1 .. SPDX-License-Identifier: GFDL-1.1-no-invariants-or-later
2
3 .. _jpeg-controls:
4
5 **********************
6 JPEG Control Reference
7 **********************
8
9 The JPEG class includes controls for common features of JPEG encoders
10 and decoders. Currently it includes features for codecs implementing
11 progressive baseline DCT compression process with Huffman entropy
12 coding.
13
14
15 .. _jpeg-control-id:
16
17 JPEG Control IDs
18 ================
19
20 ``V4L2_CID_JPEG_CLASS (class)``
21 The JPEG class descriptor. Calling
22 :ref:`VIDIOC_QUERYCTRL` for this control will
23 return a description of this control class.
24
25 ``V4L2_CID_JPEG_CHROMA_SUBSAMPLING (menu)``
26 The chroma subsampling factors describe how each component of an
27 input image is sampled, in respect to maximum sample rate in each
28 spatial dimension. See :ref:`itu-t81`, clause A.1.1. for more
29 details. The ``V4L2_CID_JPEG_CHROMA_SUBSAMPLING`` control determines
30 how Cb and Cr components are downsampled after converting an input
31 image from RGB to Y'CbCr color space.
32
33 .. tabularcolumns:: |p{7.5cm}|p{10.0cm}|
34
35 .. flat-table::
36 :header-rows: 0
37 :stub-columns: 0
38
39 * - ``V4L2_JPEG_CHROMA_SUBSAMPLING_444``
40 - No chroma subsampling, each pixel has Y, Cr and Cb values.
41 * - ``V4L2_JPEG_CHROMA_SUBSAMPLING_422``
42 - Horizontally subsample Cr, Cb components by a factor of 2.
43 * - ``V4L2_JPEG_CHROMA_SUBSAMPLING_420``
44 - Subsample Cr, Cb components horizontally and vertically by 2.
45 * - ``V4L2_JPEG_CHROMA_SUBSAMPLING_411``
46 - Horizontally subsample Cr, Cb components by a factor of 4.
47 * - ``V4L2_JPEG_CHROMA_SUBSAMPLING_410``
48 - Subsample Cr, Cb components horizontally by 4 and vertically by 2.
49 * - ``V4L2_JPEG_CHROMA_SUBSAMPLING_GRAY``
50 - Use only luminance component.
51
52
53
54 ``V4L2_CID_JPEG_RESTART_INTERVAL (integer)``
55 The restart interval determines an interval of inserting RSTm
56 markers (m = 0..7). The purpose of these markers is to additionally
57 reinitialize the encoder process, in order to process blocks of an
58 image independently. For the lossy compression processes the restart
59 interval unit is MCU (Minimum Coded Unit) and its value is contained
60 in DRI (Define Restart Interval) marker. If
61 ``V4L2_CID_JPEG_RESTART_INTERVAL`` control is set to 0, DRI and RSTm
62 markers will not be inserted.
63
64 .. _jpeg-quality-control:
65
66 ``V4L2_CID_JPEG_COMPRESSION_QUALITY (integer)``
67 Determines trade-off between image quality and size.
68 It provides simpler method for applications to control image quality,
69 without a need for direct reconfiguration of luminance and chrominance
70 quantization tables. In cases where a driver uses quantization tables
71 configured directly by an application, using interfaces defined
72 elsewhere, ``V4L2_CID_JPEG_COMPRESSION_QUALITY`` control should be set by
73 driver to 0.
74
75 The value range of this control is driver-specific. Only positive,
76 non-zero values are meaningful. The recommended range is 1 - 100,
77 where larger values correspond to better image quality.
78
79 .. _jpeg-active-marker-control:
80
81 ``V4L2_CID_JPEG_ACTIVE_MARKER (bitmask)``
82 Specify which JPEG markers are included in compressed stream. This
83 control is valid only for encoders.
84
85
86
87 .. flat-table::
88 :header-rows: 0
89 :stub-columns: 0
90
91 * - ``V4L2_JPEG_ACTIVE_MARKER_APP0``
92 - Application data segment APP\ :sub:`0`.
93 * - ``V4L2_JPEG_ACTIVE_MARKER_APP1``
94 - Application data segment APP\ :sub:`1`.
95 * - ``V4L2_JPEG_ACTIVE_MARKER_COM``
96 - Comment segment.
97 * - ``V4L2_JPEG_ACTIVE_MARKER_DQT``
98 - Quantization tables segment.
99 * - ``V4L2_JPEG_ACTIVE_MARKER_DHT``
100 - Huffman tables segment.
101
102
103
104 For more details about JPEG specification, refer to :ref:`itu-t81`,
105 :ref:`jfif`, :ref:`w3c-jpeg-jfif`.
Linux® is a registered trademark of Linus Torvalds in the United States and other countries.
TOMOYO® is a registered trademark of NTT DATA CORPORATION.