V4L2 단일 평면 포맷 구조체

영상 크기, 픽셀 포맷, 필드 순서

`v4l2_pix_format`은 단일 평면 영상의 크기, 저장 방식, 버퍼 크기와 색도 정보를 기술합니다. 애플리케이션은 희망 값을 요청하고 드라이버는 하드웨어가 지원하는 가장 가까운 실제 값을 돌려줍니다.

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

******************************
Single-planar format structure
******************************

.. tabularcolumns:: |p{4.0cm}|p{2.6cm}|p{10.7cm}|

.. c:type:: v4l2_pix_format

.. cssclass:: longtable

.. flat-table:: struct v4l2_pix_format
    :header-rows:  0
    :stub-columns: 0
    :widths:       1 1 2

    * - __u32
      - ``width``
      - Image width in pixels.
    * - __u32
      - ``height``
      - Image height in pixels. If ``field`` is one of ``V4L2_FIELD_TOP``,
	``V4L2_FIELD_BOTTOM`` or ``V4L2_FIELD_ALTERNATE`` then height
	refers to the number of lines in the field, otherwise it refers to
	the number of lines in the frame (which is twice the field height
	for interlaced formats).
    * - :cspan:`2` Applications set these fields to request an image
	size, drivers return the closest possible values. In case of
	planar formats the ``width`` and ``height`` applies to the largest
	plane. To avoid ambiguities drivers must return values rounded up
	to a multiple of the scale factor of any smaller planes. For
	example when the image format is YUV 4:2:0, ``width`` and
	``height`` must be multiples of two.

	For compressed formats that contain the resolution information encoded
	inside the stream, when fed to a stateful mem2mem decoder, the fields
	may be zero to rely on the decoder to detect the right values. For more
	details see :ref:`decoder` and format descriptions.

	For compressed formats on the CAPTURE side of a stateful mem2mem
	encoder, the fields must be zero, since the coded size is expected to
	be calculated internally by the encoder itself, based on the OUTPUT
	side. For more details see :ref:`encoder` and format descriptions.
    * - __u32
      - ``pixelformat``
      - The pixel format or type of compression, set by the application.
	This is a little endian
	:ref:`four character code <v4l2-fourcc>`. V4L2 defines standard
	RGB formats in :ref:`pixfmt-rgb`, YUV formats in
	:ref:`yuv-formats`, and reserved codes in
	:ref:`reserved-formats`
    * - __u32
      - ``field``
      - Field order, from enum :c:type:`v4l2_field`.
        Video images are typically interlaced. Applications can request to
	capture or output only the top or bottom field, or both fields
	interlaced or sequentially stored in one buffer or alternating in
	separate buffers. Drivers return the actual field order selected.
	For more details on fields see :ref:`field-order`.

행 간격, 버퍼 크기, 색공간

    * - __u32
      - ``bytesperline``
      - Distance in bytes between the leftmost pixels in two adjacent
	lines.
    * - :cspan:`2`

	Both applications and drivers can set this field to request
	padding bytes at the end of each line. Drivers however may ignore
	the value requested by the application, returning ``width`` times
	bytes per pixel or a larger value required by the hardware. That
	implies applications can just set this field to zero to get a
	reasonable default.

	Video hardware may access padding bytes, therefore they must
	reside in accessible memory. Consider cases where padding bytes
	after the last line of an image cross a system page boundary.
	Input devices may write padding bytes, the value is undefined.
	Output devices ignore the contents of padding bytes.

	When the image format is planar the ``bytesperline`` value applies
	to the first plane and is divided by the same factor as the
	``width`` field for the other planes. For example the Cb and Cr
	planes of a YUV 4:2:0 image have half as many padding bytes
	following each line as the Y plane. To avoid ambiguities drivers
	must return a ``bytesperline`` value rounded up to a multiple of
	the scale factor.

	For compressed formats the ``bytesperline`` value makes no sense.
	Applications and drivers must set this to 0 in that case.
    * - __u32
      - ``sizeimage``
      - Size in bytes of the buffer to hold a complete image, set by the
	driver. Usually this is ``bytesperline`` times ``height``. When
	the image consists of variable length compressed data this is the
	number of bytes required by the codec to support the worst-case
	compression scenario.

