4f14e3272f
The extended-controls.rst file had become too big. Split it up: each control class reference gets its own rst file, and this file just describes the Extended Control API. Each control class reference is also moved up one level into the table of contents to make it easier to find e.g. the codec control reference. Finally I rearranged the order so that all camera-related control classes are grouped together, ditto for codec/jpeg and fm-rx/tx. The ext-ctrls-codec.rst is still pretty big and it is a candidate to split up further in the future, possibly per codec. Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl> Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
113 lines
4.1 KiB
ReStructuredText
113 lines
4.1 KiB
ReStructuredText
.. 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
|
|
coding.
|
|
|
|
|
|
.. _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.
|
|
|
|
``V4L2_CID_JPEG_CHROMA_SUBSAMPLING (menu)``
|
|
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.
|
|
|
|
.. tabularcolumns:: |p{7.0cm}|p{10.5cm}|
|
|
|
|
.. flat-table::
|
|
:header-rows: 0
|
|
:stub-columns: 0
|
|
|
|
* - ``V4L2_JPEG_CHROMA_SUBSAMPLING_444``
|
|
- No chroma subsampling, each pixel has Y, Cr and Cb values.
|
|
* - ``V4L2_JPEG_CHROMA_SUBSAMPLING_422``
|
|
- Horizontally subsample Cr, Cb components by a factor of 2.
|
|
* - ``V4L2_JPEG_CHROMA_SUBSAMPLING_420``
|
|
- Subsample Cr, Cb components horizontally and vertically by 2.
|
|
* - ``V4L2_JPEG_CHROMA_SUBSAMPLING_411``
|
|
- Horizontally subsample Cr, Cb components by a factor of 4.
|
|
* - ``V4L2_JPEG_CHROMA_SUBSAMPLING_410``
|
|
- Subsample Cr, Cb components horizontally by 4 and vertically by 2.
|
|
* - ``V4L2_JPEG_CHROMA_SUBSAMPLING_GRAY``
|
|
- Use only luminance component.
|
|
|
|
|
|
|
|
``V4L2_CID_JPEG_RESTART_INTERVAL (integer)``
|
|
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 (integer)``
|
|
``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:
|
|
|
|
``V4L2_CID_JPEG_ACTIVE_MARKER (bitmask)``
|
|
Specify which JPEG markers are included in compressed stream. This
|
|
control is valid only for encoders.
|
|
|
|
|
|
|
|
.. flat-table::
|
|
:header-rows: 0
|
|
:stub-columns: 0
|
|
|
|
* - ``V4L2_JPEG_ACTIVE_MARKER_APP0``
|
|
- Application data segment APP\ :sub:`0`.
|
|
* - ``V4L2_JPEG_ACTIVE_MARKER_APP1``
|
|
- Application data segment APP\ :sub:`1`.
|
|
* - ``V4L2_JPEG_ACTIVE_MARKER_COM``
|
|
- Comment segment.
|
|
* - ``V4L2_JPEG_ACTIVE_MARKER_DQT``
|
|
- Quantization tables segment.
|
|
* - ``V4L2_JPEG_ACTIVE_MARKER_DHT``
|
|
- Huffman tables segment.
|
|
|
|
|
|
|
|
For more details about JPEG specification, refer to :ref:`itu-t81`,
|
|
:ref:`jfif`, :ref:`w3c-jpeg-jfif`.
|