106 lines
3.7 KiB
ReStructuredText
106 lines
3.7 KiB
ReStructuredText
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.. SPDX-License-Identifier: GFDL-1.1-no-invariants-or-later
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.. _jpeg-controls:
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**********************
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JPEG Control Reference
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**********************
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The JPEG class includes controls for common features of JPEG encoders
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and decoders. Currently it includes features for codecs implementing
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progressive baseline DCT compression process with Huffman entrophy
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coding.
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.. _jpeg-control-id:
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JPEG Control IDs
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================
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``V4L2_CID_JPEG_CLASS (class)``
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The JPEG class descriptor. Calling
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:ref:`VIDIOC_QUERYCTRL` for this control will
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return a description of this control class.
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``V4L2_CID_JPEG_CHROMA_SUBSAMPLING (menu)``
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The chroma subsampling factors describe how each component of an
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input image is sampled, in respect to maximum sample rate in each
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spatial dimension. See :ref:`itu-t81`, clause A.1.1. for more
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details. The ``V4L2_CID_JPEG_CHROMA_SUBSAMPLING`` control determines
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how Cb and Cr components are downsampled after converting an input
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image from RGB to Y'CbCr color space.
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.. tabularcolumns:: |p{7.5cm}|p{10.0cm}|
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.. flat-table::
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:header-rows: 0
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:stub-columns: 0
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* - ``V4L2_JPEG_CHROMA_SUBSAMPLING_444``
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- No chroma subsampling, each pixel has Y, Cr and Cb values.
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* - ``V4L2_JPEG_CHROMA_SUBSAMPLING_422``
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- Horizontally subsample Cr, Cb components by a factor of 2.
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* - ``V4L2_JPEG_CHROMA_SUBSAMPLING_420``
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- Subsample Cr, Cb components horizontally and vertically by 2.
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* - ``V4L2_JPEG_CHROMA_SUBSAMPLING_411``
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- Horizontally subsample Cr, Cb components by a factor of 4.
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* - ``V4L2_JPEG_CHROMA_SUBSAMPLING_410``
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- Subsample Cr, Cb components horizontally by 4 and vertically by 2.
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* - ``V4L2_JPEG_CHROMA_SUBSAMPLING_GRAY``
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- Use only luminance component.
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``V4L2_CID_JPEG_RESTART_INTERVAL (integer)``
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The restart interval determines an interval of inserting RSTm
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markers (m = 0..7). The purpose of these markers is to additionally
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reinitialize the encoder process, in order to process blocks of an
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image independently. For the lossy compression processes the restart
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interval unit is MCU (Minimum Coded Unit) and its value is contained
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in DRI (Define Restart Interval) marker. If
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``V4L2_CID_JPEG_RESTART_INTERVAL`` control is set to 0, DRI and RSTm
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markers will not be inserted.
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.. _jpeg-quality-control:
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``V4L2_CID_JPEG_COMPRESSION_QUALITY (integer)``
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Determines trade-off between image quality and size.
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It provides simpler method for applications to control image quality,
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without a need for direct reconfiguration of luminance and chrominance
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quantization tables. In cases where a driver uses quantization tables
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configured directly by an application, using interfaces defined
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elsewhere, ``V4L2_CID_JPEG_COMPRESSION_QUALITY`` control should be set by
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driver to 0.
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The value range of this control is driver-specific. Only positive,
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non-zero values are meaningful. The recommended range is 1 - 100,
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where larger values correspond to better image quality.
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.. _jpeg-active-marker-control:
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``V4L2_CID_JPEG_ACTIVE_MARKER (bitmask)``
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Specify which JPEG markers are included in compressed stream. This
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control is valid only for encoders.
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.. flat-table::
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:header-rows: 0
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:stub-columns: 0
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* - ``V4L2_JPEG_ACTIVE_MARKER_APP0``
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- Application data segment APP\ :sub:`0`.
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* - ``V4L2_JPEG_ACTIVE_MARKER_APP1``
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- Application data segment APP\ :sub:`1`.
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* - ``V4L2_JPEG_ACTIVE_MARKER_COM``
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- Comment segment.
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* - ``V4L2_JPEG_ACTIVE_MARKER_DQT``
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- Quantization tables segment.
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* - ``V4L2_JPEG_ACTIVE_MARKER_DHT``
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- Huffman tables segment.
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For more details about JPEG specification, refer to :ref:`itu-t81`,
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:ref:`jfif`, :ref:`w3c-jpeg-jfif`.
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