	The driver will set the value for uncompressed images.

	Clients are allowed to set the sizeimage field for variable length
	compressed data flagged with ``V4L2_FMT_FLAG_COMPRESSED`` at
	:ref:`VIDIOC_ENUM_FMT`, but the driver may ignore it and set the
	value itself, or it may modify the provided value based on
	alignment requirements or minimum/maximum size requirements.
	If the client wants to leave this to the driver, then it should
	set sizeimage to 0.
    * - __u32
      - ``colorspace``
      - Image colorspace, from enum :c:type:`v4l2_colorspace`.
        This information supplements the ``pixelformat`` and must be set
	by the driver for capture streams and by the application for
	output streams, see :ref:`colorspaces`. If the application sets the
	flag ``V4L2_PIX_FMT_FLAG_SET_CSC`` then the application can set
	this field for a capture stream to request a specific colorspace
	for the captured image data. If the driver cannot handle requested
	conversion, it will return another supported colorspace.
	The driver indicates that colorspace conversion is supported by setting
	the flag V4L2_FMT_FLAG_CSC_COLORSPACE in the corresponding struct
	:c:type:`v4l2_fmtdesc` during enumeration. See :ref:`fmtdesc-flags`.

확장 필드와 색도 정보

`priv` 뒤의 항목은 확장 필드입니다. 기능 지원을 확인하고 매직 값을 설정한 뒤 모든 확장 필드와 사용하지 않는 원시 데이터 바이트를 초기화해야만 정의된 결과를 얻을 수 있습니다.

    * - __u32
      - ``priv``
      - This field indicates whether the remaining fields of the
	struct :c:type:`v4l2_pix_format`, also called the
	extended fields, are valid. When set to
	``V4L2_PIX_FMT_PRIV_MAGIC``, it indicates that the extended fields
	have been correctly initialized. When set to any other value it
	indicates that the extended fields contain undefined values.

	Applications that wish to use the pixel format extended fields
	must first ensure that the feature is supported by querying the
	device for the :ref:`V4L2_CAP_EXT_PIX_FORMAT <querycap>`
	capability. If the capability isn't set the pixel format extended
	fields are not supported and using the extended fields will lead
	to undefined results.

	To use the extended fields, applications must set the ``priv``
	field to ``V4L2_PIX_FMT_PRIV_MAGIC``, initialize all the extended
	fields and zero the unused bytes of the
	struct :c:type:`v4l2_format` ``raw_data`` field.

