linux/linux-5.4.31/Documentation/media/uapi/v4l/vidioc-g-jpegcomp.rst

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.. 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

.. _VIDIOC_G_JPEGCOMP:

******************************************
ioctl VIDIOC_G_JPEGCOMP, VIDIOC_S_JPEGCOMP
******************************************

Name
====

VIDIOC_G_JPEGCOMP - VIDIOC_S_JPEGCOMP


Synopsis
========

.. c:function:: int ioctl( int fd, VIDIOC_G_JPEGCOMP, v4l2_jpegcompression *argp )
    :name: VIDIOC_G_JPEGCOMP

.. c:function:: int ioctl( int fd, VIDIOC_S_JPEGCOMP, const v4l2_jpegcompression *argp )
    :name: VIDIOC_S_JPEGCOMP


Arguments
=========

``fd``
    File descriptor returned by :ref:`open() <func-open>`.

``argp``
    Pointer to struct :c:type:`v4l2_jpegcompression`.


Description
===========

These ioctls are **deprecated**. New drivers and applications should use
:ref:`JPEG class controls <jpeg-controls>` for image quality and JPEG
markers control.

[to do]

Ronald Bultje elaborates:

APP is some application-specific information. The application can set it
itself, and it'll be stored in the JPEG-encoded fields (eg; interlacing
information for in an AVI or so). COM is the same, but it's comments,
like 'encoded by me' or so.

jpeg_markers describes whether the huffman tables, quantization tables
and the restart interval information (all JPEG-specific stuff) should be
stored in the JPEG-encoded fields. These define how the JPEG field is
encoded. If you omit them, applications assume you've used standard
encoding. You usually do want to add them.


.. tabularcolumns:: |p{1.2cm}|p{3.0cm}|p{13.3cm}|

.. c:type:: v4l2_jpegcompression

.. flat-table:: struct v4l2_jpegcompression
    :header-rows:  0
    :stub-columns: 0
    :widths:       1 1 2

    * - int
      - ``quality``
      - Deprecated. If
	:ref:`V4L2_CID_JPEG_COMPRESSION_QUALITY <jpeg-quality-control>`
	control is exposed by a driver applications should use it instead
	and ignore this field.
    * - int
      - ``APPn``
      -
    * - int
      - ``APP_len``
      -
    * - char
      - ``APP_data``\ [60]
      -
    * - int
      - ``COM_len``
      -
    * - char
      - ``COM_data``\ [60]
      -
    * - __u32
      - ``jpeg_markers``
      - See :ref:`jpeg-markers`. Deprecated. If
	:ref:`V4L2_CID_JPEG_ACTIVE_MARKER <jpeg-active-marker-control>`
	control is exposed by a driver applications should use it instead
	and ignore this field.


.. tabularcolumns:: |p{6.6cm}|p{2.2cm}|p{8.7cm}|

.. _jpeg-markers:

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

    * - ``V4L2_JPEG_MARKER_DHT``
      - (1<<3)
      - Define Huffman Tables
    * - ``V4L2_JPEG_MARKER_DQT``
      - (1<<4)
      - Define Quantization Tables
    * - ``V4L2_JPEG_MARKER_DRI``
      - (1<<5)
      - Define Restart Interval
    * - ``V4L2_JPEG_MARKER_COM``
      - (1<<6)
      - Comment segment
    * - ``V4L2_JPEG_MARKER_APP``
      - (1<<7)
      - App segment, driver will always use APP0


Return Value
============

On success 0 is returned, on error -1 and the ``errno`` variable is set
appropriately. The generic error codes are described at the
:ref:`Generic Error Codes <gen-errors>` chapter.