fields a cathode ray TV displays the lines interleaved, yielding the
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original frame. This curious technique was invented because at refresh
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rates similar to film the image would fade out too quickly. Transmitting
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fields reduces the flicker without the necessity of doubling the frame
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rate and with it the bandwidth required for each channel.
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It is important to understand a video camera does not expose one frame
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at a time, merely transmitting the frames separated into fields. The
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fields are in fact captured at two different instances in time. An
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object on screen may well move between one field and the next. For
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applications analysing motion it is of paramount importance to recognize
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which field of a frame is older, the *temporal order*.
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When the driver provides or accepts images field by field rather than
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interleaved, it is also important applications understand how the fields
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combine to frames. We distinguish between top (aka odd) and bottom (aka
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even) fields, the *spatial order*: The first line of the top field is
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the first line of an interlaced frame, the first line of the bottom
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field is the second line of that frame.
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However because fields were captured one after the other, arguing
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whether a frame commences with the top or bottom field is pointless. Any
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two successive top and bottom, or bottom and top fields yield a valid
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frame. Only when the source was progressive to begin with, e. g. when
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transferring film to video, two fields may come from the same frame,
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creating a natural order.
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Counter to intuition the top field is not necessarily the older field.
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Whether the older field contains the top or bottom lines is a convention
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determined by the video standard. Hence the distinction between temporal
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and spatial order of fields. The diagrams below should make this
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clearer.
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All video capture and output devices must report the current field
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order. Some drivers may permit the selection of a different order, to
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this end applications initialize the ``field`` field of struct
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:c:type:`v4l2_pix_format` before calling the
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:ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>` ioctl. If this is not desired it
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should have the value ``V4L2_FIELD_ANY`` (0).
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enum v4l2_field
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===============
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.. c:type:: v4l2_field
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.. tabularcolumns:: |p{6.6cm}|p{2.2cm}|p{8.7cm}|
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.. flat-table::
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:header-rows: 0
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:stub-columns: 0
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:widths: 3 1 4
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* - ``V4L2_FIELD_ANY``
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- 0
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- Applications request this field order when any one of the
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``V4L2_FIELD_NONE``, ``V4L2_FIELD_TOP``, ``V4L2_FIELD_BOTTOM``, or
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``V4L2_FIELD_INTERLACED`` formats is acceptable. Drivers choose
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depending on hardware capabilities or e. g. the requested image
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size, and return the actual field order. Drivers must never return
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``V4L2_FIELD_ANY``. If multiple field orders are possible the
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driver must choose one of the possible field orders during
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:ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>` or
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:ref:`VIDIOC_TRY_FMT <VIDIOC_G_FMT>`. struct
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:c:type:`v4l2_buffer` ``field`` can never be
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``V4L2_FIELD_ANY``.
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* - ``V4L2_FIELD_NONE``
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- 1
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- Images are in progressive format, not interlaced. The driver may
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also indicate this order when it cannot distinguish between
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``V4L2_FIELD_TOP`` and ``V4L2_FIELD_BOTTOM``.
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* - ``V4L2_FIELD_TOP``
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- 2
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- Images consist of the top (aka odd) field only.
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* - ``V4L2_FIELD_BOTTOM``
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- 3
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- Images consist of the bottom (aka even) field only. Applications
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may wish to prevent a device from capturing interlaced images
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because they will have "comb" or "feathering" artefacts around
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moving objects.
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* - ``V4L2_FIELD_INTERLACED``
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- 4
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- Images contain both fields, interleaved line by line. The temporal
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order of the fields (whether the top or bottom field is first
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transmitted) depends on the current video standard. M/NTSC
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transmits the bottom field first, all other standards the top
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field first.
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* - ``V4L2_FIELD_SEQ_TB``
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- 5
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- Images contain both fields, the top field lines are stored first
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in memory, immediately followed by the bottom field lines. Fields
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are always stored in temporal order, the older one first in
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memory. Image sizes refer to the frame, not fields.
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* - ``V4L2_FIELD_SEQ_BT``
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- 6
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- Images contain both fields, the bottom field lines are stored
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first in memory, immediately followed by the top field lines.
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Fields are always stored in temporal order, the older one first in
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memory. Image sizes refer to the frame, not fields.
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* - ``V4L2_FIELD_ALTERNATE``
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- 7
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- The two fields of a frame are passed in separate buffers, in
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temporal order, i. e. the older one first. To indicate the field
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parity (whether the current field is a top or bottom field) the
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driver or application, depending on data direction, must set
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struct :c:type:`v4l2_buffer` ``field`` to
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``V4L2_FIELD_TOP`` or ``V4L2_FIELD_BOTTOM``. Any two successive
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fields pair to build a frame. If fields are successive, without
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any dropped fields between them (fields can drop individually),
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can be determined from the struct
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:c:type:`v4l2_buffer` ``sequence`` field. This
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format cannot be selected when using the read/write I/O method
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since there is no way to communicate if a field was a top or
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bottom field.
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* - ``V4L2_FIELD_INTERLACED_TB``
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- 8
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- Images contain both fields, interleaved line by line, top field
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first. The top field is transmitted first.
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* - ``V4L2_FIELD_INTERLACED_BT``
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- 9
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- Images contain both fields, interleaved line by line, top field
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first. The bottom field is transmitted first.
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.. _fieldseq-tb:
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Field Order, Top Field First Transmitted
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========================================
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.. figure:: field-order_files/fieldseq_tb.*
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:alt: fieldseq_tb.pdf / fieldseq_tb.gif
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:align: center
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.. _fieldseq-bt:
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Field Order, Bottom Field First Transmitted
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===========================================
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.. figure:: field-order_files/fieldseq_bt.*
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:alt: fieldseq_bt.pdf / fieldseq_bt.gif
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:align: center
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