	When the ``priv`` field isn't set to ``V4L2_PIX_FMT_PRIV_MAGIC``
	drivers must act as if all the extended fields were set to zero.
	On return drivers must set the ``priv`` field to
	``V4L2_PIX_FMT_PRIV_MAGIC`` and all the extended fields to
	applicable values.
    * - __u32
      - ``flags``
      - Flags set by the application or driver, see :ref:`format-flags`.
    * - union {
      - (anonymous)
    * - __u32
      - ``ycbcr_enc``
      - Y'CbCr encoding, from enum :c:type:`v4l2_ycbcr_encoding`.
        This information supplements the ``colorspace`` and must be set by
	the driver for capture streams and by the application for output
	streams, see :ref:`colorspaces`. If the application sets the
	flag ``V4L2_PIX_FMT_FLAG_SET_CSC`` then the application can set
	this field for a capture stream to request a specific Y'CbCr encoding
	for the captured image data. If the driver cannot handle requested
	conversion, it will return another supported encoding.
	This field is ignored for HSV pixelformats. The driver indicates that
	ycbcr_enc conversion is supported by setting the flag
	V4L2_FMT_FLAG_CSC_YCBCR_ENC in the corresponding struct
	:c:type:`v4l2_fmtdesc` during enumeration. See :ref:`fmtdesc-flags`.
    * - __u32
      - ``hsv_enc``
      - HSV encoding, from enum :c:type:`v4l2_hsv_encoding`.
        This information supplements the ``colorspace`` and must be set by
	the driver for capture streams and by the application for output
	streams, see :ref:`colorspaces`. If the application sets the flag
	``V4L2_PIX_FMT_FLAG_SET_CSC`` then the application can set this
	field for a capture stream to request a specific HSV encoding for the
	captured image data. If the driver cannot handle requested
	conversion, it will return another supported encoding.
	This field is ignored for non-HSV pixelformats. The driver indicates
	that hsv_enc conversion is supported by setting the flag
	V4L2_FMT_FLAG_CSC_HSV_ENC in the corresponding struct
	:c:type:`v4l2_fmtdesc` during enumeration. See :ref:`fmtdesc-flags`.
    * - }
      -
    * - __u32
      - ``quantization``
      - Quantization range, from enum :c:type:`v4l2_quantization`.
        This information supplements the ``colorspace`` and must be set by
	the driver for capture streams and by the application for output
	streams, see :ref:`colorspaces`. If the application sets the flag
	``V4L2_PIX_FMT_FLAG_SET_CSC`` then the application can set
	this field for a capture stream to request a specific quantization
	range for the captured image data. If the driver cannot handle requested
	conversion, it will return another supported quantization.
	The driver indicates that quantization conversion is supported by setting
	the flag V4L2_FMT_FLAG_CSC_QUANTIZATION in the corresponding struct
	:c:type:`v4l2_fmtdesc` during enumeration. See :ref:`fmtdesc-flags`.
    * - __u32
      - ``xfer_func``
      - Transfer function, from enum :c:type:`v4l2_xfer_func`.
        This information supplements the ``colorspace`` and must be set by
	the driver for capture streams and by the application for output
	streams, see :ref:`colorspaces`. If the application sets the flag
	``V4L2_PIX_FMT_FLAG_SET_CSC`` then the application can set
	this field for a capture stream to request a specific transfer function
	for the captured image data. If the driver cannot handle requested
	conversion, it will return another supported transfer function.
	The driver indicates that xfer_func conversion is supported by setting
	the flag V4L2_FMT_FLAG_CSC_XFER_FUNC in the corresponding struct
	:c:type:`v4l2_fmtdesc` during enumeration. See :ref:`fmtdesc-flags`.

포맷 플래그

양자화만 바꾸려면 `quantization`을 `V4L2_QUANTIZATION_FULL_RANGE` 또는 `V4L2_QUANTIZATION_LIM_RANGE`로 설정하고, 다른 모든 색도 필드는 `*_DEFAULT`로 둡니다.

.. tabularcolumns:: |p{6.8cm}|p{2.3cm}|p{8.2cm}|

.. _format-flags:

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

    * - ``V4L2_PIX_FMT_FLAG_PREMUL_ALPHA``
      - 0x00000001
      - The color values are premultiplied by the alpha channel value. For
        example, if a light blue pixel with 50% transparency was described
	by RGBA values (128, 192, 255, 128), the same pixel described with
	premultiplied colors would be described by RGBA values (64, 96,
	128, 128)
    * .. _`v4l2-pix-fmt-flag-set-csc`:

      - ``V4L2_PIX_FMT_FLAG_SET_CSC``
      - 0x00000002
      - Set by the application. It is only used for capture and is
        ignored for output streams. If set, then request the device to do
	colorspace conversion from the received colorspace to the requested
	colorspace values. If the colorimetry field (``colorspace``, ``xfer_func``,
	``ycbcr_enc``, ``hsv_enc`` or ``quantization``) is set to ``*_DEFAULT``,
	then that colorimetry setting will remain unchanged from what was received.
	So in order to change the quantization, only the ``quantization`` field shall
	be set to non default value (``V4L2_QUANTIZATION_FULL_RANGE`` or
	``V4L2_QUANTIZATION_LIM_RANGE``) and all other colorimetry fields shall
	be set to ``*_DEFAULT``.

	To check which conversions are supported by the hardware for the current
	pixel format, see :ref:`fmtdesc-flags`.