linux/Documentation/userspace-api/media/v4l/vidioc-g-jpegcomp.rst

.. 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/userspace-api/media/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.