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diff --git a/Documentation/userspace-api/media/v4l/field-order.rst b/Documentation/userspace-api/media/v4l/field-order.rst new file mode 100644 index 000000000..54548ea43 --- /dev/null +++ b/Documentation/userspace-api/media/v4l/field-order.rst @@ -0,0 +1,165 @@ +.. 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 +=============== + +.. c:type:: v4l2_field + +.. tabularcolumns:: |p{5.8cm}|p{0.6cm}|p{11.1cm}| + +.. 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. + + + +.. _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 |