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.. Permission is granted to copy, distribute and/or modify this
.. document under the terms of the GNU Free Documentation License,
.. Version 1.1 or any later version published by the Free Software
.. Foundation, with no Invariant Sections, no Front-Cover Texts
.. and no Back-Cover Texts. A copy of the license is included at
.. Documentation/media/uapi/fdl-appendix.rst.
.. TODO: replace it to GFDL-1.1-or-later WITH no-invariant-sections
.. _jpeg-controls:
JPEG Control Reference
The JPEG class includes controls for common features of JPEG encoders
and decoders. Currently it includes features for codecs implementing
progressive baseline DCT compression process with Huffman entrophy
.. _jpeg-control-id:
JPEG Control IDs
``V4L2_CID_JPEG_CLASS (class)``
The JPEG class descriptor. Calling
:ref:`VIDIOC_QUERYCTRL` for this control will
return a description of this control class.
The chroma subsampling factors describe how each component of an
input image is sampled, in respect to maximum sample rate in each
spatial dimension. See :ref:`itu-t81`, clause A.1.1. for more
details. The ``V4L2_CID_JPEG_CHROMA_SUBSAMPLING`` control determines
how Cb and Cr components are downsampled after converting an input
image from RGB to Y'CbCr color space.
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- No chroma subsampling, each pixel has Y, Cr and Cb values.
- Horizontally subsample Cr, Cb components by a factor of 2.
- Subsample Cr, Cb components horizontally and vertically by 2.
- Horizontally subsample Cr, Cb components by a factor of 4.
- Subsample Cr, Cb components horizontally by 4 and vertically by 2.
- Use only luminance component.
The restart interval determines an interval of inserting RSTm
markers (m = 0..7). The purpose of these markers is to additionally
reinitialize the encoder process, in order to process blocks of an
image independently. For the lossy compression processes the restart
interval unit is MCU (Minimum Coded Unit) and its value is contained
in DRI (Define Restart Interval) marker. If
``V4L2_CID_JPEG_RESTART_INTERVAL`` control is set to 0, DRI and RSTm
markers will not be inserted.
.. _jpeg-quality-control:
``V4L2_CID_JPEG_COMPRESSION_QUALITY`` control determines trade-off
between image quality and size. It provides simpler method for
applications to control image quality, without a need for direct
reconfiguration of luminance and chrominance quantization tables. In
cases where a driver uses quantization tables configured directly by
an application, using interfaces defined elsewhere,
``V4L2_CID_JPEG_COMPRESSION_QUALITY`` control should be set by
driver to 0.
The value range of this control is driver-specific. Only positive,
non-zero values are meaningful. The recommended range is 1 - 100,
where larger values correspond to better image quality.
.. _jpeg-active-marker-control:
Specify which JPEG markers are included in compressed stream. This
control is valid only for encoders.
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- Application data segment APP\ :sub:`0`.
- Application data segment APP\ :sub:`1`.
- Comment segment.
- Quantization tables segment.
- Huffman tables segment.
For more details about JPEG specification, refer to :ref:`itu-t81`,
:ref:`jfif`, :ref:`w3c-jpeg-jfif`.